[c-nsp] MTU over L2TPv3 tunnel

[alex at alsn.be]

Can you tell me the models that support a higher MTU on a Fa port? I don't
seem to find that information on the cisco website.


Thanks

Alex

> alex at alsn.be <> wrote on Thursday, February 28, 2008 10:44 AM:
>
>> Hey,
>>
>> I have a problem that i just can't resolve. I hope someone will be
>> able to help.
>>
>> Here's the situation. I have 2 client switches (Catalyst 2400) each
>> connected on a separate border router (7200). Between the router is a
>> L2TPv3 tunnel.
>>
>> When i test the connection (RFC 2544) i get the result i expect
>> +/-90Mb/s as long as the packet size is smaller then 1518. Once the
>> packet size is 1518 the cpu of one of the two routers gets to 100%
>> and the throughput lowers to +/-30Mb/s after about 30 seconds.
>>
>> I tested the maximum MTU with a ping and found out it's at 1472.
>>
>> I tried to change the ip mtu and mss to correct values (even tried
>> lower values) but it didn't help. When i sniff the connection i
>> noticed the mss value isn't the value i put one the interface. The
>> lowest value i have seen is 1460 (wich should be allright).
>
> Well, you need to increase the IP MTU on the core/uplink interface to
> avoid fragmentation (as you've already assumed). You might fail doing so
> as you seem to be using an I/O FE port which doesn't support a higher
> MTU, but on the NPE-G1 side, you should be able to my increasing "mtu"
> on the main interface and setting "ip mtu" on the subinterfaces (if you
> want them lower than the main MTU).
> enabling PMTU within the PW-class sets the DF-bit, so packets are
> dropped if they need to be fragmented.
>
> 	oli
>