Re: BGP deployment and peering questions

["Ryan O'Connell" <ryan () complicity co uk>]

On Tue, Feb 13, 2001 at 01:00:00PM -0800, Pyda Srisuresh wrote:
> 1. What is the maximum no. of peers a core-BGP peers with externally?
>    What is a good average or median number? How does this vary with
>    Tier-1 BGP speakers vs. Tier-2 BGP speakers? Also, What is an 
>    average no. of peers a BGP border router multi-homes with? (Do not
>    include Border routers with a single ISP peer - only the multi-homed
>    border routers)

I'm not sure what the maximum number supported by various OSes is, but most
people seem to limit it to around 30-50 per router. Of course, the realisitic
limit depends on router CPU and memory hardware more than anything else - a
Cisco 3640 isn't going to be able to handle as much as a 12000GSR.

An "average" is meaningless - there are many rotuers out there that are
multihomed to only two or three ISPs and therefore only have a handful of
BGP sessions.

> 2. I understand, an AS by itself does not originate more than 
>    10,000 (UUnet being the one with this many) subnets. But, 
>    I believe, when you peer with a tier-1 ISP BGP speaker, you 
>    will get AS Paths for the entire 90,000+ routes (or whatever 
>    the maximum core routing tabel size is) exchanged at BGP
>    connection setup time. On the other hand, I believe, the number
>    of routes exchanged to be much less when you peer with a tier-2 BGP. 
>    What is a resonable average size of routing entries you could 
>    expect from a tier-2 ISP (and even a Tier-1 ISP, for that matter)?

Any ISP shuld give you the entire internet routing table - 90k+ prefixes.
However, your router will only use a certain number of those as "best" routes.
The number of "best" routes per ISP depends entirely on who the ISPs are, and
doesn't (necessarily) have any relation to what Tier the ISPs are.

> 3. Do yo have an estimate of memory requirements for some of the core
>    routers (peering with tier-1 ISPs or tier-2 ISPs)? Is there a 
>    relation with the number of BGP peers?

A Cisco 3640 with 64Mb will (Or at least did) just about handle a BGP
feed from two or three peers. Memory requirements (And CPU requirements)
increase with the number of peers, but 192Mb should be plenty for most
applications.

-- 
Ryan O'Connell - <ryan () complicity co uk> - http://www.complicity.co.uk

I'm not losing my mind, no I'm not changing my lines,
I'm just learning new things with the passage of time