Jumbo frames and VLAN MTU

[jedavis]

I have found this question posted a couple of times, but I have yet to see it answered.

On a 3750/3560 to enable jumbo frames on gigabit interfaces I enter the global config command "system mtu jumbo 9000".

Switch#sho system mtu

System MTU size is 1998 bytes

System Jumbo MTU size is 9000 bytes

System Alternate MTU size is 1998 bytes

Routing MTU size is 1500 bytes

Ok, all is well and good here.  But what is the effect, if any, of the MTU setting on the VLAN (NOT the SVI) configuration.  In other words,

Switch(config)#vlan 22

Switch(config-vlan)#mtu 9000

Switch#sho vlan mtu

VLAN    SVI_MTU    MinMTU(port)      MaxMTU(port)     MTU_Mismatch

---- ------------- ----------------  ---------------  ------------

1    1998          9000              9000              No

22     -           1998 (Fa0/9    )  9000 (Gi0/1    )  Yes

Ok, but now what happens if I set the VLAN MTU to, say... 500?

Switch(config-vlan)#mtu 576

VLAN    SVI_MTU    MinMTU(port)      MaxMTU(port)     MTU_Mismatch

---- ------------- ----------------  ---------------  ------------

1    1998          9000              9000              No

22     -           1998 (Fa0/9    )  9000 (Gi0/1    )  Yes

Nothing seems to happen as far as the VLAN display goes.  A "show interface" for g0/9 also shows an MTU of 1998.

What, then, does the VLAN MTU setting actually do?

TIA - Jeff

[matt.millington]

Jeff,

The vlan mtu setting has no effect on intra-vlan packets, but rather on inter-vlan packets. So, if the VLAN mtu was set to the default 1500, but it received a packet from another VLAN destined for VLAN 22 in your example, it would not pass it because the SVI (interface vlan22) would not view it as a valid packet size.

If you were to do a sho int vlan22, you would see the VLAN mtu of 9000. Its confusing because it seems that you're making a VLAN setting in the VLAN database, but the effect is at the VLAN's SVI, not on each port that is a member of the VLAN. In the 3750, you can only set the MTU size for switch ports globally for the whole switch. You can't set only ports in a certain VLAN, and you can't do it on a per-port basis. I believe you need the 6500 for that.

HTH,

Matt