[c-nsp] asr 920 - lower mpls mtu?

Mike mike-cisconsplist at tiedyenetworks.com
Sun Jun 19 09:39:51 EDT 2016

On 06/19/2016 05:56 AM, Lukas Tribus wrote:
> Hi Mike,
>> Trying to set up an EoMPLS tunnel, the mtu allowed for
>> 'l2 vfi somename manual' is a bit short.. only 9180 bytes as
>> opposed to 9216 for all the rest of my me3600's for example.
>> asr920(config-vfi)#mtu ?
>>     <1500-9180>  MTU size in bytes
>>     <cr>
>> I am trying to figure out why this is... am I being silly and this is
>> too high and I should select a lower global default like 9100 or ?
> I would consider setting this value to something lower, a value that is likely supported by future platforms as well, with a bit of margin. Otherwise you would have reconfigure the VPLS instance or the pseudowires each time you have a platform not supporting the MTU. The ASR920 has a lower MTU than the ME3600, the c6840x platform has an even lower MTU (9154) for example.

The strategy I was going for until now was just max mtu, which happened 
to be 9216. This is the same as discussed in cisco support forums in 
this thread:


  So, the question now is, what IS a likely baseline then? In my network 
I honestly don't think I will see much more than 1500 payload + vlan + 
mpls, but the day may come when something else comes along like a 
customer with a 9000 byte mtu requirement for their iscsi? Maybe 9100 is 
a good number?

> Slightly related: the maximum MTU on an ASR920 interface seems to be 9216, however it appears that packets get drop if higher than 9214 (ip ping to the next-hop IP, non-labeled). Anyone seen this already? If its another limitation I didn't find any documentation about it and it certainly breaks Path MTU discovery/IP fragmentation because the box thinks it can handle 9216 while it can only handle 9214.
I have just tried this (ASR920 <-> ME3600) and don't seem to have that 
problem, just FYI.

Thank you for the response.


More information about the cisco-nsp mailing list