ospf (single area ospf) - college of · pdf fileospf (single area ospf) ... ospf) are reserved...

© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

ITE PC v4.0

Chapter 1 1

OSPF (Single Area OSPF)

Routing Protocols and Concepts – Chapter 11

Modified by Tony Chen

07/01/2008

ITE PC v4.0

Chapter 1 2© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Introduction

•In this chapter, you will learn basic, single-area OSPFimplementations and configurations. •More complex OSPF configurations and concepts (multi-areas OSPF) are reserved for CCNP-level courses.

http://www.cod.edu/

ITE PC v4.0


Introduction to OSPF

Background of OSPF

Began in 1987

1989 OSPFv1 released in RFC 1131

This version was experimental & never deployed

1991 OSPFv2 released in RFC 1247

1998 OSPFv2 updated in RFC 2328

1999 OSPFv3 published in RFC 2740

http://www.cod.edu/

ITE PC v4.0


Introduction to OSPFOSPF Message Encapsulation

OSPF packet type

– There exist 5 types (next slide)

OSPF packet header

–Contains - Router ID an area IDand Type code for OSPF packet type

IP packet header

– Contains - Source IP address, Destination IP address, & Protocol field set to 89. the destination address is set to one of two multicast addresses: 224.0.0.5 or 224.0.0.6.

Data Link Frame Header

–Contains - destination MAC address is also a multicast address: 01-00-5E-00-00-05 or 01-00-5E-00-00-06.

http://www.cod.edu/

ITE PC v4.0


Introduction to OSPF5 OSPF Packet Types:

1. Hello - Hello packets are used to establish

and maintain adjacency with other OSPF

routers.

2. DBD - The Database Description (DBD)

packet contains an abbreviated list of the

sending router's link-state database and is used by receiving routers to check against the local

link-state database.

3. LSR - Receiving routers can then request more information about any entry in the DBD by

sending a Link-State Request (LSR).

4. LSU - Link-State Update (LSU) packets are

used to reply to LSRs as well as to announce new information.

–LSUs contain 7 different types of Link-State

Advertisements (LSAs).

–LSUs and LSAs are discussed in a later topic.

5. LSAck - When an LSU is received, the router

sends a Link-State Acknowledgement (LSAck)

to confirm receipt of the LSU.

http://www.cod.edu/

ITE PC v4.0


OSPF: Hello Protocol Purpose of Hello Packet

Discover OSPF neighbors & establish adjacencies

Advertise parameters on which routers must agree to become neighbors

Used by multi-access networks to elect a Designated Router and a Backup Designated Router

Type: OSPF Packet Type: Hello (1), DD (2), LS Request (3), LS Update (4), LS ACK (5)

Router ID: ID of the originating router

Area ID: area from which the packet originated

Network Mask: Subnet mask associated with the sending interface

Hello Interval: number of seconds between the sending router's hellos

Router Priority: Used in DR/BDR election (discussed later)

Designated Router (DR): Router ID of the DR, if any

Backup Designated Router (BDR): Router ID of the BDR, if any

List of Neighbors: lists the OSPF Router ID of the neighboring router(s)

http://www.cod.edu/

ITE PC v4.0


OSPF: Hello Protocol Establish adjacencies:

– They must agree on three values: Hello interval, Dead interval, and network type.

OSPF Hello Intervals

–Hello interval indicates how often an OSPF router transmits its Hello packets

–Usually multicast (224.0.0.5) for ALLSPFRouters

–sent every 10 seconds on multiaccess and point-to-point segments

–Sent every 30 seconds for NBMA segments

OSPF Dead Intervals

–This is the time that must transpire before the neighbor is considered down

–Default time is 4 times the hello interval

–For multiaccess and point-to-point segments, this period is 40 seconds.

–For NBMA networks, the Dead interval is 120 seconds.

–If the Dead interval expires before the routers receive a Hello packet, OSPF will remove that neighbor from its link-state database.

Why 10 second hello interval

communications consider better

than the 30 second routing update

for RIP?

Also need to have the same Area ID.

http://www.cod.edu/

ITE PC v4.0


OSPF: Hello Protocol To reduce the amount of OSPF traffic on

multiaccess networks, OSPF elects a Designated Router (DR) and Backup Designated Router (BDR).

