routers at the edge of 2 AS

uzair syed naveed · ‎10-25-2009

Suppose 2 routers A & B are at the edge of 2 different AS. as they have a direct link between them, what is the most likely routing protocol which tehy speak to each other across this link ?

Since its a direct link b/w them would OSPF/EIGRP be a preffered choice? since we would have to run EBGP in this case and that would increase the convergence time.

Giuseppe Larosa · ‎10-25-2009

Hello Uzair,

eBGP is by far the most common option in a scenario like yours.

Different ASes have different administrators and using an IGP is seen as an unnecessary exposure of internal details.

Using an IGP process different then the one deployed inside would require redistribution as with BGP.

BGP is preferred for its rich feature set and also because it has been designed for this kind of scenarios.

eBGP convergence can be improved using:

neigh x.x.x.x ebgp fast-external-fallover

you can also deploy more aggressive timers on a per neighbor basis

neigh x.x.x.x timers 2 6

see

neighbor [ip-address | peer-group-name] timers keepalive holdtime

http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_bgp4.html#wp1011411

neighbor ip-address fall-over [bfd | route-map map-name]

http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_bgp3.html#wp1013842

this is the modern version of old ebgp fast-external fallover that was able to tear an eBGP session when the link to neighbor fails without waiting for hold time expiration.

Hope to help

Giuseppe