[c-nsp] MTU wierdness

[Michael Robson]

> Michael,
>
> Check:
> http://www.cisco.com/en/US/partner/docs/switches/lan/catalyst6500/ios/12
> .2SX/configuration/guide/intrface.html#wp1041111
>
> http://www.cisco.com/en/US/partner/docs/ios/interface/command/reference/
> ir_l2.html#wp1030775
> http://www.cisco.com/en/US/partner/docs/ios/fundamentals/command/referen
> ce/cf_s3.html#wp1019645
>
> I think it should be in there.
>
I couldn't get to any of these, even taking into account the wrapped  
lines.


> 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



> 	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.


I finally sorted this out: If I was setting the MTU on a routed  
interface, then I could set the MTU to anything up to 9216B (using the  
mtu interface command), however, if I was trying to set the MTU an a  
switchported interface, then the mtu command would only allow me to  
change the MTU to the value defined in the global "system jumbmtu"  
command - this is a feature not a bug.


Thanks,

Michael
-- 

Michael Robson	| Tel: +44 (0) 161 275 6113
Networks		| Fax: +44 (0) 161 275 6120
Net North West	| Email: Michael.Robson at manchester.ac.uk