Re: Tunnel Interface MTU issues

Setting IP MTU has no effect on interface MTU i.e at layer 2 it will still fragment to the following values:

Ethernet 1500
Serial 1500
ATM 4470 (Cisco)
FDDI 4470
HSSI (HSA) 4470
Token Ring 4464

Setting L2 interface MTU does have an effect on IP MTU in that it automatically matches it.

IMO setting a higher MTU should be done at L2 thus anything at layer 3 will be matched, this should also reduce CPU cycles with less bursty cycles needed to recompute L2 fragments, example would be Jumbo Ethernet but the bottle neck would be any WAN links which fragment back to 1500 bytes, best to be done on P2P links if software supports it.

--
BR
Tony
Sent from my iPad
> On 17 Jan 2014, at 14:07, marco207p_at_gmail.com wrote:
> 
> 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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