[c-nsp] MTU and PMTUD

[Lukasz Bromirski]

Marcin,

I did some benchmarking back in ~2008-2009 (with 12.2.31SB and 12.2SR,
who still remembers that?), and we (Cisco) fixed numerous non optimized
paths in the routing code. But that was over a decade ago, so I’ll
limit bragging about it ;) There are two presentations, quoted from
about that time I can find some references to, which show in general
terms what improvements you can expect. XR code was however way
more optimized right from the start, and specially over last 5 years
there was a lot additional work done to optimize it.

Anyway - Cisco BGP stack is still limited to 4kB message for BGP (as
far as I know), so we still don’t implement RFC 8654 neither in XR or XE.
I can check that with developers, but last time I’ve heard it’s only on
roadmap.

In the end you’ll get about 2.5-3x improvement using MTU 9k, but the actual
BGP messages inside will still be limited to 4k. Back in the day, in my
testbed, that made 7200VXR with NPE-G1 converge in about 21 seconds versus
a minute with ~600k prefixes and 100 sessions.

Most of the benefits are coming BTW from the fact its lesser amount of
packets in BGP communication (to Ytti point) as for real-world BGP table,
the packing of attributes and updates with 4k vs 1.3-1.5k is better,
but only by so much. Again, the old numbers I’m quoting are back from the
day when IOS was monolithic, now with CSR you’re using for testing, IOS-XE
should handle that way more smoothly even under stress.

Running 9k MTU is anyway recommended for BGP RR scenarios, as many core SP
networks (and some Enterprises) run 9k MTU in the core anyway. You’ll get
less bursts for initial convergence and then ongoing session maintenance
thanks to less packets. Some very old recommendations (again, remember
ISP Best Practices guide v2.9?) about interface hold buffers and SPD tuning
will no longer apply obviously.

--
./
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