rip commands 2

7/28/2019 RIP Commands 2

1/80

RIP Commands

Use the commands in this chapter to configure and monitor Routing Information

Protocol (RIP). For RIP configuration information and examples, refer to the"Configuring RIP" chapter of the Network Protocols Configuration Guide, Part 1 .

auto-summary (RIP)

To restore the default behavior of automatic summarization of subnet routes intonetwork-level routes, use the auto-summary command in router configuration mode.To disable this function and transmit subprefix routing information across classfulnetwork boundaries, use the no form of this command.

auto-summary

no auto-summary

Syntax Description

This command has no arguments or keywords.

Defaults

Enabled (the software summarizes subprefixes to the classful network boundary when

crossing classful network boundaries).

Command Modes

Router configuration

Command History

Release Modification

10.0 This command was introduced.

Usage Guidelines

Route summarization reduces the amount of routing information in the routing tables.

RIP Version 1 always uses automatic summarization. If you are using RIP Version 2,you can turn off automatic summarization by specifying no auto-summar y. Disable

automatic summarization if you must perform routing between disconnected subnets.When automatic summarization is off, subnets are advertised.


2/80

Examples

In the following example, network numbers are not summarized automatically:

router rip

version 2no auto-summary

default-information originate

To generate a default route into RIP, use the default-information originate commandin router configuration mode. To disable this feature, use the no form of this command.

default-information originate [route-map mapname ]

no default-information originate

Syntax Description

route-map mapname

(Optional) Routing process will generate the defaultroute if the route map is satisfied.

Defaults

This command is disabled by default.

Command Modes


Command History



Usage Guidelines

The route map referenced in the default-information originate command cannot usean extended access list; it can use a standard access list.

Examples


3/80

The following example originates a default route (0.0.0.0/0) over a certain interfacewhen 172.68.0.0/16 is present. This is called "conditional default origination."

router ripversion 2network 172.68.16.0default-information originate route-map condition

!route-map condition permit 10match ip address 10set interface s1/0

!access-list 10 permit 172.68.16.0 0.0.0.255!

default-metric (RIP)

To set default metric values for RIP, use this form of the default-metric command inrouter configuration mode. To return to the default state, use the no form of thiscommand.

default-metric number

no default-metric [number ]

Syntax Description

number Default metric value.

Defaults

Built-in, automatic metric translations, as appropriate for each routing protocol. Themetric of redistributed connected and static routes is set to 0.

Command Modes


Command History



Usage Guidelines


4/80

The default-metric command is used in conjunction with the redistribute router configuration command to cause the current routing protocol to use the same metricvalue for all redistributed routes. A default metric helps solve the problem of redistributing routes with incompatible metrics. Whenever metrics do not convert, usinga default metric provides a reasonable substitute and enables the redistribution to

proceed.

Note When enabled, the default-metric command applies a metric value of 0 toredistributed connected routes. The default-metric command does not override metricvalues that are applied with the redistribute command.

Examples

The following example shows a router in autonomous system 109 using both the RIPand the OSPF routing protocols. The example advertises OSPF-derived routes using theRIP protocol and assigns the OSPF-derived routes a RIP metric of 10.

router ripdefault-metric 10redistribute ospf 109

Related Commands

Command Description

redistribute Redistributes routes from one routing domain intoanother routing domain.

flash-update-threshold

To suppress regularly scheduled flash updates, use the flash-update-threshold command

in router configuration mode. To return to the default state, use the no form of thiscommand.

flash-update-threshold seconds

no flash-update-threshold

Syntax Description

seconds The time interval in seconds for which the suppression of flash updates can be configured.


5/80

Defaults


Command Modes


Command History



Usage Guidelines

This command suppresses flash updates when the arrival of a regularly scheduledupdate matches the number of seconds that is configured with the seconds argument.The range of seconds that can be configure is from 0 to 30 seconds. If the number of seconds matches the number of seconds or is less than the number seconds that isconfigured with the seconds argument, the flash update is suppressed. If the numbers

seconds until the flash update arrives exceeds the number of seconds that is configuredwith the seconds argument, the flash update is not suppressed. The regular scheduledinterval for flash updates and the configuration of the suppression of flash updates can

be verified with the show ip protocol command.

Examples

The following example configures a router to suppress a regularly scheduled flashupdate if the update is due in 10 seconds or less:

router rip

flash-update-threshold 10

Related Commands

Command Description

show ipprotocols

Displays the parameters and current state of the activerouting protocol process.


6/80

input-queue

To adjust the depth of the Routing Information Protocol (RIP) input queue, use theinput-queue command in router configuration mode. To remove the configured depthand restore the default depth, use the no version of this command.

input-queue depth

no input-queue [ depth ]

Syntax Description

depth Numerical value associated with the depth of the RIP inputqueue. The larger the numerical value, the larger the depth of the queue. The range is 0 to 1024.

Defaults

50

Command Modes


Command History



Usage Guidelines

Consider using the input-queue command if you have a high-end router sending at highspeed to a low-speed router that might not be able to receive at the high speed.Configuring this command will help prevent the routing table from losing information.

Examples

The following example sets the depth of the RIP input queue to 100:

input-queue 100

Related Commands


7/80

Command Description

output-delay Changes interpacket delay for RIP updates sent.

ip rip authentication key-chain

To enable authentication for RIP Version 2 packets and to specify the set of keys thatcan be used on an interface, use the ip rip authentication key-chain command ininterface configuration mode. Use the no form of this command to preventauthentication.

ip rip authentication key-chain name-of-chain

no ip rip authentication key-chain [name-of-chain ]

Syntax Description

name-of-chain

Enables authentication and specifies the group of keysthat are valid.

Defaults

No authentication is provided for RIP packets.

Command Modes

Interface configuration

Command History



Usage Guidelines

If no key chain is configured with the key-chain command, no authentication is performed on the interface (not even the default authentication).

Examples


8/80

The following example configures the interface to accept and send any key belonging tothe key chain named trees :

ip rip authentication key-chain trees

Related Commands

Command Description

key chain Enables authentication for routing protocols by identifyinga group of authentication keys.

ip rip authentication mode

To specify the type of authentication used in RIP Version 2 packets, use the ip ripauthentication mode command in interface configuration mode. Use the no form of thiscommand to restore clear text authentication.

ip rip authentication mode {text | md5}

no ip rip authentication mode

Syntax Description

text Clears text authentication.

md5 Keyed MD5 authentication.

Defaults

Clear text authentication is provided for RIP packets.

Command Modes


Command History




9/80

Usage Guidelines

RIP Version 1 does not support authentication.

Examples

The following example configures the interface to use MD5 authentication:

ip rip authentication mode md5

Related Commands

Command Description

input-queue

Enables authentication for RIP Version 2 packets andspecifies the set of keys that can be used on an interface.

key chain Enables authentication for routing protocols byidentifying a group of authentication keys.

ip rip receive version

To specify a RIP version to receive on an interface basis, use the ip rip receive version command in interface configuration mode. Use the no form of this command to followthe global version rules.

ip rip receive version [1] [2]

no ip rip receive version

Syntax Description

1 (Optional) Accepts only RIP Version 1 packets on the interface.

2 (Optional) Accepts only RIP Version 2 packets on the interface.

Defaults


Command Modes


10/80


Command History



Usage Guidelines

Use this command to override the default behavior of RIP as specified by the version command. This command applies only to the interface being configured. You canconfigure the interface to accept both RIP versions.

Examples

The following example configures the interface to receive both RIP Version 1 andVersion 2 packets:

ip rip receive version 1 2

The following example configures the interface to receive only RIP Version 1 packets:

ip rip receive version 1

Related Commands

Command Description

ip rip sendversion

Specifies a RIP version to send on an interface basis.

version Specifies a RIP vesrion used globally by the router.

input-queue Enables authentication for RIP Version 2 packets andspecifies the set of keys that can be used on aninterface.

key chain Enables authentication for routing protocols byidentifying a group of authentication keys.

ip rip send version


11/80

To specify a RIP version to send on an interface basis, use the ip rip send version command in interface configuration mode. Use the no form of this command to followthe global version rules.

ip rip send version [1] [2]

no ip rip send version

Syntax Description

1 (Optional) Sends only RIP Version 1 packets out the interface.

