[nsp] Max eBGP sessions

[Niels Bakker]

* jim at powerpulse.cc (Jim Devane) [Mon 09 Jun 2003, 20:34 CEST]:
> I am trying to get a handle on a maximum number of eBGP sessions that
> would be possible. Of course, I understand this depends widely on what
> platform so I will throw 2 out there a 7507/RSP-8 256 Mb and 7204
> VXR/NPE-G1 As far as routes, only doing a minimum, pushing a default
> route out and accepting 1 /24 back in. Clearly, this is a hypothetical
> exercise but I am trying to validate some suspicions.
>  
> Again, I am only trying to get a ballpark figure to see if it matches
> with what I have in my head already. I am thinking the most would
> probably be about 40. 

I beg to differ.  Routers at exchange points routinely handle literally
hundreds of eBGP sessions, plus the full mesh to their internal network
(or one or two route reflectors).


> Is there a way that I could stretch this? Is 40 too low? Too high? I am
> thinking that I would run into a barrier on the CPU rather than memory.

BGP keepalives are once every 60 seconds (vendor C) or 30 seconds
(vendor J).  CPU is only an issue during a direct (DOS) attack on the
router itself, when it's too busy dropping packets to think about them.

Where did you come up with your limit of 40 BGP sessions per box?

The enemy here is churn, i.e. how many updates you receive, have to
filter, apply to the routing table, seed back through policy and maybe
announce to other sessions etc.  BGP runs over TCP, so if the router
can't keep up it has an easy mechanism to tell a neighbor to slow down
a bit by lowering its window size and delaying ACKs.  Flap damping also
helps battle this, by ignoring "misbehaving" prefixes for a while.


> Can someone point me to a resource or call upon experience to help me
> get a handle on this?

Do check for yourself on e.g. route-views.oregon-ix.net.  This seems to
be a 7206VXR (NPE-G1) with 75 eBGP sessions, of which about 57 are full
views (currently consisting of 105,000 to 125,000 routes).  Memory
utilization is 660 out of 1024MB.


	-- Niels.

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