changing mtu-size in MPLS networks

[pverstegen]

In what circumstances should I change the default mtu-size of tag-switching interfaces.

[pverstegen]

Martin,

Two more question:

1> In case I want to do Ethernet over MPLS on a GigE sub-interface with mtu 9216 and tag-switching mtu 9242 (+26 bytes for AToM) do I need to change all interfaces (and tag-switching mtu sizes) of all tag-switching interfaces in the MPLS core in between the two sides as well ?

2> I guess the extra 4 bytes for dot1q will be stripped when entering the MPLS core. Is that correct ?

Regards,

Peter

[mheusinger]

Hello Peter,

A1) are you sure the GE interfaces do support anything larger 9216 Byte? You should first check your hardware capabilities with respect to MTU. This needs to be done on every interface the labeled packets are possibly traversing. From the minimum MTU along the path you can then calculate the supported MTU for your customer interfaces and their frames.

Q2) I do not quite understand the question: are you refering to EoMPLS transporting customer VLANs (like with the C7200)?

Hope this helps! Please rate all posts.

Regards, Martin

[pverstegen]

Martin,

You are right. The GE interface supports max. 9216. So the labeled packet can not exceed this. I assume this means the biggest jumbo the customer can sent is 9190 bytes (9216 - 26). Am I right ?

But I do have to configure every tag-switching GigE interface for mtu 9216 (and tag-switching mtu 9216), not only the two AToM interfaces. Correct ?

Q2) I do not quite understand the question: are you refering to EoMPLS transporting customer VLANs (like with the C7200)? YES

2> I guess the extra 4 bytes for dot1q will be stripped when entering the MPLS core. Correct ?

[mheusinger]

Peter,

I would recommend to lower the customer MTU even further to have some flexibility. What, if you need to implement MPLS TE adding another label? What, if you want to implement interfaces only supporting 4470 Byte MTU? Then your network again might experience MTU issues or you are severely restricted in redesigns. Also be aware that in any case the ingress and egress PE interface for a given EoMPLS VC has to have the same MTU. And not every GE interface supports 9216 Byte MTU afaik.

A2) As you would have a lower MTU for the customer anyhow, I would not assume this to be a problem in either case. In fact I think it is sent, but I am not sure.

Regards, Martin