IpinIP tunnels - was Re: MTU problems with GRE tunnels (fwd)

Paul Mansfield paulm at uk.psi.com
Mon Jul 6 16:20:02 UTC 1998

> # > I'm experiencing problems with fragmentation due to Cisco GRE tunnel
> # > overhead: the way I understand it, the MTU if a GRE tunnel will always be
> # > less than the MTU of the underlying IP cloud (in our case 1500 bytes) due

We can confirm that IP in IP tunneling is broken too...

To quote a specific case, Sparc/Solaris networking over a tunnel with IP in IP
will break, e.g. the largest ping you can do is "ping -s xyz 1452", so you have
to reduce the MTU, we lowered ours to 1400 (arbitrary reduction) and it makes
the problem "go away". 

The router in question was running 11.1, but from what's been said, it looks
like it's generic across the range.

I think the Perl program posted was a great idea, now if only Cisco would add
a Perl interpreter to IOS :-)

P Mansfield, Senior SysAdmin PSINet, +44-1223-577577x2611/577611 fax:577600 

More information about the NANOG mailing list