2 (Optional) Sends only RIP Version 2 packets out the interface.

Defaults

The software behaves according to the router version command.

Command Modes


Command History



Usage Guidelines

Use this command to override the default behavior of RIP as specified by the router version command. This command applies only to the interface being configured.

The following example configures the interface to send both RIP Version 1 and Version2 packets out the interface:

ip rip send version 1 2

The following example configures the interface to send only RIP Version 2 packets outthe interface:

ip rip send version 2


12/80

Related Commands

Command Description

ip rip receiveversion

Specifies a RIP version to receive on aninterface basis.

version Specifies a RIP vesrion used globally by therouter.

ip split-horizon (RIP)

To enable the split horizon mechanism, use the ip split-horizon command in interfaceconfiguration mode. To disable the split horizon mechanism, use the no form of thiscommand.

ip split-horizon

no ip split-horizon

Syntax Description


Defaults

Default behavior varies with media type.

Command Modes


Command History



Usage Guidelines

For all interfaces except those for which either Frame Relay or SMDS encapsulation is

enabled, the default condition for this command is ip split-horizon ; in other words, thesplit horizon feature is active. If the interface configuration includes either the


13/80

encapsulation frame-relay or encapsulation smds commands, then the default is for split horizon to be disabled. Split horizon is not disabled by default for interfaces usingany of the X.25 encapsulations.

Note For networks that include links over X.25 PSNs, the neighbor router configuration command can be used to defeat the split horizon feature. You can as analternative explicitly specify the no ip split-horizon command in your configuration.However, if you do so you must similarly disable split horizon for all routers in anyrelevant multicast groups on that network.

Note If split horizon has been disabled on an interface and you want to enable it, use theip split-horizon command to restore the split horizon mechanism.

Note In general, changing the state of the default for the ip split-horizon command isnot recommended, unless you are certain that your application requires a change inorder to properly advertise routes. If split horizon is disabled on a serial interface (andthat interface is attached to a packet-switched network), you must disable split horizonfor all routers and access servers in any relevant multicast groups on that network.

The following simple example disables split horizon on a serial link. The serial link isconnected to an X.25 network:

interface serial 0encapsulation x25no ip split-horizon

Related Commands

Command Description

neighbor(RIP)

Defines a neighboring router with which to exchangerouting information.

neighbor (RIP)

To define a neighboring router with which to exchange routing information, use thisform of the neighbor command in router configuration mode. To remove an entry, usethe no form of this command.


14/80

neighbor ip-address

no neighbor ip-address

Syntax Description

ip-address

IP address of a peer router with which routing informationwill be exchanged.

Defaults

No neighboring routers are defined.

Command Modes


Command History



Usage Guidelines

This command permits the point-to-point (nonbroadcast) exchange of routinginformation. When it is used in combination with the passive-interface router configuration command, routing information can be exchanged between a subset of routers and access servers on a LAN.

Multiple neighbor commands can be used to specify additional neighbors or peers.

Examples

In the following example, RIP updates are sent to all interfaces on network 10.108.0.0except interface Ethernet 1. However, in this case a neighbor router configurationcommand is included. This command permits the sending of routing updates to specificneighbors. One copy of the routing update is generated per neighbor.

router ripnetwork 10.108.0.0passive-interface ethernet 1

neighbor 10.108.20.4


15/80

Related Commands

Command Description

passive-interface Disables sending routing updates on an interface.

network (RIP)

To specify a list of networks for the Routing Information Protocol (RIP) routing process, use this form of the network command in router configuration mode. Toremove an entry, use the no form of this command.

network network-number

no network network-number

Syntax Description

network-number

IP address of the network of directly connectednetworks.

Defaults

No networks are specified.

Command Modes


Command History



Usage Guidelines

The network number specified must not contain any subnet information. There is nolimit to the number of network commands you can use on the router. RIP routingupdates will be sent and received only through interfaces on this network.


16/80

RIP sends updates to the interfaces in the specified networks. Also, if an interface'snetwork is not specified, it will not be advertised in any RIP update.

Examples

The following example defines RIP as the routing protocol to be used on all interfacesconnected to networks 10.99.0.0 and 192.168.7.0:

router ripnetwork 10.99.0.0network 192.168.7.0

Related Commands

Command Description

router rip Configures the Routing Information Protocol (RIP) process.

offset-list

To add an offset to incoming and outgoing metrics to routes learned via RIP, use the offset-list command in router configuration mode. To remove an offset list, use the no form of this command.

offset-list {access-list-number | name } {in | out } offset [type number ]

no offset-list {access-list-number | name } { in | out } offset [type number ]

Syntax Description

access-list-number |

name

Standard access list number or name to be applied.Access list number 0 indicates all access lists. If offset is

0, no action is taken. For IGRP, the offset is added to thedelay component only.

in Applies the access list to incoming metrics.

out Applies the access list to outgoing metrics.

offset Positive offset to be applied to metrics for networksmatching the access list. If the offset is 0, no action is

taken.


17/80

type (Optional) Interface type to which the offset-list isapplied.

number (Optional) Interface number to which the offset-list isapplied.

Defaults


Command Modes


Command History



10.3 The type and number arguments were added.

11.2 The name argument was added.

Usage Guidelines

The offset value is added to the routing metric. An offset-list with an interface type andinterface number is considered extended and takes precedence over an offset-list that isnot extended. Therefore, if an entry passes the extended offset-list and the normaloffset-list, the extended offset-list's offset is added to the metric.

Examples

In the following example, the router applies an offset of 10 to the router's delaycomponent only to access list 21:

offset-list 21 out 10

In the following example, the router applies an offset of 10 to routes learned from

Ethernet interface 0:offset-list 21 in 10 ethernet 0


18/80

output-delay

To change the interpacket delay for RIP updates sent, use the output-delay command inrouter configuration mode. To remove the delay, use the no form of this command.

output-delay delay

no output-delay [delay ]

Syntax Description

delay Delay, in milliseconds, between packets in a multiple-packetRIP update. The range is 8 to 50 milliseconds. The default is nodelay.

Defaults

0 milliseconds

Command Modes


Command History



Usage Guidelines

Consider using this command if you have a high-end router sending at high speed to alow-speed router that might not be able to receive at the high speed. Configuring thiscommand will help prevent the routing table from losing information.

Examples

The following example sets the interpacket delay to 10 milliseconds:

output-delay 10

router rip


19/80

To configure the Routing Information Protocol (RIP) routing process, use the routerrip command in global configuration mode. To turn off the RIP routing process, use theno form of this command.

router rip

no router rip

Syntax Description


Defaults

No RIP routing process is defined.

Command Modes

Global configuration

Command History



Examples

The following example shows how to begin the RIP routing process:

router rip

Related Commands

Command Description

network (RIP)

Specifies a list of networks for the Routing InformationProtocol (RIP) process.

timers basic

To adjust RIP network timers, use the timers basic command in router configurationmode. To restore the default timers, use the no form of this command.


