[c-nsp] System MTU on trunks for Q in Q

[Dan Armstrong]

Unfortunately, we're using ME3400s, and you cannot specify MTU on a per 
interface basis... 



Ben Steele wrote:
> Your better off just running system mtu 1504(if you want to deliver 
> QinQ to customers) and then specifying the larger mtu frames on your 
> trunk interfaces, this still restricts your customer access ports to 
> 1504 while allowing you to run what you need, jumbo frame mtu on an 
> interface will override the system "baby jumbo" mtu command.
>
> On 27/03/2008, at 12:12 PM, Dan Armstrong wrote:
>
>> The reason I don't want to raise it too high - is if we're selling TLS
>> services to a customer, (ie a VLAN provisioned on 2 ports on different
>> switches, carried across our core/trunks) - I don't want them being able
>> to send any packet larger than 1500 byes.
>>
>> A bit bigger wouldn't be a problem, but if I set it to, say 9000, and
>> all of a sudden we have some jackass with a storage head that could be
>> firing 9000 byte packet across our backbone... not cool, I believe it
>> would cause havoc with "small packet" applications like VoIP, even with
>> QoS, the bit-time to send a 9000 byte packet out an interface is
>> significant.
>>
>> I've also never been too clear on the interaction between the system mtu
>> command, and the system mtu jumbo command.  I've always just made them
>> match...
>>
>>
>>
>>
>>
>> Peter Rathlev wrote:
>>> On Wed, 2008-03-26 at 19:01 -0400, Dan Armstrong wrote:
>>>
>>>> I've been bashing my head against the wall all day for a definitive
>>>> answer on this:
>>>>
>>>> On a Cisco switch that supports QinQ (3550, 3750, ME3400, 3560 etc)
>>>>
>>>>
>>>> What is the _minimum_ value I need to set the system MTU to, to do
>>>> QinQ?  1504?  1522?  1526?  1546?
>>>>
>>>> I can't seem to find one concise answer...
>>>>
>>>
>>> I'm not entirely sure what the "system mtu" specifies, i.e. if it's
>>> interface MTUs (typically excluding data link headers) or what.
>>>
>>> IPv4 packet = 1500 bytes. Ethernet header = 14 bytes. 802.1q header = 4
>>> bytes. Another one = 4 bytes more. So for "simple" QinQ of "regular"
>>> IPv4 traffic it would be max 1522 bytes of data per packets.
>>>
>>> Any special reason not to just raise it to the maximum of 2000 bytes?
>>>
>>> Regards,
>>> Peter
>>>
>>>
>>>
>>
>> _______________________________________________
>> cisco-nsp mailing list  cisco-nsp at puck.nether.net
>> https://puck.nether.net/mailman/listinfo/cisco-nsp
>> archive at http://puck.nether.net/pipermail/cisco-nsp/
>