[c-nsp] MPLS MTU / Jumbo frames etc.

[Brandon Applegate]

On Wed, 22 Jul 2009, Gert Doering wrote:

> Hi,
>
> On Wed, Jul 22, 2009 at 02:16:29PM -0400, Brandon Applegate wrote:
>> Scenario 3 really gets me though.  Why doesnt it complain and tell me icmp
>> frag to 9212 or something ?  Isnt the frame 9220 when it's all said and
>> done ?  Is the router fragmenting this in software at the 'mpls level' and
>> just not telling me ?  Should I set mtu down to 9212 or something to make
>> sure that the router NEVER frags frames ?
>
> I'd bet that the linux box is not sending full-sized 9220 packets, but
> fragmenting inside.

I'm sending full 9216 packets.  Confirmed with tcpdump as I'm sending. 
The 9220 number is what the frame looks like after 1 MPLS label.  Hence my 
confusion as to how scenario 3 is working without icmp unreachables etc 
(ala scenario 1).

>
> Unless the linux box has 10GE to the router, and is allowed to use full
> 9220 MTU (via ifconfig and/or "ip route"), it will send 1500 byte fragments.

Yes I have my MTU cranked up in linux and am doing all of this 
intentionally as a test.  Unless tcpdump is lying to me, these are 
unfragmented 9216-byte frames leaving and coming back with no 'complaints' 
in sceneraio 3.

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Brandon Applegate - CCIE 10273
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