20/80

timers basic update invalid holddown flush

no timers basic

Syntax Description

update Rate in seconds at which updates are sent. This is thefundamental timing parameter of the routing protocol. Thedefault is 30 seconds.

invalid Interval of time (in seconds) after which a route is declaredinvalid. The interval should be at least three times the valueof update time. The interval is measured from the lastupdate received for the route. The route becomes invalidwhen there is an absence of updates during the invalid timethat refresh the route. The route is marked inaccessible andadvertised as unreachable. However, the route stillforwards packets until the flush interval expires. Thedefault is 180 seconds.

holddown Interval (in seconds) during which routing informationregarding better paths is suppressed. The interval should beat least three times the value of update time. A route entersinto a holddown state when an update packet is receivedthat indicates the route is unreachable. The route is markedinaccessible and advertised as unreachable. However, theroute continues to forward packets until an update isreceived with a better metric or until the holddown timeexpires. When the holddown expires, routes advertised byother sources are accepted and the route is no longer inaccessible. The default is 180 seconds.

flush Amount of time (in seconds) that must pass before theroute is removed from the routing table. The interval is

measured from the last update received for the route. Theinterval should be longer than the larger of the invalid andholddown values. If the interval is less than the sum of theupdate and holddown values, the proper holddown intervalcannot elapse, which results in a new route being accepted

before the holddown interval expires. The default is 240seconds.

Defaults


21/80

update is 30 secondsinvalid is 180 secondsholddown is 180 seconds

flush is 240 seconds

Command ModesRouter configuration

Command History



Usage Guidelines

The basic timing parameters for RIP are adjustable. Since RIP is executing a distributed,asynchronous routing algorithm, it is important that these timers be the same for allrouters and access servers in the network.

Note The current and default timer values can be seen by inspecting the output of theshow ip protocols EXEC command. The relationships of the various timers should be

preserved as described previously.

The following example sets updates to be broadcast every 5 seconds. If a router is notheard from in 15 seconds, the route is declared unusable. Further information issuppressed for an additional 15 seconds. At the end of the suppression period, the routeis flushed from the routing table.

router riptimers basic 5 15 15 30

Note By setting a short update period, you run the risk of congesting slow-speed seriallines; however, this is not a big concern on faster-speed Ethernets and T1-rate seriallines. Also, if you have many routes in your updates, you can cause the routers to spendan excessive amount of time processing updates.

validate-update-source


22/80

To have the Cisco IOS software validate the source IP address of incoming routingupdates for RIP and IGRP routing protocols, use the validate-update-source commandin router configuration mode. To disable this function, use the no form of thiscommand.

validate-update-source

no validate-update-source

Syntax Description


Defaults

The behavior of this command is enabled by default.

Command Modes


Command History



Usage Guidelines

This command is applicable only to RIP and IGRP. The software ensures that the sourceIP address of incoming routing updates is on the same IP network as one of theaddresses defined for the receiving interface.

Disabling split horizon on the incoming interface will also cause the system to performthis validation check.

For unnumbered IP interfaces (interfaces configured as ip unnumbered ), no checkingis performed.

Examples

The following example configures a router not to perform validation checks on thesource IP address of incoming RIP updates:

router ripnetwork 10.105.0.0no validate-update-source


23/80

version

To specify a RIP version used globally by the router, use the version command inrouter configuration mode. Use the no form of this command to restore the defaultvalue.

version {1 | 2}

no version

Syntax Description

1 Specifies RIP Version 1.

2 Specifies RIP Version 2.

Defaults

The software receives RIP Version 1 and Version 2 packets, but sends only Version 1 packets.

Command Modes


Command History



Usage Guidelines

To specify RIP versions used on an interface basis, use the ip rip receive version andip rip send version commands.

Examples

The following example enables the software to send and receive RIP Version 2 packets:

version 2

Related Commands


24/80

Command Description

ip rip receiveversion

Specifies a RIP version to receive on an interface basis.

ip rip sendversion

Specifies a RIP version to send on an interface basis.

show ipprotocols

Displays the parameters and current state of theactive routing protocol process..

Configuring OSPF

This chapter describes how to configure OSPF. For a complete description of the OSPFcommands in this chapter, refer to the "OSPF Commands" chapter of the Network Protocols Command Reference, Part 1 . To locate documentation of other commandsthat appear in this chapter, use the command reference master index or search online.

Open shortest path first (OSPF) is an IGP developed by the OSPF working group of theInternet Engineering Task Force (IETF). Designed expressly for IP networks, OSPFsupports IP subnetting and tagging of externally derived routing information. OSPF alsoallows packet authentication and uses IP multicast when sending/receiving packets.

We support RFC 1253, Open Shortest Path First (OSPF) MIB, August 1991. The OSPF

MIB defines an IP routing protocol that provides management information related toOSPF and is supported by Cisco routers.

For protocol-independent features that include OSPF, see the chapter "Configuring IPRouting Protocol-Independent Features" in this document.

Cisco's OSPF Implementation

Cisco's implementation conforms to the OSPF Version 2 specifications detailed in theInternet RFC 1583. The list that follows outlines key features supported in Cisco's

OSPF implementation:


25/80

Stub areasDefinition of stub areas is supported.

Route redistributionRoutes learned via any IP routing protocol can beredistributed into any other IP routing protocol. At the intradomain level, this meansthat OSPF can import routes learned via IGRP, RIP, and IS-IS. OSPF routes can also be

exported into IGRP, RIP, and IS-IS. At the interdomain level, OSPF can import routeslearned via EGP and BGP. OSPF routes can be exported into EGP and BGP.

AuthenticationPlain text and MD5 authentication among neighboring routerswithin an area is supported.

Routing interface parametersConfigurable parameters supported include interfaceoutput cost, retransmission interval, interface transmit delay, router priority, router "dead" and hello intervals, and authentication key.

Virtual linksVirtual links are supported.

NSSA areasRFC 1587.

OSPF over demand circuitRFC 1793.

OSPF Configuration Task List

OSPF typically requires coordination among many internal routers, area border routers (routers connected to multiple areas), and autonomous system boundary routers. At aminimum, OSPF-based routers or access servers can be configured with all default

parameter values, no authentication, and interfaces assigned to areas. If you intend tocustomize your environment, you must ensure coordinated configurations of all routers.

To configure OSPF, complete the tasks in the following sections. Enabling OSPF ismandatory; the other tasks are optional, but might be required for your application.

Enable OSPF

Configure OSPF Interface Parameters

Configure OSPF over Different Physical Networks

Configure OSPF Area Parameters

Configure OSPF Not So Stubby Area (NSSA)

Configure Route Summarization between OSPF Areas

Configure Route Summarization when Redistributing Routes into OSPF

Create Virtual Links

Generate a Default Route
http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp4652http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp4671http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp4729http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp4852http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp4888http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp4920http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp4937http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp4952http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp4970http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp4652http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp4671http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp4729http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp4852http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp4888http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp4920http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp4937http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp4952http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp4970


26/80

Configure Lookup of DNS Names

Force the Router ID Choice with a Loopback Interface

Control Default Metrics

Change the OSPF Administrative Distances

Configure OSPF on Simplex Ethernet Interfaces

Configure Route Calculation Timers

Configure OSPF over On Demand Circuits

Log Neighbors Going Up or Down

Change the LSA Group Pacing

Block OSPF LSA Flooding

Ignore MOSPF LSA Packets

Monitor and Maintain OSPF

In addition, you can specify route redistribution; see the task "Redistribute RoutingInformation" in the chapter "Configuring IP Routing Protocol-Independent Features" for

information on how to configure route redistribution.

Enable OSPF

As with other routing protocols, enabling OSPF requires that you create an OSPFrouting process, specify the range of IP addresses to be associated with the routing

process, and assign area IDs to be associated with that range of IP addresses. Use thefollowing commands, starting in global configuration mode:

Step Command Purpose

1 router ospf process-id Enable OSPF routing, which placesyou in router configuration mode.

2 network addresswildcard-mask areaarea-id

Define an interface on which OSPFruns and define the area ID for thatinterface.

Configure OSPF Interface Parameters
http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp4988http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5003http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5023http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp6419http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5037http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5052http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5067http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5098http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5113http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp6663http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp6741http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5161http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp4988http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5003http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5023http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp6419http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5037http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5052http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5067http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5098http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5113http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp6663http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp6741http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5161


27/80

Our OSPF implementation allows you to alter certain interface-specific OSPF parameters, as needed. You are not required to alter any of these parameters, but someinterface parameters must be consistent across all routers in an attached network. Those

parameters are controlled by the ip ospf hello-interval , ip ospf dead-interval , and ipospf authentication-key commands. Therefore, be sure that if you do configure any of

these parameters, the configurations for all routers on your network have compatiblevalues.

