ccnp1: advanced routing v3.0 cisco networking academy program chapter 2 – single area ospf ospf...

CCNP1: Advanced Routing v3.0

CISCO NETWORKING ACADEMY PROGRAM

Chapter 2 – Single Area OSPF

OSPF

Link State Routing

Single Area and Multiarea OSPF

CCNP1: Advanced Routing


Chapter 6 –OSPF

Identify Distance Vector & Link State Routing Characteristics

Periodic updates

Topology changes trigger updates

Updates sent to neighbours

Updates contain changes only

Updates contain entire routing table

Increased memory & processing requirements

Updates consume significant bandwidth

Rapid convergence

Updates sent to all routers Support CIDR/VLSM

Slow convergence



Chapter 6 –OSPF

Summary of Link State Features• Responds quickly to network changes• Use ‘hellos’ to discover and create neighbors• Send updates when a there has been a change

in the network topology • Updates contain changes not whole routing table• Calculates shortest path to each route in the

network from a separate topology table



Chapter 6 –OSPF

Link State Operation • Routers are aware of directly connected networks known

as ‘links’• Routers send ‘hellos’ to discover neighbors• Routers send Link State Advertisements (LSAs) to other

routers informing them of their links• All routers add Link State Advertisements to their

topological database (topology table)• Shortest Path algorithm calculates best route to each

network• When link states change, LSA update are sent to all

routers which recalculate their routes



Chapter 6 –OSPF

Topological Database

• Every router advertises directly connected networks via Link State Advertisements

• Every router has it’s own view of the network – it builds a ‘topological database’

• Router A is aware of 2 paths to 192.168.157.0 – this provides redundancy should one of the routers fail



Chapter 6 –OSPF

Evaluation of Link State Routing



Chapter 6 –OSPF

Review – Link State & Distance Vector



Chapter 6 –OSPF

OSPF Overview

• Preferred to RIP on larger networks• Open Standard - IETF RFC 2328• Link State routing protocol• Interior Gateway Protocol for Autonomous

systems• Metric based on bandwidth - Cost• Supports VLSM• OSFP can use ‘areas’ to allow hierarchical

design – Multiarea OSPF



Chapter 6 –OSPF

OSPF Key WordsAdjacencies database• Directly connected routersTopological Database• Routes to every networkRouting table

– Best path to each network(chosen from topological database)

Designated Router• A router elected by all others to

represent the network area(multi-access networks only)

Area 0• backbone



Chapter 6 –OSPF

OSPF Packet Types• Type 1 – Hello

– Establishes and Maintains adjacency info w/ neighbors• Type 2 – Database description packet (DBD)

– Describes the contents of an OSPF router’s link-state database

• Type 3 – Link State Request– Requests specific pieces of a routers link-state database

• Type 4 – Link State Update (LSUs)– Transports link-state advertisements (LSAs) to neighbor

routers• Type 5 – Link-state acknowledgement (LSAKs)

– Acknowledges receipt of a neighbor’s LSA



Chapter 6 –OSPF

OSPF Hello Protocol



Chapter 6 –OSPF

Designated Router/Backup DR

• All LSA sent to DR/BDR instead of to every single router

• Reduces overhead of LSA updates

• Standard on multi-access networks

• DR is single point of failure – solution is BDR



Chapter 6 –OSPF

DR/BDR

• Once a DR is established, a new router with a higher priority or router ID will NOT become the DR or BDR.

• If DR fails, BDR takes over as DR and selection process for new BDR begins.



Chapter 6 –OSPF

Backup Designated Router

• Listens, but doesn’t act.• If LSA is sent, BDR sets a timer.• If timer expires before it sees the reply from the DR, it

becomes the DR and takes over the update process.• The process for a new BDR begins.



Chapter 6 –OSPF

DR/BDR selection• To suit the topology used the network

administrator will want to choose DR/BDR• DR/BDR election based on OSPF priority• Highest priority=DR• 2nd highest priority=BDR• Priority of 0 = DROTHER (ensures will not be DR)• Default priority = 1Router(config-if)#ip ospf priority number

Router#show ip ospf interface type number



Chapter 6 –OSPF

OSPF Loopback Address

• For OSPF to function there must always be an active interface

• Physical interfaces e.g. serial/Ethernet may not always be active – routing would fail

• Configure virtual “loopback” interface as solution• Subnet mask will always be 255.255.255.255

Router(config)#interface loopback number

Router(config-if)#ip address ip-address subnet-mask



Chapter 6 –OSPF

Drothers – All Other OSPF Routers

• All other routers, DROTHER, establish adjacencies with DR and BDR only.