Hello protocol packets contain information that is used in electing DR and BDR

–The DR is responsible for updating all other OSPF routers (called DROthers) when a change occurs in the multiaccess network.

–The BDR monitors the DR and takes over as DR if the current DR fails.

In the figure, R1, R2, and R3 are connected through point-to-point links. Therefore, no DR/BDR election occurs.

–The DR/BDR election and processes will be discussed in a later topic and the topology will be changed to a multiaccess network.

More detail discussion on the DR, BDR, DROther later. You need to know this for CCNA exam.

http://www.cod.edu/

ITE PC v4.0


Introduction to OSPFOSPF Link-state Updates

Purpose of a Link State Update (LSU)

–Used to deliver link state advertisements

Purpose of a Link State Advertisement (LSA)

–Contains information about neighbors & path costs

–An LSU packet can contain 11 different types of LSAs,

http://www.cod.edu/

ITE PC v4.0



OSPF Algorithm

OSPF routers build &

maintain link-state

database containing

LSA received from other

routers

1. Information found in

database is utilized upon

execution of Dijkstra

SPF algorithm

2. SPF algorithm used to

create SPF tree

3. SPF tree used to

populate routing table

http://www.cod.edu/

ITE PC v4.0



Administrative Distance

Default Administrative Distance for OSPF is 110

http://www.cod.edu/

ITE PC v4.0


Introduction to OSPF OSPF Authentication

–It is good practice to authenticate transmitted routing information.

–This is an interface specific configuration

–This practice ensures that routers will only accept routing information from other routers that have been configured with the same password or authentication information

?

Note: Authentication does not encrypt the router's routing table.

MD5 authentication

uses a key ID that

allows the router to

reference multiple

passwords, making

password migration

easier and more

secure.

http://www.cod.edu/

ITE PC v4.0


Basic OSPF ConfigurationLab Topology

Topology used for this chapter

–Discontiguous IP addressing

scheme

–Since OSPF is a classless routing protocol the subnet mask

is will be configured as part of

our OSPF configuration.

http://www.cod.edu/

ITE PC v4.0


Basic OSPF Configuration

The router ospf command

To enable OSPF on a router use the following command

R1(config)#router ospf process-id

Process id

A locally significant number between 1 and 65535

ID cannot be 0

http://www.cod.edu/

ITE PC v4.0


Basic OSPF Configuration OSPF network command

–Requires entering:

•network address

•wildcard mask - the inverse of the subnet mask

•area-id - area-id refers to the OSPF area. OSPF area is a group of routers that share link state

information

Router(config-router)#network network-address wildcard-ask area area-id

255.255.255.255- 255.255.255.240 --------------------

0. 0. 0. 15

Subtract the subnet mask

Wildcard mask

255.255.255.255- 255.255.255.252 --------------------

0. 0. 0. 03

Subtract the subnet mask

Wildcard mask

http://www.cod.edu/

ITE PC v4.0


Basic OSPF Configuration Cisco IOS now properly handles overlapping network ... area configuration commands.

Consider the following example:

fw#conf t

Enter configuration commands, one per line. End with CNTL/Z.

fw(config)#router ospf 100

fw(config-router)#network 0.0.0.0 255.255.255.255 area 0


13:06:57: %OSPF-6-AREACHG: 10.0.0.0 255.255.252.0 changed from area 0 to area 1


13:07:10: %OSPF-6-AREACHG: 10.0.0.0 255.255.255.248 changed from area 1 to area 2

fw(config-router)#^Z

I've entered overlapping network statements, each one with a smaller address range.

Not only does IOS detect that they overlap, it also prints nice syslog messages and reorders the commands in the running configuration. Well done !

fw#show run | begin router ospf

router ospf 100

log-adjacency-changes

network 10.0.0.0 0.0.0.7 area 2

network 10.0.0.0 0.0.3.255 area 1

network 0.0.0.0 255.255.255.255 area 0

http://blog.ioshints.info/2006/11/network-statements-in-ospf-

process-are.html

http://www.cod.edu/

ITE PC v4.0


Basic OSPF Configuration ospf network definition for adding all interfaces / default route

What’s the difference?

router ospf 1

network 0.0.0.0 0.0.0.0 area 0

vs.

router ospf 1

network 0.0.0.0 255.255.255.255 area 0

Both add all existing interfaces into area 0 and all later added

interfaces also. Both statements are valid.

http://blog.sazza.de/?p=427

http://www.cod.edu/

ITE PC v4.0



Area area-id

An OSPF area is a group of routers that share link-state

information.

