[c-nsp] Cisco 7600 TTL and MTU Failures

[Simon Leinen]

Palis Michalis writes:
> Thanks for your uselull info.. But what is actually the cause of TTL
> and MTU failures? In my case it seems tha their is a large number of
> TTL MTU failures.

I would expect three TTL failures to occur whenever someone performs a
traceroute through your router (because traceroute typically performs
three queries per TTL value).  Many people like traceroute (I sure do
:-), so seeing many TTL failures is not unusual.

On the other hand, MTU failures shouldn't be that frequent except when
you have links with <1500 byte MTU leaving the router (tunnels?).
They will happen when the router receives a packet that is too big for
the link that the packet should go out on.  The router then has to
either fragment the packet, or, when the packet had the "Don't
Fragment" bit set, discard it and generate an ICMP message (type 3
"destination unreachable" code 4 "fragmentation needed and Don't
Fragment bit set").  Today, most large packets have the "Don't
Frament" bit set to support Path MTU Discovery, so the second option
is the common one.

I ran a script over all our routers with PFC3s (27 with uptimes
ranging from days to months), and here's the overall result:

TTL failures          MTU failures        total packets
1960023145 (0.0153%)  7113151 (0.0001%)   12783231177812

So about 0.015% of our packets are due to traceroutes (more on some
routers), and MTU failures are very rare occurrences.

Regards,
-- 
Simon.