• LSAs are multicast to DR and BDR only– (224.0.0.6 - all DR routers)

• DR sends LSA to all adjacent neighbors – (224.0.0.5 - all OSPF routers)



Chapter 6 –OSPF

OSPF basic commands

Router(config)#router ospf process-id

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

EXAMPLE

Router(config)#router ospf 2

Router(config-router)#network 172.16.10.0 0.0.0.255 area 2

NOTES• process-id can be a value between 0 and 65,535• Wildcard mask NOT subnet mask used with network command



Chapter 6 –OSPF

Basic OSPF Configuration



Chapter 6 –OSPF

Configuring OSPF Authentication within a Single Area

Rtr(config)# router ospf process-id

Rtr(config-router)#network address wildcard-mask area area-id

Rtr(config-router)# area area authentication [message-digest]

Rtr(config)# interface type slot/port

Rtr(config-if)# ip ospf priority <0-255>

RTB(config-if)# ip ospf cost cost

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

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

Rtr(config-if)# ip ospf message-digest-key key-id md5 [encryption-type] password



Chapter 6 –OSPF

Steps to OSPF Operation

1. Establishing router adjacencies

2. Electing DR and BDR

3. Discovering Routes

4. Choosing Routes

5. Maintaining Routing Information



Chapter 6 –OSPF

Router States

• down• init• 2 way• exstart• exchange• loading• full

Good neighbors, no LSA sharing.

Sharing route via LSAs.



Chapter 6 –OSPF

Reaching 2-Way



Chapter 6 –OSPF

Electing a DR/BDR



Chapter 6 –OSPF

1.Establishing Adjacencies

An OSPF router tries to form an adjacency with at least one neighbor for each IP network it’s connected to.



Chapter 6 –OSPF

2.Electing a DR and BDR• On point-to-point links adjacencies are established

with all neighbors, because there is only one neighbor.• On multi-access networks,OSPF elects a DR and BDR

to limit the number of adjacencies.– Reduce routing update traffic



Chapter 6 –OSPF

3.Discovering Routes

• EXSTART state - prepare for initial database exchange of Database Description Packets (DBDs)

• master/slave relationship decided (higher router id)• EXCHANGE state - routers exchange one or more

DBDs (Database Description) packets, which is a summary of the link-state database– send LSAcks to verify– compares DBD with its own database



Chapter 6 –OSPF

3.Discovering Routes• LOADING STATE - When a slave router receives

a DBD it:– Acknowledges receipt of the DBD by sending an LSAck– Compares the information it received with the

information it has by checking the LSA sequence number

– If the DBD has a more up-to-date link-state entry, the slave router sends a link state request (LSR) to the master

– The master responds with a link state update (LSU).



Chapter 6 –OSPF

3.Discovering Routes

• When all LSRs have been satisfied for a given router, the adjacent routers are considered to be synchronized and in a FULL STATE.

• At this point all routers within the network should have identical link-state databases.



Chapter 6 –OSPF

4.Choosing Routes• OSPF bases routing metrics on cost.• Cisco routers, cost = 108/BW• BW is the configured bandwidth for an interface

and may be changed using the ip ospf cost command. The bandwidth command can also be used to change the bandwidth metric on an interface.

• ip ospf cost is used when converting the metric between routers from different vendors.



Chapter 6 –OSPF

Choosing Routes

• SPF, Shortest Path First calculations use the Dijkstra algorithm, placing itself as the root and creating a “tree diagram” of the network



Chapter 6 –OSPF

5.Maintaining Routing Info• Flooding process• When there is a state change in one of the connected

links, as soon as the router learns of this change it floods it to all adjacent neighbors (224.0.0.6 - all DR/BDRs).

• Only the changed link information is flooded, not the entire database.

• DR sends LSU (LSA) to others on the network, area (224.0.0.5 all SPF routers)

• Router which receives LSU updates links-state database, computes the SPF algorithm, and generates a new routing table

• LSAs are aged at 30 minutes and flooded every 30 minutes



Chapter 6 –OSPF

Additional Configuration

Network administrators can also configure:

• LSA update authentication• OSPF Priority at the interface• ‘Hello’ & ‘Dead’ interval timers• Default route to routers outside the

area/autonomous system



Chapter 6 –OSPF

Configuring OSPF Loopback Address and Router Priority



Chapter 6 –OSPF

Setting OSPF Priority

The priorities can be set to any value from 0 to 255. A value of 0 prevents that router from being elected. A router with the highest OSPF priority will win the election for DR.



Chapter 6 –OSPF

Modifying OSPF Cost Metric



Chapter 6 –OSPF

Configuring OSPF Authentication

The encryption setting of 7 is Cisco proprietary andwill not work properly.

Use 5 for MD5 hashinstead.



