layer 2 protocol tunnelling and mtu setting

[rathinilesh]

Hi All,

I am trying to get my head around layer 2 protocol tunnelling. I assume this can be configured using switchport access mode, switchport dot1q tunnel mode and AToM. In what cases would we need to increase the system mtu of the service provider switches?

Regards,

Nilesh

[gfcisco31]

Hello mate.

When dealing with dot1q tunnels you need to increase the system MTU to at least 1504 bytes.  This is because the switch will use extra 4 bytes to carry the METRO vlan , used to tunnel the traffic.

So, all system mtu in the service provider side needs to be increased to 1504.

hope this helps

[rathinilesh]

Hi Gustavo,

Thanks for replying. Would we need to increase the mtu in all cases or are there some exceptions when we do not need to increase the system mtu?  Would we need to increase the mtu when we do layer 2 protocol tunnelling using switchport access mode? Please also refer to the below link.

http://ieoc.com/forums/t/14647.aspx

Regards,

Nilesh

[gfcisco31]

HI mate...

We''ll do tuneling whetever we need to carry 2 vlans at the same time.

In the example you showed, they guy is doing port channel (layer3) - So he used just a simple tunnel to carry the traffic, no need for do1tq tunnel in this case.

cheers

[smehrnia]

Hi Nilesh,

you need to increase SYSTEM MTU to 1504, starting at the point (on the device) where you apply the dot1q tunnel command and along the way (providers path), because where the tunneling happenes and carried, there is an extra vlan id appended to the original one. double VLAN ID's only affect layer 2 not Layer 3

HTH,

Soroush.

[gfcisco31]

Please, rate useful comments

[rathinilesh]

guys thanks for your help...

[smehrnia]

anytime! i agree with Gustavo

Soroush.