In interface configuration mode, use any of the following commands to specify interface parameters as needed for your network:

Command Purpose

ip ospf cost cost Explicitly specify the cost of sending a packeton an OSPF interface.

ip ospf retransmit-interval seconds

Specify the number of seconds between link state advertisement retransmissions for adjacencies belonging to an OSPF interface.

ip ospf transmit-delayseconds

Set the estimated number of seconds it takesto transmit a link state update packet on anOSPF interface.

ip ospf prioritynumber

Set priority to help determine the OSPFdesignated router for a network.

ip ospf hello-intervalseconds

Specify the length of time between the hello packets that the Cisco IOS software sends onan OSPF interface.

ip ospf dead-intervalseconds

Set the number of seconds that a device's hello packets must not have been seen before itsneighbors declare the OSPF router down.

ip ospf authentication-key key

Assign a password to be used by neighboringOSPF routers on a network segment that isusing OSPF's simple password authentication.

ip ospf message-digest-key keyid md5key

Enable OSPF MD5 authentication.

ip ospf authentication Specifies the authentication type for an


28/80

[message-digest |null]

interface.

Configure OSPF over Different Physical Networks

OSPF classifies different media into the following three types of networks by default:

Broadcast networks (Ethernet, Token Ring, FDDI)

Nonbroadcast multiaccess networks (SMDS, Frame Relay, X.25)

Point-to-point networks (HDLC, PPP)

You can configure your network as either a broadcast or a nonbroadcast multiaccessnetwork.

X.25 and Frame Relay provide an optional broadcast capability that can be configuredin the map to allow OSPF to run as a broadcast network. See the x25 map and frame-relay map command descriptions in the Wide-Area Networking Command Reference for more detail.

Configure Your OSPF Network Type

You have the choice of configuring your OSPF network type as either broadcast or nonbroadcast multiaccess, regardless of the default media type. Using this feature, youcan configure broadcast networks as nonbroadcast multiaccess networks when, for example, you have routers in your network that do not support multicast addressing.You also can configure nonbroadcast multiaccess networks (such as X.25, Frame Relay,and SMDS) as broadcast networks. This feature saves you from having to configureneighbors, as described in the section " Configure OSPF for Nonbroadcast Networks ."

Configuring nonbroadcast, multiaccess networks as either broadcast or nonbroadcastassumes that there are virtual circuits from every router to every router or fully meshednetwork. This is not true for some cases, for example, because of cost constraints, or when you have only a partially meshed network. In these cases, you can configure theOSPF network type as a point-to-multipoint network. Routing between two routers notdirectly connected will go through the router that has virtual circuits to both routers.

Note that it is not necessary to configure neighbors when using this feature.

An OSPF point-to-multipoint interface is defined as a numbered point-to-point interfacehaving one or more neighbors. It creates multiple host routes. An OSPF point-to-multipoint network has the following benefits compared to nonbroadcast multiaccessand point-to-point networks:

Point-to-multipoint is easier to configure because it requires no configuration of neighbor commands, it consumes only one IP subnet, and it requires no designatedrouter election.
http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp4801http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp4801


29/80

It costs less because it does not require a fully meshed topology.

It is more reliable because it maintains connectivity in the event of virtual circuitfailure.

To configure your OSPF network type, use the following command in interfaceconfiguration mode:

Command Purpose

ip ospf network {broadcast | nonbroadcast | {point-to-multipoint [nonbroadcast] }}

Configure the OSPF network type for a specified interface.

See the " OSPF Point-to-Multipoint Example " section at the end of this chapter for anexample of an OSPF point-to-multipoint network.

Configure Point-to-Multipoint, Broadcast Networks

On point-to-multipoint, broadcast networks, there is no need to specify neighbors.However, you can specify neighbors with the neighbor command, in which case youshould specify a cost to that neighbor.

Before this feature, some OSPF point-to-multipoint protocol traffic was treated as

multicast traffic. Therefore, the neighbor command was not needed for point-to-multipoint interfaces because multicast took care of the traffic. Hellos, updates andacknowledgments were sent using multicast. In particular, multicast hellos discoveredall neighbors dynamically.

On any point-to-multipoint interface (broadcast or not), the Cisco IOS softwareassumed the cost to each neighbor was equal. The cost was configured with the ip ospf cost command. In reality, the bandwidth to each neighbor is different, so the cost should

be different. With this feature, you can configure a separate cost to each neighbor. Thisfeature applies to point-to-multipoint interfaces only.

To treat an interface as point-to-multipoint broadcast and assign a cost to each neighbor,use the following commands beginning in interface configuration mode:


1 ip ospf network point-to-multipoint

Configure an interface as point-to-multipointfor broadcast media.

2 exit Enter global configuration mode.


30/80

3 router ospf process-id

Configure an OSPF routing process and enter router configuration mode.

4 neighbor ip-address costnumber

Specify a neighbor and assign a cost to theneighbor.

5 Repeat Step 4 for each neighbor if you wantto specify a cost. Otherwise, neighbors willassume the cost of the interface, based on theip ospf cost command.

Configure OSPF for Nonbroadcast Networks

Because there might be many routers attached to an OSPF network, a designated router is selected for the network. It is necessary to use special configuration parameters in thedesignated router selection if broadcast capability is not configured.

These parameters need only be configured in those devices that are themselves eligibleto become the designated router or backup designated router (in other words, routerswith a nonzero router priority value).

To configure routers that interconnect to nonbroadcast networks, use the followingcommand in router configuration mode:

Command Purpose

neighbor ip-address [prioritynumber] [poll-interval seconds]

Configure a router interconnectingto nonbroadcast networks.

You can specify the following neighbor parameters, as required:

Priority for a neighboring router

Nonbroadcast poll interval

Interface through which the neighbor is reachable

On point-to-multipoint, nonbroadcast networks, you now use the neighbor command toidentify neighbors. Assigning a cost to a neighbor is optional.

Prior to Release 12.0, some customers were using point-to-multipoint on nonbroadcastmedia (such as classic IP over ATM), so their routers could not dynamically discover


31/80

their neighbors. This feature allows the neighbor command to be used on point-to-multipoint interfaces.

On any point-to-multipoint interface (broadcast or not), the Cisco IOS softwareassumed the cost to each neighbor was equal. The cost was configured with the ip ospf

cost command. In reality, the bandwidth to each neighbor is different, so the cost should be different. With this feature, you can configure a separate cost to each neighbor. Thisfeature applies to point-to-multipoint interfaces only.

To treat the interface as point-to-multipoint when the media does not support broadcast,use the following commands beginning in interface configuration mode:


1 ip ospf network point-to-multipoint non-broadcast

Configure an interface as point-to-multipoint for nonbroadcast media.

2 exit Enter global configuration mode.

3 router ospf process-id Configure an OSPF routing processand enter router configurationmode.

4 neighbor ip-address [costnumber]

Specify an OSPF neighbor andoptionally assign a cost to theneighbor.

5 Repeat Step 4 for each neighbor.

Configure OSPF Area Parameters

Our OSPF software allows you to configure several area parameters. These area

parameters, shown in the following table, include authentication, defining stub areas,and assigning specific costs to the default summary route. Authentication allows

password-based protection against unauthorized access to an area.

Stub areas are areas into which information on external routes is not sent. Instead, thereis a default external route generated by the area border router, into the stub area for destinations outside the autonomous system. To take advantage of the OSPF stub areasupport, default routing must be used in the stub area. To further reduce the number of link state advertisements sent into a stub area, you can configure no-summary on theABR to prevent it from sending summary link advertisement (link state advertisementsType 3) into the stub area.