In this chapter, we will configure all of the OSPF routers within

a single area. This is known as single-area OSPF.

Multi-area OSPF is covered in CCNP.

http://www.cod.edu/

ITE PC v4.0



Router ID

– This is an IP address used to identify a router

– 3 criteria for deriving the router ID

1. Use IP address configured with OSPF router-id command

-Takes precedence over loopback and physical interface addresses

2. If router-id command not used then router chooses highest IP address of any loopback interfaces

3. If no loopback interfaces are configured then the highest IP address on any active physical interface is used

The interface does not need to be enabled for OSPF, meaning that it does not need to be included in one of the OSPF network commands.

However, the interface must be active - it must be in the up state.

However!!!!!!

http://www.cod.edu/

ITE PC v4.0



Router ID

– If you are the king when the

kingdom is built, you are the

KING for life

– It means when ID is elected, it

is the ID for the router, unless

…………..

http://www.cod.edu/

ITE PC v4.0



OSPF Router ID

Commands used to verify current router ID

–Show ip protocols

–Show ip ospf

–Show ip ospf interface

http://www.cod.edu/

ITE PC v4.0



OSPF Router ID

Router ID (not configured) & Loopback addresses (configured)

–Highest loopback address will be used as router ID

–Advantage of using loopback address the loopback interface cannot fail OSPF stability

The OSPF router-id command

–Introduced in IOS 12.0

–OSPF router-id command, which is a fairly recent addition to IOS, it is more common to find loopback addresses used for configuring OSPF router IDs.

–Command syntax

Router(config)#router ospf process-id

Router(config-router)#router-id ip-address

Modifying the Router ID

–Use the command Router#clear ip ospf process

This command does not work in PT.

http://www.cod.edu/

ITE PC v4.0




The router ID is selected when OSPF is configured with its first OSPF network command.

– If the OSPF router-id command or the loopback address is configured after the OSPF network command, the router ID will be derived from the interface with the highest active IP address.


The router ID can be modified with

1. the IP address from a subsequent OSPF router-id command by reloading the router or

2. by using the following command:

Router#clear ip ospf process

3. Modifying a router ID with a new loopback or physical interface IP address may require reloading the router

http://www.cod.edu/

ITE PC v4.0



Duplicate Router IDs

When two routers have the same router ID in an OSPF domain, routing may not function properly.

– If the router ID is the same on two neighboring routers, the neighbor establishment may not occur.

When duplicate OSPF router IDs occur, IOS will display a message similar to:

– %OSPF-4-DUP_RTRID1: Detected router with duplicate router ID

To correct this problem, configure all routers so that they have unique OSPF router IDs.

Because some IOS versions do not support the router-id command, we will use the loopback address method for assigning router IDs.

http://www.cod.edu/

ITE PC v4.0


Quick Review

We just went over 3 different types of ID

ospf process-id.

– OSPF process.

– Cannot be 0

Area ID:

– OFPS area

– If it is the first, and the backbone area, it is 0

Router ID

– Router ID

– 1 IP address is elected per router,

• Highest physical address (or)

• Highest logical address (loopback)

http://www.cod.edu/

ITE PC v4.0


Basic OSPF ConfigurationVerifying OSPF

Use the show ip ospf command to verify &

trouble shoot OSPF networks:

Neighbor adjacency

Adjacency indicated by

The OSPF state of the interface is

“full state”

No adjacency indicated by -

Neighboring router’s Router ID is

not displayed

A state of full is not displayed

-Consequence of no adjacency-

No link state information exchanged

Inaccurate SPF trees & routing

tables

•Neighbor ID - The router ID of the neighboring router.•Pri - The OSPF priority of the interface..•State - The OSPF state of the interface. FULL state means that the router and its neighbor have identical OSPF link-state databases.•Dead Time - The amount of time remaining that the router will wait to receive an OSPF Hello packet from the neighbor before declaring the neighbor down. This value is reset when the interface receives a Hello packet.•Address - The IP address of the neighbor's interface to which this router is directly connected.•Interface - The interface on which this router has formed adjacency with the neighbor.

http://www.cod.edu/

ITE PC v4.0



Note:

On multiaccess networks such as

Ethernet, two routers that are adjacent

may have their states displayed as

2WAY.

