Unanswered Question
May 23rd, 2010

Dear Gurus,

Can you pls clear me up on these, does every hop uses 16 bytes for labelling? Also on point to point routers, mpls mtu is not applicable since i can ping more than what is set on the mtu.


from R1:

ping R2 size 1600 is ok

ping R3 size up to 1530 only

ping R4 size up to 1530 only


ping R1 size 1600 is ok

ping R2 size 1600 is ok

ping R4 size up to 1530 only

With this, does it mean p2p mpls router does not use the mtu. For the whole network, only 16 bytes is used for labels. pls clear me on these.



I have this problem too.
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Nagendra Kumar ... Mon, 05/24/2010 - 05:15

Hi Jeff,

Even with P2P link, MTU comes into picture. When there is a need for label imposition, each label will consume 4 bytes. Depending on what MPLS application you are running, number of labels may vary. For example, plain MPLS will need 1 label to be imposed while L3VPN needs 2 labels.

Consider the below topology,


  1500                 1500              

On the above, When no mpls is enabled between R2 and R3, if you send packet of size 1500 bytes with DF bit from R1, it will work fine. With 1501 and DF bit, it will fail.

When MPLS is enabled between R2 and R3 (if u test the same setup, enable explicit-null on R3), packet of size 1496 and DF bit will work fine. With 1497, it will fail.

So the maximum IP packet size that can be sent over an MPLS interface without fragmentation depends on what is the MTU of the interface and the number of labels to be imposed.

If the above doesnt answer your question, Please capture the related config and the output and let us know.



Chetan Kumar Ress Mon, 05/24/2010 - 08:33

Hi Jeffer

It is depending on your senario that whether MTU come in P2P picture.

For example if you have below senario then MTU will not come in picture because there is no other device which modifies the packet.


In Below Senario the MTU will not come in Picture because, As packet leave R1 there will be no modification.

But in below senario you can see :

R1 ---- Metro Ethernet Switch  ----  Metro Ethernet Switch  ---- R2

As Service Provider use metro ethernet switch & it will modify the L2 packet & add more Ethernet Header & as it will cross the Default MTU then It will start fragmantation.

& As it will reach to PE then there will be MPLS Lable that will add to your packet & There definetly MTU comes in picture.


Chetan Kumar

Olivier ARRIGHI Mon, 05/24/2010 - 11:26

A few things to keep in the back of your mind :

MPLS MTU applies only to MPLS packet, so only packets with tags

IP MTU applies to IP paquets only.

You could configure 2 values at interface level for IP and MPLS packets.

Also you may have some Penultimate hop popping in your scenario, meaning the downstream edge router might receive just an IP packet in case of a basic MPLS backbone, or a packet with 1 label only in case of L3VPN ( because of implicit null label advertisement.).

You might also add some labels in case of traffic engineering, meaning one or more label in your core network (with FRR for example).

As it is 4 bytes per label, make sure you count them all in worst case scenario

Have fun


jeffersoncbriones Mon, 05/24/2010 - 19:45

helo Guys,

This will be the complete setup, Layer 2 EoMPLS vlan based. Also there are no P routers, all PE in a sense.


I already set the mpls mtu to 1546, but for the interface mtu still 1500. From R1 when i ping R2 no problem with mtu, for R1 ping R3 i can only use 1530, so therefore 16 bytes is used? Also what do i expect when i put my cpe and use EoMplS vlan based, another 4 bytes will be added?

I hope this clears my query.


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