BGP - Local AS notes

Here is the topology:

R6[AS256]-----R9[AS9999]=====BB3[AS3561]

We will be looking at the BGP tables of R6 and BB3 as we test each option of the local-as neighbor configuration.

1) LOCAL-AS

R9 has local-as configured with as 19999. R6 sees routes as follows, with 19999 in between BB2 and R9 real AS number.

R9#show run | inc local-as
neighbor 100.100.250.250 local-as 19999
R9#

R6#show ip bgp

* 102.0.0.0/22 150.100.96.9 0 9999 19999 3561 ?
*> 150.100.69.9 0 9999 19999 3561 ?
* 102.0.16.0/20 150.100.96.9 0 9999 19999 3561 ?
*> 150.100.69.9 0 9999 19999 3561 ?
* 102.0.32.0/22 150.100.96.9 0 9999 19999 3561 ?
*> 150.100.69.9 0 9999 19999 3561 ?
* 102.0.48.0/22 150.100.96.9 0 9999 19999 3561 ?
*> 150.100.69.9 0 9999 19999 3561 ?

BB3 sees routes from R9 in the same way, as if AS19999 was connected between itself and R9:

CoreTech-BB3#show ip bgp
*> 150.100.25.0/24 100.100.250.9 0 19999 9999 256 i
*> 150.100.69.0/24 100.100.250.9 0 19999 9999 256 i
*> 150.100.91.0/24 100.100.250.9 0 0 19999 9999 i
*> 150.100.96.0/24 100.100.250.9 0 19999 9999 256 i


2) LOCAL-AS NO-PREPEND

With the no-prepend option, R6 does not see 19999 in the path:

R9#show run | inc local-as
neighbor 100.100.250.250 local-as 19999 no-prepend
R9#

R6#show ip bgp

* 102.0.0.0/22 150.100.96.9 0 9999 3561 ?
*> 150.100.69.9 0 9999 3561 ?
* 102.0.16.0/20 150.100.96.9 0 9999 3561 ?
*> 150.100.69.9 0 9999 3561 ?
* 102.0.32.0/22 150.100.96.9 0 9999 3561 ?
*> 150.100.69.9 0 9999 3561 ?
* 102.0.48.0/22 150.100.96.9 0 9999 3561 ?
*> 150.100.69.9 0 9999 3561 ?

This command has no impact on the routes BB3 receives. They look the same.

CoreTech-BB3#show ip bgp
*> 150.100.25.0/24 100.100.250.9 0 19999 9999 256 i
*> 150.100.69.0/24 100.100.250.9 0 19999 9999 256 i
*> 150.100.91.0/24 100.100.250.9 0 0 19999 9999 i
*> 150.100.96.0/24 100.100.250.9 0 19999 9999 256 i


3) LOCAL-AS NO-PREPEND REPLACE-AS

With the Replace-as, everything looks the same as the last command on R6.

R9#show run | inc local-as
neighbor 100.100.250.250 local-as 19999 no-prepend replace-as
R9#

R6#show ip bgp

* 102.0.0.0/22 150.100.96.9 0 9999 3561 ?
*> 150.100.69.9 0 9999 3561 ?
* 102.0.16.0/20 150.100.96.9 0 9999 3561 ?
*> 150.100.69.9 0 9999 3561 ?
* 102.0.32.0/22 150.100.96.9 0 9999 3561 ?
*> 150.100.69.9 0 9999 3561 ?
* 102.0.48.0/22 150.100.96.9 0 9999 3561 ?
*> 150.100.69.9 0 9999 3561 ?

However on BB3 we now see a difference as 9999 is no longer in the path.

CoreTech-BB3#show ip bgp
*> 150.100.25.0/24 100.100.250.9 0 19999 256 i
*> 150.100.69.0/24 100.100.250.9 0 19999 256 i
*> 150.100.91.0/24 100.100.250.9 0 0 19999 i
*> 150.100.96.0/24 100.100.250.9 0 19999 256 i