–This will be discussed in a DR and BDR section.

Two routers may not form an OSPF

adjacency if:

–The subnet masks do not match, causing

the routers to be on separate networks.

–OSPF Hello or Dead Timers do not match.

–OSPF Network Types do not match.

–There is a missing or incorrect OSPF network command.

•Neighbor ID - The router ID of the neighboring router.•Pri - The OSPF priority of the interface..•State - The OSPF state of the interface. FULL state means that the router and its neighbor have identical OSPF link-state databases.•Dead Time - The amount of time remaining that the router will wait to receive an OSPF Hello packet from the neighbor before declaring the neighbor down. This value is reset when the interface receives a Hello packet.•Address - The IP address of the neighbor's interface to which this router is directly connected.•Interface - The interface on which this router has formed adjacency with the neighbor.

http://www.cod.edu/

ITE PC v4.0


Verifying OSPF Show ip protocols

–OSPF process ID,

–the router ID,

–networks the router is advertising,

–the default administrative distance, 110 for OSPF.

Show ip ospf

–OSPF process ID

–router ID.

–OSPF area information

–the last time the SPF algorithm was calculated.

•R1 has participated in during the past 11 and a half hours is to send small Hello packets to its neighbors.

–SPF schedule delay

•The router waits 5000 msecs after receiving an LSU before running the SPF algorithm.

•There is an additional Hold Time of 10000 msecs between 2

SPF calculations.

Show ip ospf interface

–The quickest way to verify Hello and Dead intervals

• for OSPF routers to become neighbors, their OSPF Hello and Dead intervals must be identical.

http://www.cod.edu/

ITE PC v4.0


Configuring OSPF loopback address and router priority

The command show ip ospf interface will display the

interface priority value as well as other key information.

http://www.cod.edu/

ITE PC v4.0



Examining the routing table

Use the show ip route command to display the routing table

-An “O’ at the beginning of a route indicates that the router source is OSPF

-OSPF does not automatically summarize at major network boundaries

•Loopback interface counts as a network. •These loopback interfaces are not advertised in OSPF.•They function as router ID.

http://www.cod.edu/

ITE PC v4.0


OSPF Metric OSPF uses cost as the metric for determining

the best route

–A cost is associated with the output side of each router interface.

–The lower the cost, the more likely the interface is to be used to forward data traffic

The Cisco IOS uses the cumulative bandwidths of the outgoing interfaces from the router to the destination network as the cost value.

-Cost is based on bandwidth of an interface

Cost is calculated using the formula

108 / bandwidth

-Reference bandwidth

The 100Mbps (FastEthernet) and higher will have the same OSPF cost of 1.

This reference bandwidth can be modified using

auto-cost reference-bandwidth command

http://www.cod.edu/

ITE PC v4.0


OSPF Metric COST of an OSPF route is the accumulated value

from one router to the destination network

•For example, in the figure, the routing table on R1 shows a cost of 65 to reach the 10.10.10.0/24 network on R2.

•Because 10.10.10.0/24 is attached to a FastEthernet interface, R2 assigns the value 1as the cost for 10.10.10.0/24. •R1 then adds the additional cost value of 64 to send data across the default T1 link between R1 and R2.

64 + 1 = 65

http://www.cod.edu/

ITE PC v4.0


OSPF Metric Sometimes the actual speed of a link is different than

the default bandwidth

–This makes it imperative that the bandwidth value reflects link’s actual speed

Reason: so routing table has best path information

The show interface command will display interface’s bandwidth

–Most serial link default to 1.544Mbps

–However, some serial interfaces may default to 128 kbps.

http://www.cod.edu/

ITE PC v4.0


Modifying OSPF cost metric OSPF uses cost as the metric for determining the

best route.

