North American Network Operators Group

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Re: Scalability issues in the Internet routing system

  • From: Blaine Christian
  • Date: Thu Oct 27 10:32:09 2005

Neat! So you were thinking you would leave the actual route selection process monolithic and create separate processes per peer? I have seen folks doing something similar with separate MBGP routing instances. Had not heard of anyone attempting this for a "global" routing table with separate threads per neighbor (as opposed to per table). What do you do if you have one neighbor who wants to send you all 2M routes though? I am thinking of route reflectors specifically but also confederation EIBGP sessions.

> I think you hit the nail on the head regarding record locking. This is
> the thing that is going to bite you if anything will. I have heard
> none of the usual suspects speak up so I suspect that either this
> thread is now being ignored or no one has heard of an implementation
> like the one you just described.

In BGP there is no 'global' route (actually path) selection in BGP.
Everything is done per prefix+path. In the RIB you can just lock the prefix,
insert the new path and recalculate which one wins. Then issue the update
to the FIB, if any. Work done. Statistically there is very little
contention on the prefix and the path records. For contention two updates
for the same prefix would have to arrive at the same time from two different
peers handled by different CPU's. I'd guess the SMP scaling factor for BGP
is around 1.98. The 0.02 go lost for locking overhead and negative caching
effects. Real serialization happens only at the FIB change queue. However
serializing queues can be handled very efficiently on SMP too.

Hey Andre,

If you are intending to break the BGP process into per neighbor threads this does not sound like it would have beneficial impact on a single neighbor with the majority of the routes (thinking specifically of EIBGP and/or Route Reflectors). Was your idea specifically related to per neighbor processing or were you thinking you could break the BGP process itself into chunks?