MTU size of VLANs

[rasmus.elmholt]

Hi

I have just change the MTU size on all switches in my lab to 1550(mtu, jumbo, routing) and when I do a ping sweep I get to transfer 1550B packets.

But when i look at show vlan, the vlan MTU is still 1500. If I then do a show vlan mtu, the MTU is 1550.

So the MTU used on the system is 1550 but why is it showing 1500 in the "show vlan" command?.

Take a look at VLAN 1000 in the attached config dump from a 3560G but the output is the same for a 2960X and a 3560CX

Does anyone have an explanation?

[Reza Sharifi]

Hi,

Is this the case for only vlan 1000 or the same for all other vlans as well?

HTH

[rasmus.elmholt]

This is the same for all VLANs i have only shown VLAN 1000 to shorten the output.

[Reza Sharifi]

I think the issue is that when you issue "show vlan id 1000" it shows you just the layer-2 vlan which is 1500 vs "show vlan mtu" shows you the svi interface mtu which is 1550. Not sure if the layer-2 vlan interface can be changed to anything else but default (1500).

HTH

[rasmus.elmholt]

As fare as i can se the show vlan mtu shows both the L2 MTU and the L3 MTU(SVI_MTU)
I can change the MTU shown in the show vlan by changing the mtu under vlan(vlan 1000 mtu 1550) But the VLAN MTU have no effect on the switch.
Right now it is configured for the default 1500 on the VLAN but I am able to transmit 1550 through.

[Reza Sharifi]

Yes, I think the config change under the layer-2 vlan has no effect. The change will take place for all svis when you change it system wide and the same for physical interfaces again when you change it system wide (for the whole switch).

HTH