32/80

In router configuration mode, specify any of the following area parameters as neededfor your network:

Command Purpose

area area-id authentication Enable authentication for an OSPF area.

area area-id authenticationmessage-digest

Enable MD5 authentication for an OSPFarea.

area area-id stub [no-summary]

Define an area to be a stub area.

area area-id default-costcost

Assign a specific cost to the defaultsummary route used for the stub area.

Configure OSPF Not So Stubby Area (NSSA)

NSSA area is similar to OSPF stub area. NSSA does not flood Type 5 external link stateadvertisements (LSAs) from the core into the area, but it has the ability of importing ASexternal routes in a limited fashion within the area.

NSSA allows importing of Type 7 AS external routes within NSSA area byredistribution. These Type 7 LSAs are translated into Type 5 LSAs by NSSA ABR which are flooded throughout the whole routing domain. Summarization and filteringare supported during the translation.

Use NSSA to simplify administration if you are an Internet service provider (ISP), or anetwork administrator that must connect a central site using OSPF to a remote site thatis using a different routing protocol.

Prior to NSSA, the connection between the corporate site border router and the remoterouter could not be run as OSPF stub area because routes for the remote site cannot beredistributed into stub area. A simple protocol like RIP is usually run and handle theredistribution. This meant maintaining two routing protocols. With NSSA, you canextend OSPF to cover the remote connection by defining the area between the corporaterouter and the remote router as an NSSA.

In router configuration mode, use the following command to specify area parameters asneeded to configure OSPF NSSA:

Command Purpose

area area -id nssa [no-redistribution] [ default- Define an area to be


33/80

information-originate ] NSSA.

In router configuration mode on the ABR, use the following command to controlsummarization and filtering of Type 7 LSA into Type 5 LSA:

Command Purpose

summary address prefix mask [not advertise] [ tag tag ]

(Optional) Control the summarizationand filtering during the translation.

Implementation Considerations

Evaluate the following considerations before implementing this feature:

You can set a Type 7 default route that can be used to reach external destinations.When configured, the router generates a Type 7 default into the NSSA by the NSSAABR.

Every router within the same area must agree that the area is NSSA; otherwise, therouters will not be able to communicate with each other.

If possible, avoid using explicit redistribution on NSSA ABR because confusion mayresult over which packets are being translated by which router.

Configure Route Summarization between OSPF Areas

Route summarization is the consolidation of advertised addresses. This feature causes asingle summary route to be advertised to other areas by an ABR. In OSPF, an ABR willadvertise networks in one area into another area. If the network numbers in an area areassigned in a way such that they are contiguous, you can configure the ABR to advertisea summary route that covers all the individual networks within the area that fall into the

specified range.To specify an address range, use the following command in router configuration mode:

Command Purpose

area area-id range address mask [advertise | not-advertise]

Specify an address range for which asingle route will be advertised.


34/80

Configure Route Summarization when RedistributingRoutes into OSPF

When redistributing routes from other protocols into OSPF (as described in the chapter

"Configuring IP Routing Protocol-Independent Features"), each route is advertisedindividually in an external link state advertisement (LSA). However, you can configurethe Cisco IOS software to advertise a single route for all the redistributed routes that arecovered by a specified network address and mask. Doing so helps decrease the size of the OSPF link state database.

To have the software advertise one summary route for all redistributed routes covered by a network address and mask, use the following command in router configurationmode:

Command Purpose

summary-addressaddress mask

Specify an address and mask that coversredistributed routes, so only one summary route isadvertised.

Create Virtual Links

In OSPF, all areas must be connected to a backbone area. If there is a break in backbonecontinuity, or the backbone is purposefully partitioned, you can establish a virtual link .The two end points of a virtual link are Area Border Routers. The virtual link must beconfigured in both routers. The configuration information in each router consists of theother virtual endpoint (the other ABR), and the nonbackbone area that the two routershave in common (called the transit area ). Note that virtual links cannot be configuredthrough stub areas.

To establish a virtual link, use the following command in router configuration mode:

Command Purpose

area area-id virtual-link router-id [authentication[message-digest | null ]] [hello-interval seconds][retransmit-interval seconds] [transmit-delay seconds][dead-interval seconds] [[authentication-key key] |[message-digest-key keyid md5 key]]

Establish avirtual link.

To display information about virtual links, use the show ip ospf virtual-links EXECcommand. To display the router ID of an OSPF router, use the show ip ospf EXECcommand.


35/80

Generate a Default Route

You can force an autonomous system boundary router to generate a default route into anOSPF routing domain. Whenever you specifically configure redistribution of routes intoan OSPF routing domain, the router automatically becomes an autonomous system

boundary router. However, an autonomous system boundary router does not, by default,generate a default route into the OSPF routing domain.

To force the autonomous system boundary router to generate a default route, use thefollowing command in router configuration mode:

Command Purpose

default-information originate

[always] [metric metric-value][metric-type type-value] [route-map map-name]

Force the autonomous system

boundary router to generate adefault route into the OSPFrouting domain.

See the discussion of redistribution of routes in the "Configuring IP Routing Protocol-Independent Features" chapter.

Configure Lookup of DNS Names

You can configure OSPF to look up Domain Naming System (DNS) names for use inall OSPF show command displays. This feature makes it easier to identify a router, because it is displayed by name rather than by its router ID or neighbor ID.

To configure DNS name lookup, use the following command in global configurationmode:

Command Purpose

ip ospf name-lookup Configure DNS name lookup.

Force the Router ID Choice with a Loopback Interface

OSPF uses the largest IP address configured on the interfaces as its router ID. If theinterface associated with this IP address is ever brought down, or if the address isremoved, the OSPF process must recalculate a new router ID and resend all its routinginformation out its interfaces.

If a loopback interface is configured with an IP address, the Cisco IOS software will usethis IP address as its router ID, even if other interfaces have larger IP addresses. Sinceloopback interfaces never go down, greater stability in the routing table is achieved.


36/80

OSPF automatically prefers a loopback interface over any other kind, and it chooses thehighest IP address among all loopback interfaces. If no loopback interfaces are present,the highest IP address in the router is chosen. You cannot tell OSPF to use any

particular interface.

To configure an IP address on a loopback interface, use the following commands,starting in global configuration mode:


1 interfaceloopback 0

Create a loopback interface, which places youin interface configuration mode.

2 ip address

address mask

Assign an IP address to this interface.

Control Default Metrics

In Cisco IOS Release 10.3 and later, by default, OSPF calculates the OSPF metric for an interface according to the bandwidth of the interface. For example, a 64K link gets ametric of 1562, while a T1 link gets a metric of 64.

The OSPF metric is calculated as ref-bw divided by bandwidth , with ref-bw equal to

108 by default, and bandwidth determined by the bandwidth command. The calculationgives FDDI a metric of 1. If you have multiple links with high bandwidth, you mightwant to specify a larger number to differentiate the cost on those links. To do so, use thefollowing command in router configuration mode:

Command Purpose

ospf auto-cost reference-bandwidthref-bw

Differentiate high bandwidthlinks.

Change the OSPF Administrative Distances

An administrative distance is a rating of the trustworthiness of a routing informationsource, such as an individual router or a group of routers. Numerically, anadministrative distance is an integer between 0 and 255. In general, the higher the value,the lower the trust rating. An administrative distance of 255 means the routinginformation source cannot be trusted at all and should be ignored.

OSPF uses three different administrative distances: intra-area, inter-area, and external.Routes within an area are intra-area; routes to another area are inter-area; and routes


37/80

from another routing domain learned via redistribution are external. The default distancefor each type of route is 110.

To change any of the OSPF distance values, use the following command in router configuration mode:

Command Purpose

distance ospf {[intra-area dist1] [inter-areadist2] [external dist3]}

Change the OSPFdistance values.

For an example of changing administrative distance, see the section " Changing OSPFAdministrative Distance " at the end of this chapter.

