Re: Tunnel Interface MTU issues

Dave,

There a few things you can try:
* using the command, tcp adjust-mss 1460 "or which ever mtu you find that
works"
* using PMTUD "Path MTU Discovery" this however requires that you have ICMP
reachability from end to end, you will also need to allow ICMP packets to the
senders of the traffic, as the router will respond to the sender requesting
them to modify their sending size.

This link will explain it:
http://www.cisco.com/en/US/tech/tk827/tk369/technologies_white_paper09186a008
00d6979.shtml#t9

Link to commands:
http://www.cisco.com/en/US/docs/ios/12_2/ipaddr/command/reference/1rfip2.html
#

This link gives you examples of PMTUD:
http://www.cisco.com/en/US/docs/ios/12_2/ipaddr/command/reference/1rfip2.html
#wp1101164

General Good to Read information on PMTUD;
http://www.cisco.com/en/US/tech/tk870/tk877/tk880/technologies_tech_note09186
a008011a218.shtml

Best Regards,
Joe Sanchez

> On Jan 16, 2014, at 5:24 PM, Dave Serra <maybeedave_at_yahoo.com> wrote:
>
> Hey Guys,
>
> I am having MTU issues on a GRE tunnel. Users are sending packets
> with the df bit set and they are being dropped by the Tun0 interface. When
> doing pings I can see that I can only send packets out if they are 1476
(1477
> packets are dropped). I am desperately trying to figure out how to raise
the
> "Tunnel transport MTU" from 1476 but it does nothing with the command "ip
mtu
> 1500" on the tunnel interace. Can someone please help?
>
>
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Received on Fri Jan 17 2014 - 08:07:34 ART