Fragmentation Problem In L2MPLS

Unanswered Question
May 12th, 2009


In case of l3 mpls CE packets without DF bit gets fragmented but in case of l2mpls fragmentation never occurs. Whats the reason for this?


shivlu jain

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Giuseppe Larosa Tue, 05/12/2009 - 08:50

Hello Shivlu,

>> but in case of l2mpls fragmentation never occurs. Whats the reason for this?


neither the L2 carried PDU neither the carrying MPLS PDU provide a fragmentation mechanism.

L2 MPLS services require MTU tuning on the data path the MPLS MTU has to be big enough to host the carried L2 PDU and all the MPLS stack.

For making a comparison if you put an 1518 ethernet frame inside an L2TPv3 PDU it will be fragmented in travelling in an IP only network with IP MTU 1500 bytes.

But in this case the IP fragmentation services are used.

Hope to help


Harold Ritter Tue, 05/12/2009 - 12:53


The reason is that the fragmentation taking place for L3VPN is IP fragmentation. Core performs IP fragmentation on the packet and forwards the fragments using the egress label or label stack.

On the other hand, this can't be done for L2VPN as the packet payload is a L2 protocol (Ethernet, PPP, ATM, HDLC, etc).


shivlu jain Tue, 05/12/2009 - 21:01


I am unable to understand why ip fragmentation comes in L3VPN. In case of L3VPN the incoming and outgoing label are used for forwarding.

If we check the l2mpls case, the only difference is that the transport header is attached in it.


shivlu jain

Harold Ritter Wed, 05/13/2009 - 03:12


If fragmentation is needed for L3VPN, we simply take the payload (IP packet), we fragment it and forward it using the normal label (or label stack). This is a simple procedure and is based on existing standards.

For L2VPN on the other hand, we would need to start looking deeper in the payload to determined if the protocol transported by the L2 protocol is IP, fragment the IP packets, modify existing L2 header and create new ones. This procedure would be fairly complicated and is not standardized.



This Discussion