Unanswered Question
Jun 3rd, 2010


I have a stack of two 3750G, with two lyr2 VLANs configured: one for data (VLAN 100) the second for iSCSI traffic (VLAN20).

I know I can set the global mtu to 9000 (system mtu jumbo 9000), but read that you may also set mtu per VLAN, through the vlan database system:

Switch# vlan database

Switch(vlan)# vlan 20 mtu 9000

Switch(vlan)# exit

I'd like to keep the config as clean as I can, and if jumbo frames are only needed for the iSCSI VLAN, I'd rather have it done that way. However,

after setting up the VLAN mtu (and reloading the switches), I did a show int on a iSCSI interface (switchport access vlan 20), and it only shows MTU as 1500.....


CORE#sh int g1/0/2

GigabitEthernet1/0/2 is up, line protocol is up (connected)

  Hardware is Gigabit Ethernet, address is 0013.60a8.cb02 (bia 0013.60a8.cb02)

  MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,

     reliability 255/255, txload 1/255, rxload 1/255


same thing with "sh vlan mtu":


CORE# sh vlan mtu

VLAN    SVI_MTU    MinMTU(port)      MaxMTU(port)     MTU_Mismatch

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

1    1500          1500              1500              No

20     -           1500              1500              No

100    -           1500              1500              No

but sh vlan id 20:
CORE#sh vlan id 20

VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------
20   iSCSI                            active    Gi1/0/1, Gi1/0/2, Gi1/0/3, Gi1/0/4, Gi1/0/5, Gi1/0/6, Gi1/0/7
                                                Gi1/0/8, Gi2/0/1, Gi2/0/2, Gi2/0/3, Gi2/0/4, Gi2/0/5, Gi2/0/6
                                                Gi2/0/7, Gi2/0/8

VLAN Type  SAID       MTU   Parent RingNo BridgeNo Stp  BrdgMode Trans1 Trans2
---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------
20   enet  100020     9000  -      -      -        -    -        0      0
So... which is it? I'm not sure what to "trust" at this point. I don't think setting the global config for jumbo MTU will harm anything (no routing protocols on these switches), but I'm not 100% sure, and as I mentioned before, if I don;t need it - I don;t want it....
If anyone can shed some light on this, it'll be much appreciated!
I have this problem too.
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Network Engineers Thu, 06/03/2010 - 13:11

Had to set MTU globally on the switch.... the per VLAN didn't work. I hope to see per-interface MTU in future IOS releases.


Giuseppe Larosa Thu, 06/03/2010 - 13:39

Hello D,

your test remembered me  of an older thread

you can set interface MTU with mtu command in interface mode

you can set system wide MTU in global config with system mtu

setting the MTU under vlan environment as your test shows has an effect only on the fields of VTP advertisement of vlan 20.

When VTP advertises a new vlan an MTU field is present together with a vlan name and vlan number

At those times I had doubts about the effect of the command under vlan context, but now that I see your sh vlan id 20 I think I've understood this

Hope to help


Network Engineers Thu, 06/03/2010 - 13:48

Hi Giuseppe,

Thx for your reply. I saw that setting the MTU for VLAN will be carried over VTP, but what I don't see it having any effect, or the point of it at all..

>>you can set interface MTU with mtu command in interface mode<< -- not on 3750, to the best of my knowledge.



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