Cost is calculated using the formula 108/bandwidth, where bandwidth is expressed in bps. (Cost = 100,000,000/Bandwidth)

The Cisco IOS automatically determines cost based on the bandwidth of the interface.

It is essential for proper OSPF operation that the correct interface bandwidth is set.

Router(config)#interface serial 0/0Router(config-if)#bandwidth 64

The default bandwidth for Cisco serial interfaces is 1.544 Mbps, or 1544 kbps.

2A/S 2T

COD has these 2 types

of serial cards in the

lab

http://www.cod.edu/

ITE PC v4.0


OSPF Metric: Bandwidth Remember, this bandwidth value

does not actually affect the speed of the link; it is used by some routing protocols to compute the routing metric.

–It is important that the bandwidth value reflect the actual speed of the link so that the routing table has accurate best path information.

The figure displays the routing table for R1.

–R1 believes that both of its serial interfaces are connected to T1 links,

•one of the links is a 64 kbps link

•the other one is a 256 kbps link.

–This results in R1's routing table having two equal-cost paths to the 192.168.8.0/30 network, when Serial 0/0/1 is actually the better path.

How to modify the cost of all the links?

http://www.cod.edu/

ITE PC v4.0


Basic OSPF ConfigurationModifying the Cost of a link

Both sides of a serial link should be configured with the same bandwidth

–Commands used to modify bandwidth value

Bandwidth command

–Example: Router(config-if)#bandwidthbandwidth-kbps

ip ospf cost command – allows you to directly specify interface cost

-Example:R1(config)#interface serial 0/0/0

R1(config-if)#ip ospf cost 1562

http://www.cod.edu/

ITE PC v4.0


Modifying the Cost of the link

Difference between bandwidth command & the ip ospf cost command

–Ip ospf cost command

Sets cost to a specific value

–Bandwidth command

Link cost is calculated

http://www.cod.edu/

ITE PC v4.0


OSPF and Multiaccess NetworksChallenges in Multiaccess Networks

OSPF defines five network types:

–Point-to-point

•network there are only two devices on the network, one at each end.

–Broadcast Multiaccess

•a network with more than two devices on the same shared media.

•all devices on the network see all broadcast frames.

–Nonbroadcast Multiaccess (NBMA)

•networks include Frame Relay, ATM, and X.25 networks.

–Point-to-multipoint

•networks include Frame Relay, ATM, and X.25 networks.

–Virtual links

•Virtual links are a special type of link that can be used in multi-area OSPF.

http://www.cod.edu/

ITE PC v4.0


OSPF in Multiaccess Networks 2 challenges presented by

multiaccess networks

–Multiple adjacencies

–Extensive LSA flooding

The creation of an adjacency between every pair of routers in a network would create an unnecessary number of adjacencies.

–This would lead to an excessive number of LSAs passing between routers on the same network.

•5 routers in the figure will need 10 adjacencies,

•10 routers would require 45 adjacencies.

•20 routers would require 190 adjacencies

http://www.cod.edu/

ITE PC v4.0


OSPF in Multiaccess Networks

Extensive flooding of LSAs

For every LSA sent out there must be an acknowledgement

of receipt sent back to transmitting router.

consequence: lots of bandwidth consumed and chaotic

traffic

Solution:

http://www.cod.edu/

ITE PC v4.0


Steps in the operation of OSPF OSPF routers send Hello packets on OSPF enabled interfaces.

On multi-access networks, the routers elect a DR and BDR. On these networks other routers become adjacent to the DR.

To reduce the number of adjacencies each router must form, OSPF

calls one of the routers the designated router. A designated router is

elected as routers are forming adjacencies, and then all other routers

establish adjacencies only with the designated router. This simplifies

the routing table update procedure and reduces the number of link-state

records in the database. The designated router plays other important

roles as well to reduce the overhead of a OSPF link-state procedures.

For example, other routers send link-state advertisements it to the

designated router only by using the all-designated-routers multicast

address of 224.0.0.6.

http://www.chebucto.ns.ca/Chebucto/Technical/

Manuals/Max/max6000/isptele/maxospf.htm

To reduce the number of adjacencies

traffics

http://www.cod.edu/

ITE PC v4.0


Steps in the operation of OSPF

OSPF routers send Hello

packets on OSPF enabled

interfaces.

