MTU on FE/GigE trunk ports

Answered Question
Dec 5th, 2003
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Is setting the 'mpls mtu' sufficient for increasing the MTU size for tagged packets or do I need to modify the MTU on the master interface also ? We are using trunking (dot1q) as well.


Also, once I've set the mpls MTU, will I need to go to all the ethernet switches and configure jumbo frame support ?


What the max MTU size I should set ?


Thanks for any help.


= K

Correct Answer by Harold Ritter about 13 years 8 months ago

Because of the penultimate hop popping behavior the IGP lable would most likely be an implicit null (unless you change the default behavior) leaving you with only your service label (assuming that you are doing MPLS VPN).

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Harold Ritter Fri, 12/05/2003 - 08:20
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From the router perspective, setting the "mpls mtu" is all you need.


You need to make sure the switch is configured to support baby giants. The following URL can help you with that task.


http://www.cisco.com/en/US/products/hw/switches/ps700/products_configuration_example09186a008010edab.shtml


Tha maximum MTU you should set is dependent on what type of MPLS application you are running (number of labels involved). It is safe though to set it to 1524, which would basically allow you to hold 6 labels (more than you really need bu won't hurt).


Hope this helps,


kschafer Fri, 12/05/2003 - 08:33
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Thanks for the note.


Now it turns out that I've got an issue with my Cat4000 switches not being able to support baby-giants. Argh.


I may be able to use the suggested work-around by enabling trunking to allow for an extra 4 bytes. (1 label)


If all my MPLS tag-switched interfaces are in the same subnet, it's likely 1 label is all I'm going to need right ?


= K

Correct Answer
Harold Ritter Fri, 12/05/2003 - 09:46
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  • Cisco Employee,

Because of the penultimate hop popping behavior the IGP lable would most likely be an implicit null (unless you change the default behavior) leaving you with only your service label (assuming that you are doing MPLS VPN).

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