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

Jack Bates jbates at brightok.net
Sat Nov 6 15:56:13 CDT 2010

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.


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