dot1q,isl and mtu issue

[sarahr202]

Hi everybody

How is everybody doing?

My book says when dot1q header in inserted in ethernet frame, it could result in larger mtu than allowed (MTU 1500 bytes).

What about when we use ISL? 

Does using isl ( 30 bytes) not result in larger mtu than allowed ?

Or

When switch receives a frame encapsulated in isl, it hands it over to isl which removes the encapsulation and hands it over to ethernet layer. So ethernet layer does not know anything about isl encapsulation b/c it has been removed; so no mtu issue arise.

thanks and have a nice weekend.

[Peter Paluch · Accepted solution]

Hi Sarah,

My understanding is that if a device supports ISL, it also has to support frames that are oversized by the regular Ethernet standard. True, the ISL will add 30 bytes of overhead to each encapsulated frame, resulting in frames growing up to 1548 bytes, but the devices supporting ISL must be prepared for that. In this sense, MTU issues should not arise at all - devices supporting ISL should be capable of handling oversized frames, and devices using plain Ethernet framing do not see any difference. Connecting an ISL-configured port to an Ethernet port is not going to produce useful results.

Best regards,

Peter

[Mohamed Sobair · Accepted solution]

Hello Sarah,

I totally agree with Peter Here. A device Supporting ISL should be capable of handling a larger frame size So no MTU issue arise here.

MTU issue arises when we add dot1q tag to the original Ethernet Frame, and Practicaly Speaking from my experience, we need to increase the MTU Size whenever we have QinQ tunneling in place resulting in total MTU of 1504.

Practicaly speaking, no issue arises as well with normal dot1q tag with the original Ethernet Header of 1518. So the 1500 Byte MTU is sufficient to handle it.

Regards,

Mohamed

[sarahr202]

Hi Mohammed.

"Practicaly speaking, no issue arises as well with normal dot1q tag with  the original Ethernet Header of 1518. So the 1500 Byte MTU is sufficient  to handle it."

"MTU issue arises when we add dot1q tag to the original Ethernet Frame,  and Practicaly Speaking from my experience, we need to increase the MTU  Size whenever we have QinQ tunneling in place resulting in total MTU of  1504."

I am confused. Please break it down for my small brain

thanks.

[Reza Sharifi]

Hi Sarah,

Please have a look at this document.  It does a pretty good job of breaking it down and explaining QinQ, ISL, 802.1q,tagging etc...

http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094665.shtml

Have a noce weekend

HTH

[Mohamed Sobair]

Thank Reza for the Link.

SARAH,

In a simple describtion:

An MTU of 1500 is sufficient to handle and ethernet frame with its header included. ((An MTU of 1500 is taking the consideration of ethernet frame and header in its calculation)).

Whenever we have a situation where we use dot1q tunneling, we need to increase the layer-2 MTU by 4 byte for the additional Vlan Tag ,therfore the MTU should be 1504.

Thanks and Have a nice Weekend.

Regards,

Mohamed