VLAN MTU setting vs system mtu and system mtu jumbo

Unanswered Question
Jun 19th, 2008

On a Catalyst 2960G switch I configured

system mtu jumbo 9000

but only today I discovered that the VLAN that should allow jumbo frames had 1500 as its mtu setting all the time! However there has been no trouble in passing jumbo frames over this vlan.

So my question: what does the vlan mtu setting precisely do?

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
aghaznavi Wed, 06/25/2008 - 12:04

You can issue the "vlan vlan-id mtu 2018 'command to increase the maximum transmission unit (mtu) size upto 2018 bytes, so that the switch does not drop the large frames.

donghoon1 Wed, 06/25/2008 - 23:28

Actually, whether I issue commands

vlan ... mtu 1500

vlan ... mtu 2018

vlan ... mtu 7200

vlan ... mtu 9000

the switch's behavior seems to be unaffected. Only the command "system jumbo mtu ..." affects whether large frames are passed or not.

So my question is: can anyone confirm that the "vlan ... mtu ..." is obsolete on the 2960G?

Edison Ortiz Wed, 06/25/2008 - 14:11

2960G is a layer2 switch so packets aren't actually going via the SVI.

Packets would only traverse via the SVI if you had a Layer3 switch.

Your packet flow is strictly Layer2 so you are only using the physical ports on the data flow.




Please rate helpful posts

donghoon1 Wed, 06/25/2008 - 23:31

Sorry Edison, I don't understand the point of your reply. The VLAN tag is part of the ethernet header right?

Edison Ortiz Thu, 06/26/2008 - 05:35

When you verify the Vlan isn't configure for a large MTU size, where exactly are you looking?

If you are looking in the interface Vlan, then you are looking at a Layer3 interface.

The 2960G does not use the Layer3 interface Vlan for passing traffic in behalf the users that are connected to it.

It passes the traffic directly via the physical ports (Layer2).

It has nothing to do with the Vlan tag.



Pravin Phadte Thu, 06/26/2008 - 06:10


Correct me if i am wrong. Both the setting are differ from each other.

system mtu jumbo 9000 :- sets the mtu for the physical interface on the switch.

Configuring MTU for Vlans may mismatch with the mtu set for the ports.

As per my oberavtions you can set system mtu for all the interfaces as jumbo. Depending on the output for the vlan mtu whith the higest size set the mtu for all vlans with the same.

Observe the output for the commands:

show system mtu

sh vlan mtu

Hope this helps.




This Discussion