[c-nsp] BGP Balanced

[Gert Doering]

Hi,

On Tue, Aug 24, 2004 at 09:38:37PM -0400, Alex Rubenstein wrote:
> Not being a wise-ass, but what is the difference between per-prefix, and
> per-destination?

prefix: routing table entry (/0.../32)
destination: a single destination IP (/32)

So, for example, if you have a customer with a single /24 that has two
bundled E1s, "per-prefix" would always route all the traffic over just 
one of the E1s (which was actually the case for fast switching in older 
IOS versions), as there is just *one* prefix.  With netflow-switching,
one could do per-flow to remedy that, but balancing was usually not
perfect either.

"per-destination" actually (as I learnt yesterday) does the load sharing
decision on a <source ip + destination ip> basis.  So if you have many
different conversations, load balancing will be pretty good.  If you have
only one single machine "left" and "right" talking to each other (like
"one big FTP download"), the traffic will take only a single path and
not the aggregate bandwidth.

"per-packet" will exactly do that: send one packet to E1 #1, the next
to E1 #2, and so on.  With that, you can get packet reordering, which will
hurt TCP throughput - but even a single TCP session can fill both pipes.

> And, I believe what CEF *actually* does is per prefix, meaning that, if
> there is a CEF route for a /17, all traffic for that /17 goes on whatever
> link is cached for that /17 until a route update occurs.

Don't "believe", measure.

Besides that: there is no "caching" in CEF.  Which is the whole point of 
CEF - the FIB is based on L3 routing and L2 adjacency information, and
every packet can be forwarded at the same speed, without any "first packet
slow, subsequent packets fast" caching effects (exceptions: punting due 
to missing ARP entry, etc.).

What you describe is the pre-CEF route cache ("fast switching").

gert
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Gert Doering - Munich, Germany                             gert at greenie.muc.de
fax: +49-89-35655025                        gert at net.informatik.tu-muenchen.de