[c-nsp] Adjusting MTU on 802.1q links

[David Hubbard]

From: Phil Mayers
> 
> On 03/12/10 13:49, Matthew Huff wrote:
> > I don't know why it never occurred to me, but on 802.1q trunk links,
> > non-native vlans are encapsulated within 802.1q headers, therefore
> > max packets would have to be fragmented. On trunks that support it,
> > should standard practice to bump up the mtu on both sides to account
> > for the 802.1q header.
> 
> No. 802.1q trunks do this automatically i.e. bump MTU from 
> 1518 to 1522 
> to account for the extra space. I've never seen a switch 
> platform that 
> needed any special config for this to work.

Would that include a jumbo frame environment?  I'm currently
trying to troubleshoot a performance issue of a Cisco UCS
vmware cluster to EMC CX4 storage array using iscsi and 9000
byte MTU.  I've got MTU set to 9000 on the EMC, UCS and the
vmware side, and both sides are also tagging into a 4900M
they both connect to for the storage vlan; all vlan's and
interfaces on the 4900M are set to 9000 as well.  I figure
since it's all tagged end to end and all 9000 that I'm good
either way but just thought I'd check when I saw this thread.

David