[c-nsp] gre tunnel mtu mismatch

[Gert Doering]

Hi,

On Wed, Oct 03, 2018 at 02:42:22PM -0700, Mike wrote:
> On the me3600, the transport MTU is 8976 bytes, while on the ASR920 it's 
> 1476. The me3600 has most of it's interfaces with a 9216 bytes, while on 
> the asr920 the single internet facing interface has a 1500byte mtu.
> 
> 
> 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
> 
> 
>      This is silly. I have no choice in the matter; the me3600 has a big 
> mtu and the asr920 has a smaller one, and no way to influence the 
> selection it seems. Surely there needs to be some way to connect 
> disparate equipment over a gre tunnel?

Well, reasonable equipment supports user-settable MTU... does "mtu 1476"
work?

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...

gert
-- 
"If was one thing all people took for granted, was conviction that if you 
 feed honest figures into a computer, honest figures come out. Never doubted 
 it myself till I met a computer with a sense of humor."
                             Robert A. Heinlein, The Moon is a Harsh Mistress

Gert Doering - Munich, Germany                             gert at greenie.muc.de
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