BGP - local-as option

BGP local-as option allows a router to appear as if it is in another AS. Suppose we have a frame-relay cloud with 3 routers all EBGP peers with each other:

R6: 172.14.45.6 (AS 65000)
R5: 172.14.45.5 (AS 65005)
R4: 172.14.45.4 (AS 345)

We can configure R6 to use the local-as option to appear to be from AS 65006 to R5, but remain in AS65000 for R4. Here's how:

R6(config-router)#router bgp 65000
R6(config-router)#neighbor 172.14.45.5 local-as 65006

On R5:

R5(config)#router bgp 65005
R5(config-router)#neighbor 172.14.45.6 remote-as 65006

Let's take a look at the neighbor summary:

R5# show ip bgp summary | be Ne
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
172.14.45.6 4 65006 220 221 244 0 0 00:03:39 9

R4#show ip bgp summary | be Ne
172.14.45.6 4 65000 176 146 69 0 0 00:00:08 11

Notice the different AS numbers. Also notice the AS path from R5's view:

R5# show ip bgp | inc 172.14.45.6
*> 6.0.3.0/24 172.14.45.6 0 0 65006 65000 i
*> 6.0.4.0/24 172.14.45.6 0 0 65006 65000 i
*> 6.0.5.0/24 172.14.45.6 0 0 65006 65000 i

And the AS path from R6's view also includes the local-AS number:

R6#show ip bgp | be Ne
Network Next Hop Metric LocPrf Weight Path
*> 2.2.2.2/32 172.14.45.5 0 65006 65005 65002 i
*> 5.5.5.5/32 172.14.45.5 0 0 65006 65005 i

The routes appear to magically pass through 65006. We can prevent R6 from prepending the local-as number on routes received from R6 with the no-prepend option

R6(config)#router bgp 65000
R6(config-router)#neighbor 172.14.45.5 local-as 65006 no-prepend

65006 is no longer in the AS Path:

R6#show ip bgp | be Ne
Network Next Hop Metric LocPrf Weight Path
*> 2.2.2.2/32 172.14.45.5 0 65005 65002 i
*> 5.5.5.5/32 172.14.45.5 0 0 65005 i

With the replace-AS we can prevent R5's real BGP AS number from appearing in the AS path on routes from R6 to R5:

R6(config)#router bgp 65000
R6(config-router)#neighbor 172.14.45.5 local-as 65006 no-prepend replace-as

R5# show ip bgp | be Ne
Network Next Hop Metric LocPrf Weight Path
*> 6.0.3.0/24 172.14.45.6 0 0 65006 i
*> 6.0.4.0/24 172.14.45.6 0 0 65006 i
*> 6.0.5.0/24 172.14.45.6 0 0 65006 i

Lastly, we can configure R6 to accept connections to either AS 65000 or AS 65006 with the dual-as option:

R6(config)#router bgp 65000
R6(config-router)#neighbor 172.14.45.5 local-as 65006 no-prepend replace-as dual-as

R5#show ip bgp summary

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
172.14.45.6 4 65006 268 284 343 0 0 00:00:08 9

R5#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R5(config)#router bgp 65005
R5(config-router)#neighbor 172.14.45.6 remote-as 65000
R5(config-router)#
.Jul 14 21:34:34.273: %BGP-5-ADJCHANGE: neighbor 172.14.45.6 Down Remote AS changed
R5(config-router)#
.Jul 14 21:34:36.505: %BGP-5-ADJCHANGE: neighbor 172.14.45.6 Up
R5(config-router)#
R5(config-router)#^Z
R5#show ip bgp summary
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
172.14.45.6 4 65000 270 286 0 0 0 00:00:09 0