Chapter 6 –OSPF

Configuring timers

Rtr(config-if)# ip ospf hello-interval secondsRtr(config-if)# ip ospf dead-interval seconds

• For OSPF routers to be able to exchange information, the must have the same hello intervals and dead intervals.

• By default, the hello interval is 4 times the dead interval, so the a router has four chances to send a hello packet being declared dead. (not required)

Defaults• On broadcast networks hello interval = 10 seconds, dead

interval 40 seconds.• On non-broadcast networks hello interval = 30 seconds, dead

interval 120 seconds.



Chapter 6 –OSPF

Configuring OSPF Timers Example



Chapter 6 –OSPF

OSPF - Propagating a Default Route

Default routes are used if the destination network is not in the routing table.The border router (Router B) is the default router.Router B must use the command ‘default-information originate’ to propagate default information to the rest of the OSPF network.



Chapter 6 –OSPF

OSPF Default Router Example

Router(config)# int s0/0Router(config-if)# ip address 172.16.16.2 255.255.255.252Router(config-if)# no shutRouter(config-if)# exitRouter(config)# ip route 0.0.0.0 0.0.0.0 172.16.16.1Router(config)# router ospf 1Router(config-router)# network 192.168.1.0 0.0.0.3 area 0Router(config-router)# network 192.168.1.128 0.0.0.63 area 0Router(config-router)# default-information originate

Notice the 172. network is not included in the OSPF configuration because youare not running OSPF to the Internet provider.



Chapter 6 –OSPF

OSPF Areas• Every OSPF router must belong to at least one

area• Every OSPF network must have an Area 0

(backbone area)• All other Areas should “touch” Area 0

– There are exceptions to this rule

• Routers in the same area have the same link-state information



Chapter 6 –OSPF

OSPF uses Areas

• Hierarchical routing enables you to separate large internetworks (autonomous system) into smaller internetworks that are called areas.

• With this technique, routing still occurs between the areas (called inter-area routing), but many of the smaller internal routing operations, such as recalculating the database, are restricted within an area.



Chapter 6 –OSPF

OSPF Areas



Chapter 6 –OSPF

OSPF Router Types



Chapter 6 –OSPF

OSPF Router TypesInternalInternal: Routers with all their interfaces within the same

area BackboneBackbone: Routers with at least one interface connected

to area 0 ASBRASBR: (Autonomous System Boundary Router): Routers

that have at least one interface connected to an external internetwork (another autonomous system)

ABRABR: (Area Border Router): Routers with interfaces

attached to multiple areas.



Chapter 6 –OSPF

Area Types

• Standard• Backbone• Stub

– Stub– Totally Stubby Area (TSA)– Not-so-stubby-area (NSSA)



Chapter 6 –OSPF

Area Types



Chapter 6 –OSPF

Stub, Totally Stubby and Not-so-stubby

• A stub area does not accept information about routes external to the AS

• A totally stubby area, which is a Cisco specific feature, blocks external Type 5 LSAs and summary, Type 3 and Type 4, LSAs from entering the area.

• An NSSA does not allow Type 5 LSAs but does allow Type 7 LSAs, which can carry external routing information and be flooded throughout the NSSA.



Chapter 6 –OSPF

Area Types

• Key difference between area types:– How they handle external routes (E1 and E2).– External routes are injected into OSPF by ASBR– Type 1 (E1): cost metric increments as route is

passed through OSPF domain– Type 2 (E2): cost metric remains the same as route is

passed through OSPF domain



Chapter 6 –OSPF

LSA Types



Chapter 6 –OSPF

LSA Types (con’t)

• Type 6 MOSPF (Multicast OSPF) Not supported by Cisco. – MOSPF enhances OSPF by letting routers use their

link-state databases to build multicast distribution trees for the forwarding of multicast traffic.



Chapter 6 –OSPF

LSA Types (con’t)

• Type 7 NSSA External Link Entry– Originated by an ASBR connected to an NSSA.

• Type 7 messages can be flooded throughout NSSAs and translated into LSA Type 5 messages by ABRs. Routes learned via Type-7 LSAs are denoted by either a “N1” or and “N2” in the routing table. (Compare to E1 and E2).



Chapter 6 –OSPF

Multi-area Example

ABR

ASBR

RIP



Chapter 6 –OSPF

Stub Example

ABR

ASBR

No Type 5 LSAs

Route to 0.0.0.0/0 via RTB



Chapter 6 –OSPF

Totally Stubby Example

ABR

ASBR

No Type 3, 4, or 5 LSAs

Route to 0.0.0.0/0 via RTB

(no more IA routes)

Totally Stubby Area

no summary



Chapter 6 –OSPF

NSSA Example

ABR

ASBR

No Type 5 LSAs

Type 7 okay



Chapter 6 –OSPF

NSSA Example

ABR

ASBR

No Type 3, 4 or 5 LSAs

Type 7 okay

no summary



Chapter 6 –OSPF

Configuring Summary Routes

• Interarea route summarization(at ABR)

Router(config-router)# area [area-id] range [address] [mask]

Router(config-router)# area 1 range 192.168.16.0 255.255.252.0

• External route summarization(at ASBR)

Router(config-router)# summary-address [address] [mask]

Router(config-router)# summary-address 200.9.0.0 255.255.0.0



Chapter 6 –OSPF

Virtual Links

• A virtual link has the following two requirements:

– It must be established between two routers that share a common area.

