Multiple BGP Routes in FIB

Fri Sep 8 08:23:45 UTC 2006

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withdraws a route it only specifies the prefix being withdrawn and not 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