Hi Glenn, Glen Kent wrote:
Hi,
There is an interesting discussion going on in the IDR WG and i am cross posting a mail on Nanog to hear from the operators, if what is described below, a common practise followed by them:
I don't think its correct to advertise one while using both for forwarding. NOTE: I am assuming that the routes share the same path length but have different AS Paths (as mentioned by you earlier in this mail)
I think this is being done by many providers.
Consider two paths for nlri X
as_path 1 {x y z} next_hop n1 as_path 2 {m n z} next_hop n2
Are you suggesting that providers are installing ecmp routes for X with next-hops n1 and n2, while advertising only one of the paths to their IBGP peers?
Yes.
Do providers really do this? Would they install multiple BGP Paths with different AS Paths (but same length) in their FIB, and yet advertise only one?
Is the the right thing to do?
I believe the problem is with the BGP withdrawal mechanism. When BGP withdraws a route it only specifies the prefix being withdrawn and not the path. In this case, if the peer advertised both paths {x y z} and {m n z} for a single prefix it would be impossible to withdraw only one of the paths. I guess, even when using ECMP, BGP still really only considers there to be one best route. Everything else is local FIB manipulations based on local policy (in a similar vein to policy routing - the BGP advertisements don't always reflect which way the traffic will actually be routed). Sam