vlan database mtu versus system/interface mtu

[hakan.topcu]

Hi, I'm currently dealing with jumbo frame configurations, and I'm not sure about the function of the mtu size value under the vlan configuration or database mode? I have found only a short statement on cisco.com: "… With the VLAN database setting, the switch checks for an MTU value greater than 1500. If the switch detects a higher value, puts the VLAN in a non-operational state. Therefore, to support large frames, you only need to change the interface MTU value and not the VLAN database MTU value..." (http://www.cisco.com/en/US/products/hw/switches/ps700/products_configuration_example09186a008010edab.shtml)

For me, this sounds confusing. Because to support jumbo frames, I configure the MTU only at global system mtu, and/or at the interface level (depending on switch model). This results in a higher value than the vlan mtu. Can anyone please explain the function of the vlan mtu and the relationship to the mtu of L2 and L3 (SVI) ports?

Thanks & regards

Hakan

[Giuseppe Larosa]

Hello Hakan,

the vlan mtu given in vlan database mode is for sure the value that is sent inside VTP messages when advetising the vlan.

it should also provide a per vlan mtu to be used for L2 traffic.

>> Can anyone please explain the function of the vlan mtu and the relationship to the mtu of L2 and L3 (SVI) ports?

good question.

mtu ?

<576-18190> Value of VLAN Maximum Tranmission Unit

we know that SVI vlan object is a distinct object from layer2 broadcast domain.

system mtu has a global effect.

mtu at interface is clearly local effect.

so what happens if we leave all parameters unchanged and we change only vlan mtu?

let's suppose we have increased only for vlan 10

if on a trunk port a frame of 2000 bytes is received the system should look at the vlan-id value before deciding if the frame size is acceptable or not.

this doesn't look like efficient.

also the interface needs at low level to be configured to reorganize its packet buffers to be able to handle bigger frames.

Hope to help

Giuseppe

[hakan.topcu]

Hello,

thank you Guiseppe for your reply. But honestly, the function of the vlan mtu is furthermore not clearly for me. Cisco states also the following regarding vlan config mode: "mtu = Specifies the maximum transmission unit (packet size, in bytes) that the VLAN can use; valid values are from 576 to 18190."

(http://www.cisco.com/en/US/docs/ios/12_3t/lanswitch/command/reference/lan_v1gt.html).

"... that the VLAN can use ..."!?!? that's what's confusing me still. If I raise the MTU on the interfaces in a VLAN, I can communicate with frames that are bigger than the VLAN MTU is configured for. This means, the VLAN can use it.

So, once again a new question: Against what is the VLAN MTU compared, and in which situation?

Thanks in advance. Hakan.