Configure OSPF on Simplex Ethernet Interfaces

Because simplex interfaces between two devices on an Ethernet represent only onenetwork segment, for OSPF you must configure the transmitting interface to be a

passive interface. This prevents OSPF from sending hello packets for the transmittinginterface. Both devices are able to see each other via the hello packet generated for thereceiving interface.

To configure OSPF on simplex Ethernet interfaces, use the following command inrouter configuration mode:

Command Purpose

passive-interface typenumber

Suppress the sending of hello packets throughthe specified interface.

Configure Route Calculation Timers

You can configure the delay time between when OSPF receives a topology change andwhen it starts a shortest path first (SPF) calculation. You can also configure the holdtime between two consecutive SPF calculations. To do this, use the following commandin router configuration mode:

Command Purpose

timers spf spf-delay spf-holdtime Configure route calculation timers.
http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp6616http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp6616http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp6616http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp6616


38/80

Configure OSPF over On Demand Circuits

The OSPF on demand circuit is an enhancement to the OSPF protocol that allowsefficient operation over on demand circuits like ISDN, X.25 SVCs and dial-up lines.This feature supports RFC 1793, Extending OSPF to Support Demand Circuits.

Prior to this feature, OSPF periodic hello and link state advertisements (LSAs) updateswould be exchanged between routers that connected the on demand link, even when nochanges occurred in the hello or LSA information.

With this feature, periodic hellos are suppressed and the periodic refreshes of LSAs arenot flooded over the demand circuit. These packets bring up the link only when they areexchanged for the first time, or when a change occurs in the information they contain.This operation allows the underlying datalink layer to be closed when the network topology is stable.

This feature is useful when you want to connect telecommuters or branch offices to anOSPF backbone at a central site. In this case, OSPF for on demand circuits allows the

benefits of OSPF over the entire domain, without excess connection costs. Periodicrefreshes of hello updates, LSA updates, and other protocol overhead are preventedfrom enabling the on demand circuit when there is no "real" data to transmit.

Overhead protocols such as hellos and LSAs are transferred over the on demand circuitonly upon initial setup and when they reflect a change in the topology. This means thatcritical changes to the topology that require new SPF calculations are transmitted inorder to maintain network topology integrity. Periodic refreshes that do not include

changes, however, are not transmitted across the link.

To configure OSPF for on demand circuits, use the following commands, beginning inglobal configuration mode:


1 router ospf process-id Enable OSPF operation.

2 interface type number Enter interface configuration mode.

3 ip ospf demand-circuit

Configure OSPF on an on demandcircuit.

If the router is part of a point-to-point topology, then only one end of the demand circuitmust be configured with this command. However, all routers must have this featureloaded.

If the router is part of a point-to-multipoint topology, only the multipoint end must beconfigured with this command.


39/80

For an example of OSPF over an on-demand circuit, see the section " OSPF over On-Demand Routing Example " at the end of this chapter.

Implementation Considerations

Evaluate the following considerations before implementing this feature:

Because LSAs that include topology changes are flooded over an on demand circuit,it is advised to put demand circuits within OSPF stub areas, or within NSSAs to isolatethe demand circuits from as many topology changes as possible.

To take advantage of the on demand circuit functionality within a stub area or NSSA, every router in the area must have this feature loaded. If this feature is deployedwithin a regular area, all other regular areas must also support this feature before thedemand circuit functionality can take effect. This is because type 5 external LSAs areflooded throughout all areas.

You do not want to do on a broadcast-based network topology because the overhead protocols (such as hellos and LSAs) cannot be successfully suppressed, which meansthe link will remain up.

Configuring the router for an OSPF on-demand circuit with an asynchronousinterface is not a supported configuration. The supported configuration is to use dialer interfaces on both ends of the circuit. For more information, refer to the following TACURL:

/en/US/tech/tk365/technologies_tech_note09186a008009481b.shtml#reason5

Log Neighbors Going Up or Down

To configure the router to send a syslog message when an OSPF neighbor goes up or down, use the following command in router configuration mode:

Command Purpose

ospf log-adjacency-changes

Send syslog message when an OSPF neighbor goes up or down.

Configure this command if you want to know about OSPF neighbors going up or downwithout turning on the debugging command debug ip ospf adjacency . The ospf log-adjacency-changes command provides a higher level view of such changes with lessoutput.

Change the LSA Group Pacing


40/80

The OSPF LSA group pacing feature allows the router to group together OSPF link state advertisements (LSAs) and pace the refreshing, checksumming, and agingfunctions. The group pacing results in more efficient use of the router.

The router groups together OSPF LSAs and paces the refreshing, checksumming, and

aging functions so that sudden hits on CPU usage and network resources are avoided.This feature is most beneficial to large OSPF networks.

OSPF LSA group pacing is enabled by default. For typical customers, the default group pacing interval for refreshing, checksumming, and aging is appropriate and you neednot configure this feature.

Original LSA Behavior

Each OSPF LSA has an age, which indicates whether the LSA is still valid. Once theLSA reaches the maximum age (one hour), it is discarded. During the aging process, theoriginating router sends a refresh packet every 30 minutes to refresh the LSA. Refresh

packets are sent to keep the LSA from expiring, whether there has been a change in thenetwork topology or not. Checksumming is performed on all LSAs every 10 minutes.The router keeps track of LSAs it generates and LSAs it receives from other routers.The router refreshes LSAs it generated; it ages the LSAs it received from other routers.

Prior to the LSA group pacing feature, the Cisco IOS software would performrefreshing on a single timer, and checksumming and aging on another timer. In the caseof refreshing, for example, the software would scan the whole database every 30minutes, refreshing every LSA the router generated, no matter how old it was. illustrates

all the LSAs being refreshed at once. This process wasted CPU resources because onlya small portion of the database needed to be refreshed. A large OSPF database (severalthousand LSAs) could have thousands of LSAs with different ages. Refreshing on asingle timer resulted in the age of all LSAs becoming synchronized, which resulted inmuch CPU processing at once. Furthermore, a huge number of LSAs could cause asudden increase of network traffic, consuming a large amount of network resources in ashort period of time.

Figure 21 OSPF LSAs on a Single Timer without Group Pacing

Solution

This problem is solved by each LSA having its own timer. Again using the example of refreshing, each LSA gets refreshed when it is 30 minutes old, independent of other LSAs. So CPU is used only when necessary. However, LSAs being refreshed atfrequent, random intervals would require many packets for the few refreshed LSAs the

router must send out. That would be inefficient use of bandwidth.


41/80

Therefore, the router delays the LSA refresh function for an interval of time instead of performing it when the individual timers are reached. The accumulated LSAs constitutea group, which is then refreshed and sent out in one packet or more. Thus, the refresh

packets are paced, as are the checksumming and aging. The pacing interval isconfigurable; it defaults to 4 minutes, which is randomized to further avoid

synchronization.

illustrates the case of refresh packets. The first timeline illustrates individual LSAtimers; the second timeline illustrates individual LSA timers with group pacing.

Figure 22 OSPF LSAs on Individual Timers with Group Pacing

The group pacing interval is inversely proportional to the number of LSAs the router isrefreshing, checksumming, and aging. For example, if you have approximately 10,000LSAs, decreasing the pacing interval would benefit you. If you have a very smalldatabase (40 to 100 LSAs), increasing the pacing interval to 10 to 20 minutes might

benefit you slightly.

The default value of pacing between LSA groups is 240 seconds (4 minutes). The rangeis 10 seconds to 1800 seconds (half an hour). To change the LSA group pacing interval,use the following command in router configuration mode:

Command Purpose

timers lsa-group-pacing seconds Change the group pacing of LSAs.

For an example, see the section " LSA Group Pacing Example " at the end of thischapter.

Block OSPF LSA Flooding

By default, OSPF floods new LSAs over all interfaces in the same area, except theinterface on which the LSA arrives. Some redundancy is desirable, because it ensures


42/80

robust flooding. However, too much redundancy can waste bandwidth and mightdestabilize the network due to excessive link and CPU usage in certain topologies. Anexample would be a fully meshed topology.