On multi-access networks, the

routers elect a DR and BDR.

On these networks other

routers become adjacent to the

DR.

http://www.cod.edu/

ITE PC v4.0


Steps in the operation of OSPF

To reduce the number of adjacencies traffics

http://www.cisco.com/warp/public/104/11.html

http://www.cod.edu/

ITE PC v4.0



Solution to LSA flooding issue is the use of

–Designated router (DR)

–Backup designated router (BDR)

•this solution is analogous to electing someone in the room to go around and learn everyone's names and then announce these names to everyone in the room at once.

–DROther

• All other routers become DROthers (this indicates a router that is neither the DR or the BDR).

•DROthers only form full adjacencies with the DR and BDR in the network.

DR & BDR

–On multiaccess networks, OSPF elects a Designated Router (DR) to be the collection and distribution point for LSAs sent and received.

–A Backup Designated Router (BDR) is also elected in case the Designated Router fails.

–DR & BDR are elected to send & receive LSA

http://www.cod.edu/

ITE PC v4.0



DR & BDR & DROther

–Routers on a multiaccess network elect a DR and BDR.

–DR & BDR are elected to send & receive LSA

–DROthers only form full adjacencies with the DR and BDR in the network.

Sending & Receiving LSA

–DRothers send LSAs via multicast 224.0.0.6 to DR & BDR (ALLDRouters - All DR routers)

–DR forward LSA via multicast address 224.0.0.5 to all other routers (AllSPFRouters - All OSPF routers).

http://www.cod.edu/

ITE PC v4.0



DR/BDR Election Process

DR/BDR elections DO

NOT occur in point-to-

point networks

DR/BDR elections will take

place on multiaccess

networks as shown below

http://www.cod.edu/

ITE PC v4.0



Criteria for getting elected DR/BDR

1. DR: Router with the highest OSPF interface priority.

2. BDR: Router with the second highestOSPF interface priority.

3. If OSPF interface priorities are equal, the

highest router ID is used to break the tie.

http://www.cod.edu/

ITE PC v4.0


Criteria for getting elected DR/BDR

1. DR: Router with the highest OSPF interface priority.

2. BDR: Router with the second highestOSPF interface priority.

3. If OSPF interface priorities are equal, the

highest router ID is used to break the tie.

Example:

– The OSPF for all interface priority is 1.

– The OSPF router ID is used to elect the DR and BDR.

• RouterC with the highest router ID, becomes the DR

• RouterB, with the second highest router ID, becomes the BDR.

• Because RouterA is not elected as either the DR or BDR, it becomes the DROther.

DROthers only form FULL adjacencies with the DR and BDR, but will still form a neighbor adjacency with any DROthers that join the network. When two DROther routers form a neighbor adjacency, the neighbor state is displayed as 2WAY.

You need 4 routers topology to see this “2way” adjacency.

http://www.cod.edu/

ITE PC v4.0


OSPF network types (cont.)

Real DR and BDR election process

The first router up on the network is the DR.

The second router up on the network is the BDR.

If the DR fails then the BDR becomes DR and another router is elected the BDR.

The DR does not change just because another router comes on line with a higher priority or a higher router id.

If both the existing DR and BDR fail and a new DR must be elected, the router with the highest priority is elected DR.

If there's a tie, the router with the highest router id is elected DR.

http://www.cod.edu/

ITE PC v4.0


Timing of DR/BDR Election (This is really of how the election works)

Election occurs as soon as 1st router has its

OSPF enabled on multiaccess network. This

can happen when

1. When the routers are powered-on

• it is possible that a router with a lower router

ID will become the DR. This could be a lower-

end router that took less time to boot.

2. when the OSPF network command for that

interface is configured.

When a DR is elected it remains as the DR until

one of the following occurs

-The DR fails.

-The OSPF process on the DR fails.

-The multiaccess interface on the DR fails.

http://www.cod.edu/

ITE PC v4.0



DR Fails

–If the DR fails, the BDR assumes the role of

DR and an election is held to choose a new

BDR.

