Redistribute OSPF E2 Routes in BGP

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May 10, 2012 1:17 AM
May 10th, 2012
Introduction

OSPF External Routes (E2) are advertised into OSPF Autonomous System with a particular cost and the cost is not changed throughout the AS. The routes are learned via redistribution of other routing protocol or static route or a connected subnet. The router performing redistribution is the Autonomous System Border Router (ASBR). By default, all OSPF routes learned via redistribution are of E2 type.

Redisribution of OSPF Routes (E1 or E2) into BGP can be performed via two ways, this document focus on E2 routes.

a) Command " redistribute ospf 1 match external 2"

b) Using Route-Map Match Statement "match route-type external type-2"

Using Route Maps gives more felxibility to the user, as one can match the routes based on different criteria (access-list, tag).

Prerequisite
  • Understanding of OSPF Routing Protocol
  • Understanding of Border Gateway Protocol (BGP)
Background Summary

In this document the focus is on route-map to match the E2 types routes and then redistributing them into other routing system. Three Routers, R1, R2 and R3 are connected via Serial Interface. Router R1 and R2 are participating in OSPF Area 0 while Router R2 and R3 are forming IBGP peering under AS 100.

On R1, Loopback Interfaces (Lo 2 and Lo 3), are redsitributed in OSPF. A route-map "connected is configured" to match the specified interfaces and then redistributed via command "redictribute connected route-map connected". On router R2 these loopback prefixes are learned of type E2. On R2, a route-map (route-map E2OSPF in this document, see configuration on Router R2) is configured to match only the E2 type prefixes and redistribiuted into BGP system.

Note: All configurations are tested on C3745 router operating on IOS verison 12.4(15)T14.

Topology Diagram

Topology Diagram.bmp

Configuration
R1R2R3

hostname R1

!

ip cef

!

no ip domain lookup

!

interface Loopback0

ip address 1.1.1.1 255.255.255.255

!

interface Loopback2

ip address 2.2.2.2 255.255.255.255

!

interface Loopback3

ip address 3.3.3.3 255.255.255.255

!

interface Serial0/0

ip address 10.12.12.1 255.255.255.252

clock rate 2000000

!

router ospf 10

router-id 1.1.1.1

log-adjacency-changes

redistribute connected subnets route-map CONNECTED

network 10.12.12.1 0.0.0.0 area 0

!

route-map CONNECTED permit 10

match interface Loopback2 Loopback3

!

end

hostname R2

!

ip cef

!

no ip domain lookup

!

interface Loopback0

ip address 22.22.22.22 255.255.255.255

!

interface Serial0/0

ip address 10.12.12.2 255.255.255.252

clock rate 2000000

!

interface Serial0/1

ip address 192.23.23.1 255.255.255.252

clock rate 2000000

!

router ospf 10

router-id 22.22.22.22

log-adjacency-changes

network 10.12.12.2 0.0.0.0 area 0

!

router bgp 100

no synchronization

bgp router-id 22.22.22.22

bgp log-neighbor-changes

network 192.23.23.1 mask 255.255.255.255

redistribute ospf 10 route-map E2OSPF

neighbor 192.23.23.2 remote-as 100

neighbor 192.23.23.2 next-hop-self

no auto-summary

!

route-map E2OSPF permit 10

match route-type external type-2

!

end

hostname R3

!

ip cef

!

no ip domain lookup

!

interface Loopback0

ip address 33.33.33.33 255.255.255.255

!

interface Serial0/0

ip address 192.23.23.2 255.255.255.252

clock rate 2000000

!

router bgp 100

no synchronization

bgp router-id 33.33.33.33

bgp log-neighbor-changes

network 192.23.23.2 mask 255.255.255.255

neighbor 192.23.23.1 remote-as 100

no auto-summary

!

end

Verification
Command show ip route ospf

show ip route ospf.bmp

Command show route-map {route-map name}

route-map.bmp

Command show ip route BGP

BGP Route.bmp

References

Understanding Redistribution of OSPF Routes into BGP

IP Routing: OSPF Configuration Guide 12.4

IP Routing: BGP Configuration Guide 12.4

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