[c-nsp] 7200s (VXRs and not) and MPLS capabilities

[Nathan]

On Thu, Jun 12, 2008 at 1:23 AM, Sean Shepard
<sean.shepard at ewavepartners.com> wrote:
> (2) If you are using an older IOS, there is a bug with PPP where the router
> won't negotiate (I believe it is) maximum RECEIVE unit (MRU). This has kept
> us from using some devices on bonded MLPPP connections as the PPP link won't
> come up with the MRU conflict. I believe it was fixed somewhere in the 12.3
> chain.

Situation: classic client DSL using PPP with MTU smaller than 1500

There is a bug in the 12.2T IOSes where a downgoing too big packet
generates an ICMP "must fragment" packet, as it should, but the ICMP
message, in itself correctly made, is routed in the same direction as
the original packet, downstream, instead of being routed backwards to
the source IP of the too-big packet.

If the CPE routes the ICMP packet back upstream according to the
destination IP on the packet, then things "work". If the CPE decides
/not/ to route the ICMP packet back upstream ("Hey, I'm not a router,
I have one outside IP, and I drop things coming from the outside that
are not for that me"), then you're dealing with a PMTU blackhole.

This is nasty because IIRC debug ip icmp on the PE says it's sending
the ICMP message, but doesn't mention that it chose the wrong outgoing
interface.

At the time I searched CCO for the bug, didn't find it, tested on
12.3, could not reproduce, and therefore upgraded to 12.3.

-- 
HTH
Nathan