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

[Ben Steele]

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
>>
>>
>>
>
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