[c-nsp] gre tunnel mtu mismatch

[Gert Doering]

Hi,

On Wed, Oct 03, 2018 at 03:46:30PM -0600, Raymond Burkholder wrote:
> On 2018-10-03 3:42 p.m., Mike wrote:
> > I have an ME3600 and an ASR920. Im trying to run ospf over a gre 
> > tunnel and having issues because OSPF seems to see a different tunnel 
> > mtu on either end of the link.
> 
> on other cisco gear I've worked, there has been a parameter in ospf to 
> ignore mtu, I believe.

Yes, but this will cause huge problems later on.  If one side assumes
it can stuff an 8kbyte OSPF update packet down a link, which then gets
fragmented by intermediate IP gear (fragmenting of the outer layer IP
packet) and possibly not reassembled correctly on the receiving side
(ASR920 and fragmented GRE?  See my doubting face...), while the other
side expect no more than 1476 - this won't end well.

The OSPF MTU check is particularily there to avoid "this will blow up
in your face later" situations.

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
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 630 bytes
Desc: not available
URL: <https://puck.nether.net/pipermail/cisco-nsp/attachments/20181004/0b41f4a8/attachment.sig>