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MPLS VPN: size of datagram???????

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  • MPLS VPN: size of datagram???????

    Hi all,
    Imagine the following VPN scenario:


    CE----PE---P---P---PE----CE

    I have FastEthernet in all the links, and tag-switching mtu 1508 at the PEs and Ps interfaces.


    Due to the label stacking(2 labels) we have additional 8 bytes in the MTU size. So, at the CPE the maximum datagram size I can send is 1500 (What I did was pings with 1500 and with the DF=1).

    My question is:
    Why pings fail with datagram size of 1504 from PE to PE?

    From what I understand there will be only one label (4 bytes) pushed in the packet, therefore the mtu size will be 1508...
    Thanks in advance,
    Cuộc đời như gió như ngàn cơn sóng, khi buồn khi vui.

  • #2
    1500 byte ping packet + 18 bytes of standard ethernet header + 4 bytes of MPLS label = 1522 <> 1508

    Try to change tag-switching mtu to 1522 to see if it helps.
    Last edited by ptran; 10-12-2006, 03:03 AM.
    Trần Nhân Hòa
    CMND #0146257668

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