–In the figure, RouterC fails and the former

BDR, RouterB, becomes DR. The only other

router available to be BDR is RouterA.

http://www.cod.edu/

ITE PC v4.0



New Router

–If a new router enters the network after

the DR and BDR have been elected, it

will not become the DR or the BDR even

if it has a higher OSPF interface priority

or router ID than the current DR or BDR.

•If the current DR fails, the BDR will

become the DR, and the new router

can be elected the new BDR.

•After the new router becomes the

BDR, if the DR fails, then the new

router will become the DR.

•The current DR and BDR must both

fail before the new router can be

elected DR or BDR.

http://www.cod.edu/

ITE PC v4.0



Old DR Returns

–A previous DR does not regain DR

status if it returns to the network.

•In the figure, RouterC has finished a

reboot and becomes a DROther even

though its router ID, 192.168.31.33, is

higher than the current DR and BDR.

http://www.cod.edu/

ITE PC v4.0



BDR Fails

–If the BDR fails, an election is held

among the DRothers to see which router

will be the new BDR.

•In the figure, the BDR router fails.

•An election is held between RouterC

and RouterD.

•RouterD wins the election with the

higher router ID.

http://www.cod.edu/

ITE PC v4.0



New DR Fails

–In the figure, RouterB fails. Because RouterD is the current BDR, it is promoted to DR. RouterC becomes the BDR.

So, how do you make sure that the routers you want to be DR and BDR win the election? Without further configurations, the solution is to either:

–Boot up the DR first, followed by the BDR, and then boot all other routers, or

–Shut down the interface on all routers, followed by a no shutdown on the DR, then the BDR, and then all other routers.

OR: use the priority command set not desired DR and BDR to 0

http://www.cod.edu/

ITE PC v4.0


OSPF in Multiaccess NetworksOSPF Interface Priority

Manipulating the DR/BDR election process continued

–Use the ip ospf priority interface command.

–Example:Router(config-if)#ip ospf priority {0 - 255}

Priority number range 0 to 255

–0 means the router cannot become the DR or BDR

–1 is the default priority value

»router ID determined the DR and BDR

http://www.cod.edu/

ITE PC v4.0


OSPF in Multiaccess NetworksOSPF Interface Priority

Modify Priority–Router(config-if)#ip ospf priority {0 - 255}

Force Election

–After doing a shutdown and a no shutdown on the FastEthernet 0/0 interfaces of all three routers, we see the result of the change of OSPF interface priorities.

–The show ip ospf neighbor command on RouterC now shows that RouterA (Router ID 192.168.31.11) is the DR with the highest OSPF interface priority of 200

–RouterB (Router ID 192.168.31.22) is still the BDR with the next highest OSPF interface priority of 100.

–Notice from RouterA's output of show ip ospf neighbor that it does not show a DR, because RouterA is the actual DR on this network.

DR

BDR

http://www.cod.edu/

ITE PC v4.0


More OSPF Configuration

Redistributing an OSPF Default Route

Topology includes a link to ISP

–Router connected to ISP

Called an autonomous system border router

Used to propagate a default route

–Example of static default route

R1(config)#ip route 0.0.0.0 0.0.0.0 loopback 1

–Requires the use of the default-information originate command

–Example of default-information originate command

R1(config-router)#default-information originate

In this topology, the Loopback1 (Lo1) simulate the connection to another router.

http://www.cod.edu/

ITE PC v4.0


Redistributing an OSPF Default Route

The default route in R2 and R3 with the routing source OSPF, but with the additional code, E2. For R2, the route is:

O*E2 0.0.0.0/0 [110/1] via 192.168.10.10, 00:05:34, Serial0/0/1

E2 denotes that this route is an OSPF External Type 2 route. the cost of an E2 route is always the external cost, irrespective of the interior cost to reach that route. (CCNP)

http://www.cod.edu/

ITE PC v4.0



Fine-Tuning OSPF

Since link speeds are

getting faster it may be

necessary to change

reference bandwidth values

–Do this using the auto-cost

reference-bandwidth

command

–Example:

R1(config-router)#auto-

cost reference-bandwidth

10000•the default value is equivalent to 100. To increase it to 10GigE speeds, you would need to change the reference bandwidth to 10000.•Again, make sure you configure this command on all routers in the OSPF routing domain.

http://www.cod.edu/

ITE PC v4.0



Fine-Tuning OSPF

R1(config-router)#auto-

cost reference-bandwidth

10000

•the default value is equivalent to 100. To increase it to 10GigE speeds, you would need to change the reference bandwidth to 10000.

