why we need mpls mtu command?

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Jul 2nd, 2008
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if interface mtu is not big enough to take mpls packet. we just increase interface mtu. why we need mpls mtu command.

if we just only increase mpls mtu.there is problem if mpls mtu biger than interface mtu. so it seem mpls mtu command is useless.

why we need mpls mtu command!

thank you!


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lee.reade Wed, 07/02/2008 - 06:32
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Its really for optimisation purposes, if you think that an a client might send traffic at 1500 bytes, the default MTU, then once you add you MPLS LDP tag of 4 bytes and MPLS VPN tag of 4 bytes, you have exceeded the MTU of the interface.

Therefore the router will fragment the packets into two separate packets, this is bad all round. So what you would do is specify an MPLS MTU of 1508 bytes (ldp tag & mpls vpn tag =8) so that MPLS packets are not fragmented. Once the packets get to the other end they are 1500 bytes and still within the default IP MTU of 1500 bytes, and will not be fragemented at that end either.



flyjunli Wed, 07/02/2008 - 18:54
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thank you!

i know this behavior, but i can just increase interface mtu to meet this requirement. why there is a new command :mpls mtu to achive this.

thank you!


vdadlaney Wed, 07/02/2008 - 19:36
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Hi Jun Li,

I am not quite sure why you mention that if the mpls mtu is bigger than the IP or Hardware mtu it is a problem? Can you please specify where you see this behaviour.

With regards to the various MTU's. You can solve the MTU issue by increasing the IP MTU or hardware/interface MTU however that is not recommended and the reason for that is because your IGP relies on IP MTU as well. If you change your IP MTU your IGP will flap and can cause an outage. Hence you just modify the MPLS Mtu which only affects MPLS labelled packets by modifying only the MPLS MTU. I hope that I was able to help. Someone please correct me if I am wrong. Thx

flyjunli Wed, 07/02/2008 - 19:54
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i test it in netowork,like this\

topology is simple

7609-1pe--sip GE spa----7609-2p--pos---7609-3p--flexwan E1-----7604-1pe--ge--ce

i config mpls mtu 1524 between 7609-1 and 7609-2 . and keep interface mtu 1500 default.

ping packet from 7609-1 to 7604-1 loopback 0.

ping 1500 byte packet is ok, but ping 1501 byte packet is totally lost.then i config mtu 1524 between 7604-1pe and 7609-3, it is useless,not work, i can't see packet coming from 7609-1 on 7604-1.

then i add config mtu 1524 between 7609-1pe and 7609-2. config mtu 1500 between 7604-1 and 7609-3,ping 1501 bytes from 7609-1 to 7604-1 loopback0 is ok. but i can see fragment from show ip traffic command in 7609-3.

i have a question, why we need mpls mtu command. if we don't change interface mtu,just only config mpls mtu 1524, it doesn't work, if we just change mpls mtu,how it work in the ios. if we config interface mtu 1524,interface mtu size is big enough, it seems mpls mtu command is useless, we don't need mpls mtu command, just increase interface mtu 1524 is ok.

why we need mpls mtu command. we just only change interface mtu is enough.

when i config mpls mtu override 1524,this is a warning in ios:

Setting the mpls mtu to 1524 on interface serial1/0/0:0, which is higher than the interface MTU

1500. This could lead to packet forwarding problems including packet drops.

You must set the MPLS MTU values equal to or lower than the interface MTU values.

thank you!


flyjunli Wed, 07/02/2008 - 19:59
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i am interesting on ospf issue for mtu change. If i config same mtu size on direct link 2 routers,is there any problem in ospf?

thank you!

jun li

Route_reflector Wed, 07/02/2008 - 19:38
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increasing the interface mtu may lead to get customer traffic as well to the maximum mtu (If customer facing interface also having higher than 1500 mtu), thereby again no enough space to add MPLS Label - and thus need fragmentation.

In this scenario, We need to keep actual traffic in 1500 MTU, but Labeled traffic to above that.

Hence either we can configure 'mpls mtu' or 'mtu adaptive' feature.

vdadlaney Thu, 07/03/2008 - 20:35
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If you configure the IP MTU to be the same size on both sides of the OSPF link than it should not be a problem. However if one side is set to be bigger than the other than you will run into a neighbor adjacency issue wheere the neighbor will be in Exstart/Exchange state. Here's a link for your reference


If your goal is to label switch traffic than please modify the MPLS Mtu. If you have only ip traffic than modify the IP Mtu.

When you ping from your 7609-1PE to 7604-1 are you using label switching or it just that you are pinging the loopback of either side. Note that unlabelled IP traffic will not be affected by the MPLS MTU. HTH. Thx

flyjunli Fri, 07/04/2008 - 17:08
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i use lable switch ping from 7609-1pe to 7604-1, i can see lable。

in document on cco , mpls mtu is better keep same size or small thank interface mtu.if mpls mtu more than interface mtu.

traffic may drop.

thank you!



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