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BGP Routing Clarification

Hi anyone please help to clarify the following BGP routing;

RouterA;

s0 - 172.16.11.1

e0 - 172.16.15.1

loopback0 - 172.16.111.1

RouterB;

s0 - 172.16.11.2

e0 - 10.1.10.1

loopback0 - 172.16.11.2

RouterA and RouterB are connected by different AS numbers

At RouterA;

L1 - RouterA(config)#router bgp 1

L2 - RouterA(config-router)#network 172.16.0.0 mask 255.255.255.0

L3 - RouterA(config-router)#network 10.1.10.0 mask 255.255.255.0

L4 - RouterA(config-router)#neighbor 172.16.11.2 remote-as 2

L5 - RouterA(config-router)#redist static

L6 - RouterA(config-router)#no synch

L7 - RouterA(config-router)#exit

L8 - RouterA(config)#ip route 0.0.0.0 0.0.0.0 s0

At RouterB;

L1 - RouterB(config)#router bgp 2

L2 - RouterB(config-router)#network 172.16.0.0 mask 255.255.255.0

L3 - RouterB(config-router)#network 10.1.10.0 mask 255.255.255.0

L4 - RouterB(config-router)#neighbor 172.16.11.1 remote-as 1

L5 - RouterB(config-router)#redist static

L6 - RouterB(config-router)#no synch

L7 - RouterB(config-router)#exit

L8 - RouterB(config)#ip route 0.0.0.0 0.0.0.0 s0

Questions;

(1) In L4 of RouterA, what does it mean by "remote-as 2" ?

(2) In L5 of RouterA, what does it mean by "no synch"

(3) In L8 of RouterA, why we still need to specify the static default route ? The s0 here is referred to s0 of RouterA or s0 of RouterB ?

Please help to clarify.

Thank you.

1 REPLY

Re: BGP Routing Clarification

L4

This specifies to which (router / AS) . a BGP connection should be initiated. WHen you specify the command neighbor remote-as

is the ip address of the remote router to which BGP connection is required

is the AS number in which this remote router belongs (in your case 2)

L5- no synch.

no synch can be expanded as "no synchronization" ..... Describing why you need it here could take a bit of time....You might want to surf this link

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ics/icsbgp4.htm#xtocid204397

L8 - default route

the serial 0 at the end of the default route always refers to the local routers serial interface. Please note that you are using loopback ip addresses as the peer ip addresses in the BGP commands. For router A, the peer ip address in the neighbor command is the loopback address of router B, which is not directly connected to router A. So router A still needs to know how to reach that loopback address. THat is the reason why a default route is used pointing to serial0 of router A. Same is the case in router B.

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