– One of these two routers must be connected to the backbone.



Chapter 6 –OSPF

Virtual Links

RTA(config)#router ospf 1

RTA(config-router)#network 192.168.0.0 0.0.0.3 area 51

RTA(config-router)#network 192.168.1.0 0.0.0.3 area 3

RTA(config-router)#area 3 virtual-link 10.0.0.1

...

RTB(config)#router ospf 1

RTB(config-router)#network 192.168.1.0 0.0.0.3 area 3

RTB(config-router)#network 192.168.2.0 0.0.0.3 area 0

RTB(config-router)#area 3 virtual-link 10.0.0.2



Chapter 6 –OSPF

Why Virtual Link?

• Temporary fix when two existing OSPF networks merge (company merger, etc.)

• Backup path



Chapter 6 –OSPF

Common OSPF Configuration Issues



Chapter 6 –OSPF

OSPF Interface InformationRtr# show ip ospf interfaceEthernet0 is up, line protocol is up Internet Address 206.202.2.1/24, Area 1 Process ID 1, Router ID 1.2.202.206, Network Type BROADCAST, Cost: 10 Transmit Delay is 1 sec, State BDR, Priority 1 Designated Router (ID) 2.2.202.206, Interface address 206.202.2.2 Backup Designated router (ID) 1.2.202.206, Interface address 206.202.2.1 Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:00 Neighbor Count is 1, Adjacent neighbor count is 1 Adjacent with neighbor 2.2.202.206 (Designated Router) Suppress hello for 0 neighbor(s)Serial0 is up, line protocol is up Internet Address 206.202.1.2/24, Area 1 Process ID 1, Router ID 1.2.202.206, Network Type POINT_TO_POINT, Cost: 64 Transmit Delay is 1 sec, State POINT_TO_POINT, Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:04 Neighbor Count is 1, Adjacent neighbor count is 1 Adjacent with neighbor 2.0.202.206 Suppress hello for 0 neighbor(s)



Chapter 6 –OSPF

Verifying OSPF Configuration

• show ip protocol• show ip route• show ip ospf interface• shop ip ospf• show ip ospf neighbor detail• show ip ospf database



Chapter 6 –OSPF

The debug and clear Commands for OSPF Verification



Chapter 6 –OSPF

NBMA

• Non-Broadcast Multi-access Access Networks.– Frame Relay– X.25

• Without broadcasts and multicasts, DR/BDR election is problematic



Chapter 6 –OSPF

Network TypesCisco routers can treat NBMA interfaces using any of the following:Non-Broadcast• OSPF is aware that multicast packets cannot be sent over the

interface and sends OSPF packets directly to neighbors using unicast addresses.

• DR and BDR are not elected by default• DR represent the NBMA cloud as a transit network, using

network LSAs• Suitable only for when the VCs are fully meshed

Broadcast• OSPF treats the interface as belonging to a broadcast segment,

thus using multicasts to send OSPF packets.• DR and BDR are elected• Suitable only for when the VCs are fully meshed.

NBMA Networks and OSPF



Chapter 6 –OSPF

Network TypesCisco routers can treat NBMA interfaces using any of the following:

Point-to-multipoint• OSPF treats the interface as a placeholder for a set of point-to-point

adjacencies.• No DR/BDR is elected• Very much like point-to-point interfaces, except that every router

announces a host route to its own IP address.

Point-to-point• OSPF treats the interface as a set of point-to-point adjacencies• No DR/BDR is elected.

NBMA Point-to-Point and Multipoint Networks



Chapter 6 –OSPF

NBMA Solution

• Full meshFull mesh

•Full Mesh network is on one subnet

The OSPF neighbor command tells a router about the IP addresses of its neighbors so that it can exchange routing information without multicasts.



Chapter 6 –OSPF

NBMA Solution• Point-to-pointPoint-to-point uses subinterfaces

• Point-to-point networks are all on different subnets



Chapter 6 –OSPF

NMBA Solution

• Point-to-multipointPoint-to-multipoint

•Point-to-Multipoint network is on one subnet

The broadcast keyword permits the router to send broadcasts by way of the specified DLCI to the mapped neighbor or neighbors.

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