You can block OSPF flooding of LSAs two ways, depending on the type of networks:

On broadcast, nonbroadcast, and point-to-point networks, you can block floodingover specified OSPF interfaces.

On point-to-multipoint networks, you can block flooding to a specified neighbor.

On broadcast, nonbroadcast, and point-to-point networks, to prevent flooding of OSPFLSAs, use the following command in interface configuration mode:

Command Purpose

ospf database-filter allout

Block the flooding of OSPF LSA packets tothe interface.

On point-to-multipoint networks, to prevent flooding of OSPF LSAs, use the followingcommand in router configuration mode:

Command Purpose

neighbor ip-addressdatabase-filter all out

Block the flooding of OSPF LSA packetsto the specified neighbor.

For an example of blocking LSA flooding, see the section " Block LSA FloodingExample " at the end of this chapter.

Ignore MOSPF LSA Packets

Cisco routers do not support LSA Type 6 (MOSPF), and they generate syslog messagesif they receive such packets. If the router is receiving many MOSPF packets, you mightwant to configure the router to ignore the packets and thus prevent a large number of syslog messages. To do so, use the following command in router configuration mode:

Command Purpose

ospf ignore lsamospf

Prevent the router from generating syslog messageswhen it receives MOSPF LSA packets.


43/80

For an example of suppressing MOSPF LSA packets, see the section " Ignore MOSPFLSA Packets Example " at the end of this chapter.

Monitor and Maintain OSPF

You can display specific statistics such as the contents of IP routing tables, caches, anddatabases. Information provided can be used to determine resource utilization and solvenetwork problems. You can also display information about node reachability anddiscover the routing path your device's packets are taking through the network.

To display various routing statistics, use the following commands in EXEC mode:

Command Purpose

show ip ospf [process-id] Display general information aboutOSPF routing processes.

show ip ospf [process-idarea-id] database

show ip ospf [process-idarea-id] database [router][link-state-id]

show ip ospf [process-idarea-id] database [router][self-originate]

show ip ospf [process-idarea-id] database [router][adv-router [ip-address]]

show ip ospf [process-idarea-id] database [network][link-state-id]

show ip ospf [process-idarea-id] database [summary][link-state-id]

show ip ospf [process-idarea-id] database [asbr-summary] [link-state-id]

show ip ospf [process-id]database [external] [link-state-id]

show ip ospf [process-id

Display lists of information related tothe OSPF database.


44/80

area-id] database [database-summary]

show ip ospf border-routers Display the internal OSPF routing tableentries to Area Border Router (ABR)and Autonomous System BoundaryRouter (ASBR).

show ip ospf interface[interface-name]

Display OSPF-related interfaceinformation.

show ip ospf neighbor [interface-name] [neighbor-id] detail

Display OSPF-neighbor information ona per-interface basis.

show ip ospf request-list[nbr] [intf] [intf-nbr]

Display a list of all LSAs requested bya router.

show ip ospf retransmission-list [nbr][intf] [intf-nbr]

Display a list of all LSAs waiting to beretransmitted.

show ip ospf virtual-links Display OSPF-related virtual linksinformation.

OSPF Configuration Examples

The following sections provide OSPF configuration examples:

OSPF Point-to-Multipoint Example

OSPF Point-to-Multipoint, Broadcast Example

OSPF Point-to-Multipoint, Nonbroadcast Example

Variable-Length Subnet Masks Example

OSPF Routing and Route Redistribution Examples

Route Map Examples

Changing OSPF Administrative Distance
http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5239http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5302http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5354http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5397http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5414http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5623http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp6616http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5239http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5302http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5354http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5397http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5414http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5623http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp6616


45/80

OSPF over On-Demand Routing Example

LSA Group Pacing Example

Block LSA Flooding Example

Ignore MOSPF LSA Packets Example

OSPF Point-to-Multipoint Example

In , Mollie uses DLCI 201 to communicate with Neon, DLCI 202 to Jelly, andDLCI 203 to Platty. Neon uses DLCI 101 to communicate with Mollie and DLCI 102 tocommunicate with Platty. Platty communicates with Neon (DLCI 401) and Mollie(DLCI 402). Jelly communicates with Mollie (DLCI 301).

Figure 23 OSPF Point-to-Multipoint Example

Mollie's Configuration

hostname mollie!interface serial 1

ip address 10.0.0.2 255.0.0.0ip ospf network point-to-multipointencapsulation frame-relayframe-relay map ip 10.0.0.1 201 broadcastframe-relay map ip 10.0.0.3 202 broadcastframe-relay map ip 10.0.0.4 203 broadcast

!router ospf 1

network 10.0.0.0 0.0.0.255 area 0

Neon's Configuration

hostname neon!interface serial 0

ip address 10.0.0.1 255.0.0.0ip ospf network point-to-multipointencapsulation frame-relay

frame-relay map ip 10.0.0.2 101 broadcastframe-relay map ip 10.0.0.4 102 broadcast

!
http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp6463http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5726http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp6724http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp6769http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp6463http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp5726http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp6724http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp6769


46/80

router ospf 1network 10.0.0.0 0.0.0.255 area 0

Platty's Configuration

hostname platty!interface serial 3

ip address 10.0.0.4 255.0.0.0ip ospf network point-to-multipointencapsulation frame-relayclock rate 1000000frame-relay map ip 10.0.0.1 401 broadcastframe-relay map ip 10.0.0.2 402 broadcast

!router ospf 1

network 10.0.0.0 0.0.0.255 area 0

Jelly's Configurationhostname jelly!interface serial 2

ip address 10.0.0.3 255.0.0.0ip ospf network point-to-multipointencapsulation frame-relayclock rate 2000000frame-relay map ip 10.0.0.2 301 broadcast

!router ospf 1

network 10.0.0.0 0.0.0.255 area 0

OSPF Point-to-Multipoint, Broadcast Example

The following example illustrates a point-to-multipoint network with broadcast:

interface Serial0ip address 10.0.1.1 255.255.255.0encapsulation frame-relayip ospf cost 100ip ospf network point-to-multipointframe-relay map ip 10.0.1.3 202 broadcastframe-relay map ip 10.0.1.4 203 broadcastframe-relay map ip 10.0.1.5 204 broadcastframe-relay local-dlci 200

!router ospf 1

network 10.0.1.0 0.0.0.255 area 0neighbor 10.0.1.5 cost 5neighbor 10.0.1.4 cost 10

The following example shows the configuration of the neighbor at 10.0.1.3:

interface serial 0

ip address 10.0.1.3 255.255.255.0ip ospf network point-to-multipointencapsulation frame-relay


47/80

frame-relay local-dlci 301frame-relay map ip 10.0.1.1 300 broadcastno shut

!router ospf 1network 10.0.1.0 0.0.0.255 area 0

The output shown for neighbors in the first configuration above looks like this:

Router# show ip ospf neighborNeighbor ID Pri State Dead Time AddressInterface4.1.1.1 1 FULL/ - 00:01:50 10.0.1.5Serial03.1.1.1 1 FULL/ - 00:01:47 10.0.1.4Serial02.1.1.1 1 FULL/ - 00:01:45 10.0.1.3Serial0

The route information in the first configuration above looks like this:

Router# show ip routeCodes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B -BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter areaN1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGPi - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * -

candidate default

U - per-user static route, o - ODRGateway of last resort is not setC 1.0.0.0/8 is directly connected, Loopback0

10.0.0.0/8 is variably subnetted, 4 subnets, 2 masksO 10.0.1.3/32 [110/100] via 10.0.1.3, 00:39:08, Serial0C 10.0.1.0/24 is directly connected, Serial0O 10.0.1.5/32 [110/5] via 10.0.1.5, 00:39:08, Serial0O 10.0.1.4/32 [110/10] via 10.0.1.4, 00:39:08, Serial0

