When does this cmd need?

[yuanguitao]

Well, here is my lab NP.  but i find it can reflect so many problems.

As you know, we often use loopback interface to establish ibgp peer in one AS, and use connected interface to establish ebgp peer between diferent AS(s).

1. just like my lab, i use loopback int 0 of R2 and loopback int 0 of R3 to establish neighbor. then i should configure an IGP protocol or static route for TCP connection, or 2.2.2.2 can not reach 3.3.3.3. and now i was confused that  why CISCO officail file suggest us to do like this and not use connected interfaces.
2. when we use loopback interface between two AS(s), we must add cmd: neighbor x.x.x.x ebgp-multihop 0~255, which specify the least hops before destination. so message from R1 can reach R2 successfully. but why could we never use this cmd when we establish neighbor between R2 and R3 even we use loopback interface, three shake-hands also can be done.

[pankaj kumar]

Hi Wei yao,

just like my lab, i use loopback int 0 of R2 and loopback int 0 of R3 to establish neighbor. then i should configure an IGP protocol or static route for TCP connection, or 2.2.2.2 can not reach 3.3.3.3. and now i was confused that  why CISCO officail file suggest us to do like this and not use connected interfaces.
 

we use the loopback interface because they are very much reliable.loopback comes up as soon as device starts there are no chance like loose cable problem so its good to form neighborship with the help of loopback in case we have redundant links, if we form neighborship with the help of the physical interface then if one of the redendant  interface which is used to form neighborship goes down it means neighbor ship goes down but the neighbor is still reachable with another interface.

when we use loopback interface between two AS(s), we must add cmd: neighbor x.x.x.x ebgp-multihop 0~255, which specify the least hops before destination. so message from R1 can reach R2 successfully. but why could we never use this cmd when we establish neighbor between R2 and R3 even we use loopback interface, three shake-hands also can be done.

The ebgp-multihop command is used to disable the connect check mechansim and this can be used when we form the neighborship with the help of loopback or when we are no able to form the neighborship with the help of directly connected interface.

Regards,

Wei yao.

[cadet alain]

Hi,

Question 1 was already answered  by kumar so I will simply add my contribution for question 2

but why could we never use this cmd when  we establish neighbor between R2 and R3 even we use loopback interface,  three shake-hands also can be done.

It is not needed for iBGP because the outgoing TTL is 255 for iBGP but in case of eBGP it is 1 and furthermore there are no connected check for iBGP.

Regards

Alain

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