[c-nsp] DMVPN Rollout -- MTU questions

[John Kougoulos]

On Thu, 22 May 2008, Eric Cables wrote:

>
> The above, however, doesn't seem to work in some cases.  Users as these
> sites complain of intermittent connectivity problems, which seem to be
> solved rather quickly by reducing the IP MTU, and configuring TCP
> adjust-mss.  I do have concern as to why PTMUD isn't working as expected
> (sending ICMP unreachables to the client to adjust their MTU accordingly),
> and exactly what values to set both IP MTU to, as well as TCP adjust-mss,
> assuming it's necessary.
>

unless you have lots of large UDP packets (near 1500bytes), I prefer to 
use "ip mtu 1500" on tunnel interface, with "ip tcp adjust-mss 1360", 
or something similar.

This way TCP packets don't go over 1400bytes and the service is MTU 
transparent. Of course you will have fragmentations in case of large 
non-TCP packets, but let's hope/assume that these will not be too many....

Best Regards,
John