BGP Local-AS command

Local-as is useful when we have a change in the AS number of the BGP domain. However, we do not have a change to change the AS number in the bgp process yet. This command will help the neighbor think that it connects to the old AS number.

Followings are the example of the local-as and its option in BGP.
R1 = AS 100
R2 = AS 200
R3 = AS 300
Configure the network to have R1 thinks that R2 is in AS 65001 (Use local-AS)

R1

router bgp 100
 no synchronization
 bgp log-neighbor-changes
 network 1.1.1.0 mask 255.255.255.0
 network 2.2.2.0 mask 255.255.255.0
 network 3.3.3.0 mask 255.255.255.0
 neighbor 192.168.1.2 remote-as 65001
 no auto-summary

Local AS

R2
router bgp 200
 no synchronization
 bgp log-neighbor-changes
 neighbor 192.168.1.1 remote-as 100
 neighbor 192.168.1.1 local-as 65001
 neighbor 192.168.2.2 remote-as 300
 no auto-summary


R3#sh ip bgp
BGP table version is 46, local router ID is 192.168.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 1.1.1.0/24       192.168.2.1                            0 200 65001 100 i
*> 2.2.2.0/24       192.168.2.1                            0 200 65001 100 i
*> 3.3.3.0/24       192.168.2.1                            0 200 65001 100 i

When having a "no prepend", R2 will not prepend local-as to the update forwarding to R3. Therefore, the R3 will see only the real AS from R2 (without the 65001)


Local AS with no prepend
R2

router bgp 200
 no synchronization
 bgp log-neighbor-changes
 neighbor 192.168.1.1 remote-as 100
 neighbor 192.168.1.1 local-as 65001 no-prepend
 neighbor 192.168.2.2 remote-as 300
 no auto-summary

R3#sh ip bgp
BGP table version is 40, local router ID is 192.168.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 1.1.1.0/24       192.168.2.1                            0 200 100 i
*> 2.2.2.0/24       192.168.2.1                            0 200 100 i
*> 3.3.3.0/24       192.168.2.1                            0 200 100 i

With the no prepend and replace-as, (neighbor to R3) R2 will not prepend the local as but instead replace the real AS with the local as value and send an update to R3. Therefore, R3 will see 65001 (local-as value) and 100 (AS at R1)


Local AS with no prepend & replace as


R2
router bgp 200
 no synchronization
 bgp log-neighbor-changes
 neighbor 192.168.1.1 remote-as 100
 neighbor 192.168.1.1 local-as 65001 no-prepend replace-as
 neighbor 192.168.2.2 remote-as 300
 neighbor 192.168.2.2 local-as 65001 no-prepend replace-as
 no auto-summary

R3#sh ip bgp

BGP table version is 28, local router ID is 192.168.2.2

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*> 1.1.1.0/24       192.168.2.1                            0 65001 100 i

*> 2.2.2.0/24       192.168.2.1                            0 65001 100 i

*> 3.3.3.0/24       192.168.2.1                            0 65001 100 i