BGP Local-AS to save the day

local-as

You are the network engineer for ACME Inc and your company has decided to change Autonomous System (AS) number, not sure what the reason for the change but you’re just doing what you are being told.

There’s two upstream connection to AS 900 and AS 800, according to ISP AS 900 it would take a month for them to accept your new AS while ISP AS 800 will be cutting over to the new AS tonight but you need to go to a dinner party this evening as its you and your wife’s Anniversary and you wouldn’t want to miss that.

The goal is to configure the network now according to management to make the necessary changes, use the new AS internally, R1 is acting as a route-reflector in your network, peer with AS 900 still using AS 17111 and prep your peering with AS 800 to accept both the old and new AS in the event they do the cut-ver tonight while you get to enjoy dinner with the misis.

Solution: configure your edge routers with local-as. neighbor x.x.x.x local-as <oldAS> no-append replace-as {dual-as}. Configure the router facing AS 900 (R2) to maintain bgp peering by using the previous AS. Configure R3 with dual-as, so when AS 800 make their changes they would maintain the peering.

R1:

!
no router bgp 17111
router bgp 17146
neighbor 155.1.123.2 remote-as 17146
neighbor 155.1.123.3 remote-as 17146
neighbor 155.1.123.2 route-reflector-client
neighbor 155.1.123.3 route-reflector-client
!

R2:

!
no router bgp 17111
router bgp 17146
neighbor 155.1.123.1 remote-as 17146
neighbor 155.1.27.7 remote-as 900
neighbor 155.1.27.7 local-as 17111 no-append replace-as
!

R3:

!
no router bgp 17111
router bgp 17146
neighbor 155.1.123.1 remote-as 17146
neighbor 155.1.38.8 remote-as 800
neighbor 155.1.38.8 local-as 17111 no-append replace-as dual-as

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