MTU - MPLS DS3 Setup Question

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Feb 13th, 2008
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I'm not sure if I am in the correct section.

We have an MPLS network with DS3s. From what I understand (and correct me if I am wrong), MPLS puts 4 bytes on the packet. Our serial has an MTU size of 4470. Our Ethernet has an MTU size of 1500.

Do I need to change the MTU size, and if so on which interface do I need to make this change. My thinking is to change it on the serial interface (MTU => 4466).

Am I right in my assumption?

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arun kumar Wed, 02/13/2008 - 21:03
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MPLS will add 4 bytes extra to the plain IP packet. Aplications of MPLS (MPLS-VPN, AToM, MPLS TE, MPLS QoS) will add stacks of labels to IP packet. So if you are running MPLS applications on your network then your Layer 2 MTU of the physical media should be capable of handling these extra stack of labels.

In your case since DS3 is having MTU of 4470 and most data packet comes from Ethernet, it is capable of carrying MPLS Labelled packets. You need to increase the MPLS MTU of your Ethernet media = Physical MTU + No. of Label stacks (depend on applications).. It is advisable to configure the MPLS MTU on ethernet interface to 1546 (1500 bytes + 46 bytes extra ...though it is more for basic MPLS, MPLS VPN considering the future development it will be better to keep the MPLS MTU to a higher value).

Note: MPLS MTU command is applicable only for the labelled packets. If any unlabelled packet with MTU more than of 1500 bytes is entering Ethernet interface then it ll get fragmented or dropped (depends on DF bit flag in IP header)

int ethernet0

mpls mtu 1546


dphills18 Fri, 02/15/2008 - 07:20
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Thanks, I am going to give this a try. We are having issues with a database query across the MPLS cloud, and I can't seem to put my finger on the issue. The query works fine on the LAN, but not via the MPLS cloud.

mmelbourne Wed, 02/20/2008 - 05:28
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Is it recommended to have an higher MPLS MTU (e.g. 1546) configured on a interface with a fixed lower interface MTU, e.g. 1500?

Will MPLS packets greater than 1500 bytes traverse the interface, given that MPLS packets cannot be fragmented?

kpobst Fri, 02/15/2008 - 18:57
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You need to add the bytes to your mtu. It is also recommended that have all your mtu settings match. We have all our interfaces(DS3 and Eth) set to 1546.

dphills18 Wed, 02/20/2008 - 07:04
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Hmm, that's interesting. Take the DS3 down to 1546. I may give it a try. Do you know if the interface drops when the MTU is set. Also, are you referring to the interface MTU or the MPLS MTU on every interface.

One more thing. We do not run MPLS internally (only once it hits the cloud). Does the 1546 apply to our internal Ethernets in this case.


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