R1 Before, the cost to 10.10.10.0/24 is 1172. After configuring a new reference bandwidth, the cost for the same route is now 117287.

http://www.cod.edu/

ITE PC v4.0



Fine-Tuning OSPF

Modifying OSPF timers

–Reason to modify timers

Faster detection of network failures

–Manually modifying Hello & Dead intervals

Router(config-if)#ip ospf hello-interval seconds

Router(config-if)#ip ospf dead-interval seconds

–Point to be made

Hello & Dead intervals must be the same between

neighbors

http://www.cod.edu/

ITE PC v4.0


The End

Questions?

What will be the result of the DR and BDR elections for this single area

OSPF network? (Choose three.)

HQ will be DR for 10.4.0.0/16.

Router A will be DR for 10.4.0.0/16.

HQ will be BDR for 10.4.0.0/16.

Router A will be DR for 10.5.0.0/16.

Remote will be DR for 10.5.0.0/16.

Remote will be BDR for 10.5.0.0/16.

*. Decision process:

1. Which segment will have election?

2. Priority?

3. Router ID (each router will only has 1

ID)?

1. Set using “router-ID” command

2. Highest Loopback IP address?

3. Highest physical IP address

(include serial interface)?

http://www.cod.edu/

ITE PC v4.0


The End

Questions?

The routers in the diagram are configured as shown. The loopback interface on

router R1 is labeled as lo0. All OSPF priorities are set to the default except for

Ethernet0 of router R2, which has an OSPF priority of 2. What will be the result

of the OSPF DR/BDR elections on the 192.1.1.0 network? (Choose two.)

R1 will be the DR

R1 will be the BDR

R2 will be the DR

R2 will be the BDR

R3 will be the DR

R3 will be the BDR



2. Priority?

3. Router ID (each router will only has 1 ID)?



3. Highest physical IP address (include

serial interface)?

http://www.cod.edu/

ITE PC v4.0


Questions?



2. Priority?





serial interface)?

http://www.cod.edu/

ITE PC v4.0


Questions?



2. Priority?





serial interface)?

Answer?????

http://www.cod.edu/

ITE PC v4.0


Questions?

HQ will be DR for

10.4.0.0/16

Router A will be DR for

10.4.0.0/16.

HQ will be BDR for

10.4.0.0/16.

Router A will be DR for

10.5.0.0/16

Remote will be DR for

10.5.0.0/16.

Remote will be BDR for

10.5.0.0/16



2. Priority?





serial interface)?

Answer?????

http://www.cod.edu/

ITE PC v4.0


Summary

RFC 2328 describes OSPF link state concepts and operations

OSPF Characteristics

–A commonly deployed link state routing protocol

–Employs DRs & BDRs on multi-access networks

DRs & BDRs are elected

DR & BDRs are used to transmit and receive LSAs

–Uses 5 packet types:

1: HELLO

2: DATABASE DESCRIPTION

3: LINK STATE REQUEST

4: LINK STATE UPDATE

5: LINK STATE ACKNOWLEDGEMENT

http://www.cod.edu/

ITE PC v4.0


Summary

OSPF Characteristics

–Metric = cost

Lowest cost = best path

Configuration

–Enable OSPF on a router using the following command

R1(config)#router ospf process-id

–use the network command to define which interfaces will

participate in a given OSPF process

R1(config-router)#network network-address

wildcard-mask area area-id

http://www.cod.edu/

ITE PC v4.0


Summary

Verifying OSPF configuration

–Use the following commands

show ip protocol

show ip route

show ip ospf interface

show ip ospf neighbor

http://www.cod.edu/

ITE PC v4.0


http://www.cod.edu/

ospf (single area ospf) - college of · pdf fileospf (single area ospf) ... ospf) are reserved...

Documents