bgp route reflectors

8/13/2019 BGP Route Reflectors

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BGP Route Reflectors

Hopefully you recall that when running iBGP, a full mesh is required between all iBGP

speaking peers within an autonomous system (AS). This can quickly lead to a huge number

of peeringsand the associated management overheadas we add more iBGP speakers.

The total number of peerings needed to satisfy the full mesh requirement is illustrated by the

following formula, where n is the number of iBGP speakers:

n(n-1)/2

Thus:

5 routers = 10 peerings 10 routers = 45 peerings 25 routers = 300 peerings

Would you want to manage all that? Me either. Enter route reflectors.

Route reflectors are one solution for keeping all of this under control. Route reflectors ease

the full-mesh limitation and allows one router to advertise, or reflect, iBGP learned routes toother iBGP speakers. This has the end result of reducing the number of iBGP peers within

our AS.

Heres the topology well be working with:
http://evilrouters.net/http://evilrouters.net/


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In this scenario, well be running OSPF as our IGP and all routers will run iBGP and are

members of AS 65000. R3 will serve as our route reflector, and all other routers only need to

establish a BGP peering with R3. On R1 and R2 well advertised the networks represented by

the loopback0 interfaces as well.

Lets get basic connectivity established.

R1# configure terminalR1(config)# interface serial 0/1R1(config-if)# ip address 198.18.13.1 255.255.255.0R1(config-if)# no shutdownR2# configure terminalR2(config)# interface serial 0/1R2(config-if)# ip address 198.18.23.2 255.255.255.0R2(config-if)# no shutdownR3# configure terminalR3(config)# interface serial 1/2R3(config-if)# clock rate 128000

R3(config-if)# ip address 198.18.13.3 255.255.255.0R3(config-if)# no shutdownR3(config-if)# interface serial 1/3R3(config-if)# clock rate 128000R3(config-if)# ip address 198.18.23.3 255.255.255.0R3(config-if)# no shutdownR3(config-if)# interface serial 1/0R3(config-if)# encapsulation frame-relayR3(config-if)# no shutdownR3(config-if)# interface serial 1/0.34 point-to-pointR3(config-subif)# frame-relay interface-dlci 304R3(config-fr-dlci)# ip address 198.18.34.3 255.255.255.0R3(config-subif)# interface serial 1/0.35 point-to-point

R3(config-subif)# frame-relay interface-dlci 305R3(config-fr-dlci)# ip address 198.18.35.3 255.255.255.0R4# configure terminalR4(config)# interface serial 0/0R4(config-if)# encapsulation frame-relayR4(config-if)# no shutdownR4(config-if)# interface serial 0/0.34 point-to-pointR4(config-subif)# frame-relay interface-dlci 403R4(config-fr-dlci)# ip address 198.18.34.4 255.255.255.0R5# configure terminalR5(config)# interface serial 0/0R5(config-if)# encapsulation frame-relayR5(config-if)# no shutdown

R5(config-if)# interface serial 0/0.35 point-to-pointR5(config-subif)# frame-relay interface-dlci 503R5(config-fr-dlci)# ip address 198.18.35.5 255.255.255.0

At this point, make sure you can ping all routers from R3 and vice versa.

Go ahead and configure the loopback interfaces on R1 and R2.

R1(config-if)# interface loopback 0R1(config-if)# ip address 198.18.111.1 255.255.255.255R2(config-if)# interface loopback 0R2(config-if)# ip address 198.18.222.2 255.255.255.255


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*>i198.18.111.1/32 198.18.13.1 0 100 0 i

*>i198.18.222.2/32 198.18.23.2 0 100 0 i

R5(config-router)# do show ip bgp | begin Network

Network Next Hop Metric LocPrf Weight Path

*>i198.18.111.1/32 198.18.13.1 0 100 0 i

*>i198.18.222.2/32 198.18.23.2 0 100 0 i

Thats all there is to it! Weve taken what normally would take a total of 10 BGP sessions

and accomplished the same thing with just four. Later, well talkabout BGP confederations,

another technique for reducing the iBGP mesh (mess).

Related posts:

1. Advertising a default route over BGP2. Cisco BGP bug crashes 12.4(23)3. No, You Route the Packet!4. Configuring BGP maximum-prefix5. Configuring OSPF Authentication (Dynamips)
http://evilrouters.net/2009/02/25/advertising-a-default-route-over-bgp/http://evilrouters.net/2009/02/25/advertising-a-default-route-over-bgp/http://evilrouters.net/2008/12/06/cisco-bgp-bug-crashes-12423/http://evilrouters.net/2008/12/06/cisco-bgp-bug-crashes-12423/http://evilrouters.net/2011/08/11/no-you-route-the-packet-routing-loop/http://evilrouters.net/2011/08/11/no-you-route-the-packet-routing-loop/http://evilrouters.net/2009/07/21/configuring-bgp-maximum-prefix/http://evilrouters.net/2009/07/21/configuring-bgp-maximum-prefix/http://evilrouters.net/2008/11/26/configuring-ospf-authentication-dynamips/http://evilrouters.net/2008/11/26/configuring-ospf-authentication-dynamips/http://evilrouters.net/2008/11/26/configuring-ospf-authentication-dynamips/http://evilrouters.net/2009/07/21/configuring-bgp-maximum-prefix/http://evilrouters.net/2011/08/11/no-you-route-the-packet-routing-loop/http://evilrouters.net/2008/12/06/cisco-bgp-bug-crashes-12423/http://evilrouters.net/2009/02/25/advertising-a-default-route-over-bgp/

bgp route reflectors

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