[c-nsp] Missing 'packet-too-big' ICMPs on 12.2(28)SB6

[Gert Doering]

Hi,

On Mon, Sep 10, 2007 at 08:55:35PM +0200, Gerald Krause wrote:
> The only problem with our config could be the fact that we have
> configured a MTU of 1530 byte (because of MPLS) on the FEs. But even
> if this could be problematic I would expect that in the case that a
> paket arrive which can't be delivered through the VirtualInterfaces
> (MTU 1448 byte) the system have to mention that in the logging buffer.
> Thats why I have enabled ICMP debugging and related CEF debugging
> but I never see 'packet-too-big' ICMP messages, only 'echo' and
> 'host/port unreachable' messages are logged. But I See that CEF is
> aware of the fact that it can't deliver the packet without
> fragmentation:

I've seen massive problems with IPv6 MTU and CEF on a 7301 with 
12.2(31)SB2 - short version: the router seems to incorrectly count
the size of the header, and drops packets bigger than 1482 bytes
(1500 - 18 byte ethernet header).

No ICMP either, packet just gone.

Maybe this is related...

(I have no smartnet on that box, so I can't easily open a TAC case 
on this.)

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