OSPF Point-to-Multipoint, Nonbroadcast Example

The following example illustrates a point-to-multipoint network with nonbroadcast:

interface Serial0ip address 10.0.1.1 255.255.255.0ip ospf network point-to-multipoint non-broadcastencapsulation frame-relayno keepaliveframe-relay local-dlci 200frame-relay map ip 10.0.1.3 202frame-relay map ip 10.0.1.4 203frame-relay map ip 10.0.1.5 204no shut!router ospf 1

network 10.0.1.0 0.0.0.255 area 0neighbor 10.0.1.3 cost 5neighbor 10.0.1.4 cost 10


48/80

neighbor 10.0.1.5 cost 15

The following example is the configuration for the router on the other side:

interface Serial9/2ip address 10.0.1.3 255.255.255.0encapsulation frame-relayip ospf network point-to-multipoint non-broadcastno ip mroute-cacheno keepaliveno fair-queueframe-relay local-dlci 301frame-relay map ip 10.0.1.1 300no shut!router ospf 1network 10.0.1.0 0.0.0.255 area 0

The output shown for neighbors in the first configuration above looks like this:

Router# show ip ospf neighborNeighbor ID Pri State Dead Time AddressInterface4.1.1.1 1 FULL/ - 00:01:52 10.0.1.5Serial03.1.1.1 1 FULL/ - 00:01:52 10.0.1.4Serial02.1.1.1 1 FULL/ - 00:01:52 10.0.1.3Serial0

Variable-Length Subnet Masks Example

OSPF, static routes, and IS-IS support variable-length subnet masks (VLSMs). WithVLSMs, you can use different masks for the same network number on differentinterfaces, which allows you to conserve IP addresses and more efficiently use availableaddress space.

In the following example, a 30-bit subnet mask is used, leaving two bits of addressspace reserved for serial line host addresses. There is sufficient host address space for two host endpoints on a point-to-point serial link.

interface ethernet 0ip address 131.107.1.1 255.255.255.0

! 8 bits of host address space reserved for ethernets

interface serial 0ip address 131.107.254.1 255.255.255.252

! 2 bits of address space reserved for serial lines

! Router is configured for OSPF and assigned AS 107router ospf 107! Specifies network directly connected to the router

network 131.107.0.0 0.0.255.255 area 0.0.0.0


49/80

OSPF Routing and Route Redistribution Examples

OSPF typically requires coordination among many internal routers, area border routers,and autonomous system boundary routers. At a minimum, OSPF-based routers can beconfigured with all default parameter values, with no authentication, and with interfaces

assigned to areas.

Three examples follow:

The first is a simple configuration illustrating basic OSPF commands.

The second example illustrates a configuration for an internal router, ABR, andASBRs within a single, arbitrarily assigned, OSPF autonomous system.

The third example illustrates a more complex configuration and the application of various tools available for controlling OSPF-based routing environments.

Basic OSPF Configuration Example

The following example illustrates a simple OSPF configuration that enables OSPFrouting process 9000, attaches Ethernet 0 to area 0.0.0.0, and redistributes RIP intoOSPF, and OSPF into RIP:

interface ethernet 0ip address 130.93.1.1 255.255.255.0ip ospf cost 1

!


!router ospf 9000

network 130.93.0.0 0.0.255.255 area 0.0.0.0redistribute rip metric 1 subnets

!router rip

network 130.94.0.0redistribute ospf 9000default-metric 1

Basic OSPF Configuration Example for Internal Router, ABR, andASBRs

The following example illustrates the assignment of four area IDs to four IP addressranges. In the example, OSPF routing process 109 is initialized, and four OSPF areasare defined: 10.9.50.0, 2, 3, and 0. Areas 10.9.50.0, 2, and 3 mask specific addressranges, while Area 0 enables OSPF for all other networks.

router ospf 109network 131.108.20.0 0.0.0.255 area 10.9.50.0network 131.108.0.0 0.0.255.255 area 2network 131.109.10.0 0.0.0.255 area 3

network 0.0.0.0 255.255.255.255 area 0!


50/80

! Interface Ethernet0 is in area 10.9.50.0:interface ethernet 0

ip address 131.108.20.5 255.255.255.0!! Interface Ethernet1 is in area 2:interface ethernet 1





ip address 10.1.0.1 255.255.0.0

Each network area router configuration command is evaluated sequentially, so theorder of these commands in the configuration is important. The Cisco IOS softwaresequentially evaluates the addres s/wildcard-mask pair for each interface. See the "OSPFCommands" chapter of the Network Protocols Command Reference, Part 1 for moreinformation.

Consider the first network area command. Area ID 10.9.50.0 is configured for theinterface on which subnet 131.108.20.0 is located. Assume that a match is determinedfor interface Ethernet 0. Interface Ethernet 0 is attached to Area 10.9.50.0 only.

The second network area command is evaluated next. For Area 2, the same process isthen applied to all interfaces (except interface Ethernet 0). Assume that a match isdetermined for interface Ethernet 1. OSPF is then enabled for that interface and Ethernet1 is attached to Area 2.

This process of attaching interfaces to OSPF areas continues for all network areacommands. Note that the last network area command in this example is a special case.With this command, all available interfaces (not explicitly attached to another area) areattached to Area 0.

Complex Internal Router, ABR, and ASBRs Example

The following example outlines a configuration for several routers within a single OSPFautonomous system. provides a general network map that illustrates this exampleconfiguration.

Figure 24 Sample OSPF Autonomous System Network Map


51/80

In this configuration, five routers are configured with OSPF:

Router A and Router B are both internal routers within Area 1.

Router C is an OSPF area border router. Note that for Router C, Area 1 is assignedto E3 and Area 0 is assigned to S0.

Router D is an internal router in Area 0 (backbone area). In this case, both network router configuration commands specify the same area (Area 0, or the backbone area).

Router E is an OSPF autonomous system boundary router. Note that BGP routes areredistributed into OSPF and that these routes are advertised by OSPF.


52/80

Note It is not necessary to include definitions of all areas in an OSPF autonomoussystem in the configuration of all routers in the autonomous system. You must onlydefine the directly connected areas. In the example that follows, routes in Area 0 arelearned by the routers in Area 1 (Router A and Router B) when the area border router (Router C) injects summary link state advertisements (LSAs) into Area 1.

The OSPF domain in BGP autonomous system 109 is connected to the outside worldvia the BGP link to the external peer at IP address 11.0.0.6.

Router AInternal Router



Router BInternal Router



Router CABR



router ospf 999network 131.108.1.0 0.0.0.255 area 1network 131.108.2.0 0.0.0.255 area 0

Router DInternal Router



router ospf 50network 131.108.2.0 0.0.0.255 area 0network 10.0.0.0 0.255.255.255 area 0

Router EASBR



53/80


router ospf 65001network 10.0.0.0 0.255.255.255 area 0redistribute bgp 109 metric 1 metric-type 1

router bgp 109network 131.108.0.0network 10.0.0.0neighbor 11.0.0.6 remote-as 110

Complex OSPF Configuration for ABR Examples

The following example configuration accomplishes several tasks in setting up an ABR.These tasks can be split into two general categories:

Basic OSPF configuration

Route redistribution

The specific tasks outlined in this configuration are detailed briefly in the followingdescriptions. illustrates the network address ranges and area assignments for theinterfaces.

Figure 25 Interface and Area Specifications for OSPF Example Configuration

The basic configuration tasks in this example are as follows:

Configure address ranges for Ethernet 0 through Ethernet 3 interfaces.

Enable OSPF on each interface.

Set up an OSPF authentication password for each area and network.


54/80

Assign link state metrics and other OSPF interface configuration options.

Create a stub area with area id 36.0.0.0. (Note that the authentication and stub options of the area router configuration command are specified with separate area command entries, but can be merged into a single area comm

rip commands 2

Documents