MPLS MRU

Unanswered Question
Mar 22nd, 2012

Hello Experts,

I am facing problem in understanding MRU for mpls.

R3-----------------R2------------------R1

All links are configured with 1500 MTU. The prefix in question is 202.123.47.1 configured on loopback at R1.

Where there is PoP label the MRU is 1504 and where there is Swap the MRU=1500 which is equal to MTU.

SO is it means R2 telline R3 that if you have packet or destination 202.123.47.1 i can handle of size 1504....i don't know what i am saying

but i am confused. Also is it advertised between routers ?

At R1 the output is:

R1#sh mpls forwarding-table 202.123.47.1 32 detail

Local  Outgoing      Prefix            Bytes Label   Outgoing   Next Hop

Label  Label or VC   or Tunnel Id      Switched      interface

None   No Label      202.123.47.1/32   0             aggr-punt

        MAC/Encaps=0/0, MRU=0, Label Stack{}

        No output feature configured

At R2 the output is:

R2#sh mpls forwarding-table 202.123.47.1 32 detail

Local  Outgoing      Prefix            Bytes Label   Outgoing   Next Hop

Label  Label or VC   or Tunnel Id      Switched      interface

17     Pop Label     202.123.47.1/32   0             Gi1/0      192.168.14.1

        MAC/Encaps=14/14, MRU=1504, Label Stack{}

        CA000E40001CCA010E40001C8847

        No output feature configured

At R3 the output is:

R3#sh mpls forwarding-table 202.123.47.1 32 det

Local  Outgoing      Prefix            Bytes Label   Outgoing   Next Hop

Label  Label or VC   or Tunnel Id      Switched      interface

18     17            202.123.47.1/32   0             Gi1/0      192.168.14.5

        MAC/Encaps=14/18, MRU=1500, Label Stack{17}

        CA010E400038CA0203A4001C8847 00011000

        No output feature configured

Thanks in advance.

Regards # Mahesh

I have this problem too.
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kishore.chennupati Fri, 04/13/2012 - 20:41

hi mahesh,

I am not an expert but I will try and answer your query. As you might know MRU is the maximum receive unit and this is the packet size that the router can forward unfragmented. and also MRU is per FEC

In your case. between R2 and R1, there is a implicit null label. So, when a packet is destined for R1 from R3, then R2 will pop that label and forward it to R1 unlabeled. So, R2 has an MRU of 1504 to get the labeled packet from R3 and it pops that label and sends it to R1.

if you had a MPLS VPN then R2 will have an MRU of 1508. In case of R3 the MRU and the MTU are the same. as it just forwards an IP packet and not a labeled packet

Does this make sense?

HTH

Kishore

Mahesh Gohil Fri, 04/20/2012 - 00:35

Hi Kishore,

Thanks for your reply!

R2 perspective it is clear to me now: R2 expecting 1504 packet size for pop label but R3 is little bit confusing to me.

why R3 forward just ip packet and not mpls packet?

Regards # Mahesh

kishore.chennupati Sat, 04/21/2012 - 06:35

hi Mahesh,

R3 wil send a labeled packet and not an ip packet .R2 will advertise its label for the prefix to R3.

R3#sh mpls forwarding-table 202.123.47.1 32 det

Local  Outgoing      Prefix            Bytes Label   Outgoing   Next Hop

Label  Label or VC   or Tunnel Id      Switched      interface

18    17            202.123.47.1/32   0             Gi1/0      192.168.14.5

        MAC/Encaps=14/18, MRU=1500, Label Stack{17}

        CA010E400038CA0203A4001C8847 00011000

        No output feature configured

As you can see R2 sends label 17 to R3 and R3 uses that to forward traffic to R2

Hope this helps

Kishore

Mahesh Gohil Tue, 04/24/2012 - 00:38

OK!

so let me put my question again. Why MRU is 1500 in case of R3

Regards # Mahesh

Ivan Krimmel Tue, 04/24/2012 - 03:57

Mahesh,

MRU value from "show mpls forwarding-table detail" shows the MPLS MTU

size(L3 information) which is the maximum size of IP + MPLS Labels. It informs the LSR how big a received labeled packet of a certain FEC can be that can still be forwarded out of this LSR without fragmenting it.

So in your case:

R1 has got MRU = 0, it is a connected prefix so it is some sort of special case

R2, being in the middle and doing swap, has got MRU = 1504, meaning it can forward 1500 bytes as payload plus one label on top. It actually will get label 17 and then do a pop.

R3 has got MRU = 1500, because it'll need to push one label on 1500 bytes of payload, as instructed by it's upstream LSR - R2.

I hope this is clear now.

Regards,

Ivan.

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