[c-nsp] MTU wierdness

[Jeff Fitzwater]

	Once you define the L2 MTU, packets on that VLAN can traverse any  
ports on that VLAN up to that MTU, but if you need to route them and  
retain the L2 MTU then the L3 SVI must have the same MTU.  You can  
have the SVI different, say 1500, as long as you understand that the  
packets will be fragged if larger than 1500, or dropped if the DF bit  
is set.   If you have defined an SVI to a 9k+ MTU, that will force the  
L2 interfaces on that vlan to be the same since they must carry that  
size packets.


Well its sounds good anyway, but nobody knows everything ;~)



Jeff Fitzwater
OIT Networking & Communications Systems
Princeton University

On Jul 24, 2009, at 9:08 AM, Aaron Millisor wrote:

> It is likely that you have configured an SVI or a VLAN on the 6509  
> for 9216 already.
>
> If any VLAN that crosses the switchport is 9216, then you can't  
> adjust the MTU of the port to a value below 9216.
>
> Do a 'show vlan' and also check all the SVI's for an MTU higher than  
> 1504, then either reduce the MTU in those locations or I think you  
> could also restrict the large VLAN from being sent on the trunk
>
> --
> Aaron Millisor
>
>
>
> Michael Robson wrote:
>> I have a 6509 (with Sup720-3B) that contains 2 x WS-X6704-10GE  
>> blades where I am trying to set the MTU to be 1504 on each of these  
>> interfaces. On one blade it will only allow me to set the MTU to  
>> 9216 if the interface is a switchport, the 1504 MTU size only  
>> becomes an option when it is changed to a routed port. Since this  
>> is not the case on other 6509s we have, anyone have an idea why  
>> this might be happening (it maybe worth noting that, at present,  
>> one of the other ports is a routed port with MTU of 9216)?
>> Thanks,
>> Michael
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