[c-nsp] gre tunnel mtu mismatch

Thu Oct 4 15:47:05 EDT 2018

On Thu, Oct 4, 2018, at 13:33, Gert Doering wrote:
> On Wed, Oct 03, 2018 at 02:42:22PM -0700, Mike wrote:
> >
> > I have tried setting in the tunnel interface "ip mtu 1476" but I get an 
> > error:
> > 
> > ip mtu 1476
> > IP MTU not supported on interface Tunnel0
> 
> Well, reasonable equipment supports user-settable MTU... does "mtu 1476" work?

No. But "tunnel path-mtu-discovery" does and adjusts the MTU to "Real life". there are alos options for min MTU & co.

On an ASR920 in my network:
#sh  int tun 10050 | i MTU
  MTU 17916 bytes, BW 100 Kbit/sec, DLY 50000 usec,
  Path MTU Discovery, ager 10 mins, min MTU 92
  Tunnel transport MTU 9126 bytes
#

ping size 1500 df works. The other end is PPPoE (over a NNI that hopefully supports big enough frames)

> Besides this, I'm not sure if GRE is fully supported on the ME3600 anyway
> (as in "the forwarding hardware can utilize it", not in "the CPU knows
> how to tunnel a packet").  It's a switch...

That's the joker that needs to be discovered :)

-- 
R.-A. Feurdean