Playing with BGP Local-AS
In some situation you might have to change the BGP AS number used by a router. When the router peers with several other routers it is not always easy to change all the peering at the same time… Luckily you may do it one by one with the “local-as” neighbor command under bgp process.
This small article shows the different options of local-as command and their impact on the received and advertised routes.
First let’s have a look at the test setup before we use local-as
Now let’s say we want to change the local AS from 12 to 1200 without having to reconfigure the complete AS… We can achieve this with local-as. With local-as alone, the real AS# and the local AS# are in the AS_PATH
R2(config-router)#do sh run | s router bgp
router bgp 12
no synchronization
bgp log-neighbor-changes
neighbor 10.10.12.1 remote-as 12
neighbor 10.10.23.3 remote-as 34
neighbor 10.10.23.3 local-as 1200
no auto-summary
R1(config-router)#do sh ip bgp
BGP table version is 11, local router ID is 11.11.11.11
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
*> 11.11.11.11/32 0.0.0.0 0 32768 i
*> 44.44.44.44/32 10.10.12.2 0 0 1200 34 i
R4(config-router)#do sh ip bgp
BGP table version is 10, local router ID is 44.44.44.44
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
*> 11.11.11.11/32 10.10.34.3 0 0 1200 12 i
*> 44.44.44.44/32 0.0.0.0 0 32768 i
With local-as no-prepend, same results for the advertise routes. Received routes’s AS_PATH do not contain the “fake” AS number. “no-prepend” is used to prevent incoming eBGP updates to be prepended with the configured local AS#. From that perspective, it looks a bit like confederations.
R2(config-router)#do sh run | s router bgp
router bgp 12
no synchronization
bgp log-neighbor-changes
neighbor 10.10.12.1 remote-as 12
neighbor 10.10.23.3 remote-as 34
neighbor 10.10.23.3 local-as 1200 no-prepend
no auto-summary
R1(config-router)#do sh ip bgp
BGP table version is 11, local router ID is 11.11.11.11
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
*> 11.11.11.11/32 0.0.0.0 0 32768 i
*> 44.44.44.44/32 10.10.12.2 0 0 34 i
R4(config-router)#do sh ip bgp
BGP table version is 10, local router ID is 44.44.44.44
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
*> 11.11.11.11/32 10.10.34.3 0 0 1200 12 i
*> 44.44.44.44/32 0.0.0.0 0 32768 i
With local-as no-prepend replace-as, received routes’s AS_PATH do not contain the “fake” AS number. The real AS number do not appear in the AS_PATH of the advertised routes, it is replaced by the configure local AS#. In that perspective, it looks a bit like confederations…
R2(config-router)#do sh run | s router bgp
router bgp 12
no synchronization
bgp log-neighbor-changes
neighbor 10.10.12.1 remote-as 12
neighbor 10.10.23.3 remote-as 34
neighbor 10.10.23.3 local-as 1200 no-prepend replace-as
no auto-summary
R1(config-router)#do sh ip bgp
BGP table version is 11, local router ID is 11.11.11.11
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
*> 11.11.11.11/32 0.0.0.0 0 32768 i
*> 44.44.44.44/32 10.10.12.2 0 0 34 i
R4(config-router)#do sh ip bgp
BGP table version is 10, local router ID is 44.44.44.44
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
*> 11.11.11.11/32 10.10.34.3 0 0 1200 i
*> 44.44.44.44/32 0.0.0.0 0 32768 i
