North American Network Operators Group

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Re: Multiple BGP Routes in FIB

  • From: Sam Stickland
  • Date: Fri Sep 08 04:27:28 2006

Hi Glenn,

Glen Kent wrote:

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

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 When BGP withdraws a route it only specifies the prefix being withdrawn and not the path.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).