RINA - scott whaps at the nanog hornets nest :-)

Dan White dwhite at olp.net
Sat Nov 6 17:10:21 CDT 2010

On 06/11/10 15:56 -0500, Jack Bates wrote:
>On 11/6/2010 3:36 PM, Richard A Steenbergen wrote:
>>#2. The major vendors can't even agree on how they represent MTU sizes,
>>so entering the same # into routers from two different vendors can
>>easily result in incompatible MTUs. For example, on Juniper when you
>>type "mtu 9192", this is INCLUSIVE of the L2 header, but on Cisco the
>>opposite is true. So to make a Cisco talk to a Juniper that is
>>configured 9192, you would have to configure mtu 9178. Except it's not
>>even that simple, because now if you start adding vlan tagging the L2
>>header size is growing. If you now configure vlan tagging on the
>>interface, you've got to make the Cisco side 9174 to match the Juniper's
>>9192. And if you configure flexible-vlan-tagging so you can support
>>q-in-q, you've now got to configure to Cisco side for 9170.
>I agree with the rest, but actually, I've found that juniper has a 
>manual physical mtu with a separate logical mtu available, while 
>cisco sets a logical mtu and autocalculates the physical mtu (or 
>perhaps the physical is just hard set to maximum). It depends on the 
>equipment in cisco, though. L3 and L2 interfaces treat mtu 
>differently, especially noticeable when doing q-in-q on default 
>switches without adjusting the mtu. Also noticeable in mtu setting 
>methods on a c7600(l2 vs l3 methods)
>In practice, i think you can actually pop the physical mtu on the 
>juniper much higher than necessary, so long as you set the family 
>based logical mtu's at the appropriate value.

Cisco calls this 'routing mtu' and 'jumbo mtu' on the platform we have to
distinguish between layer 3 mtu (where packets which exceed that size get
fragmented) and layer 2 mtu (where frames that exceed that size get dropped
on the floor as 'giants').

We always set layer 2 mtu as high as we can on our switches (9000+), and
strictly leave everything else (layer 3) at 1500 bytes. In my experience,
setting two hosts to differing layer 3 MTUs will lead to fragmentation at
some point along the routing path or within one of the hosts.

With Path MTU Discovery moved to the end hosts in v6, the concept of a
standardized MTU should go away, and open up much larger MTUs. However,
that may not happen until dual stacked v4/v6 goes away.

Dan White

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