Csico 3640 => MTU/ISIS/MPLS

Discussion in 'Cisco' started by Mag, Feb 8, 2009.

  1. Mag

    Mag Guest

    Hi

    i want change the MTU of my MPLS interface on a PE cisco 3640.

    mpls mtu 1548

    and same on the P router

    but after this, ISIS don't see any equipement ...

    anyone know why ?

    thanks
     
    Mag, Feb 8, 2009
    #1
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  2. Mag

    Thrill5 Guest

    "Mag" <> wrote in message
    news:498ee697$0$27493$...
    > Hi
    >
    > i want change the MTU of my MPLS interface on a PE cisco 3640.
    >
    > mpls mtu 1548
    >
    > and same on the P router
    >
    > but after this, ISIS don't see any equipement ...
    >
    > anyone know why ?
    >
    > thanks


    On an Etherenet or Fast Ethernet interface the maximum MTU size is 1500.
    The maximum MPLS MTU size is 1476 because of the additional IP and MPLS
    header (20 byte IP header and 4-byte MPLS header.) If this is a serial
    interface you can use larger MTU sizes, but it must be set to the same value
    on BOTH sides. If one side is set to 1500 and the othe is set ot 1548, the
    side with an MTU of 1500 will drop any packet that it receives that is
    larger than that value.
     
    Thrill5, Feb 8, 2009
    #2
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  3. Mag

    Guest

    On Feb 8, 5:05 pm, Mag <> wrote:

    > i want change the MTU of my MPLS interface on a PE cisco 3640.
    >
    > mpls mtu 1548
    >
    > and same on the P router
    >
    > but after this, ISIS don't see any equipement ...


    OSPF has a special interface configuration command

    ip ospf mtu-ignore

    without it OSPF doesn't work if mtu of one device doesn't match mtu on
    other devices. I guess IS-IS might have something similar.

    But mpls mtu 1548 is not enough. You need your hardware to support
    lager packets too. And here we come to the bad thing: FastEthernet
    interfaces on 3640 doesn't allow packets to be larger than 1500 bytes.
     
    , Mar 5, 2009
    #3
  4. Mag

    Guest

    On Mar 5, 2:14 pm, wrote:

    > > i want change the MTU of my MPLS interface on a PE cisco 3640.
    > > mpls mtu 1548
    > > and same on the P router
    > > but after this, ISIS don't see any equipement ...

    >
    > OSPF has a special interface configuration command
    >
    > ip ospf mtu-ignore
    >
    > without it OSPF doesn't work if mtu of one device doesn't match mtu on
    > other devices. I guess IS-IS might have something similar.
    >
    > But mpls mtu 1548 is not enough. You need your hardware to support
    > lager packets too. And here we come to the bad thing: FastEthernet
    > interfaces on 3640 doesn't allow packets to be larger than 1500 bytes.


    Good news:

    mpls mtu N

    automatically increases interface mtu. Just make sure, your switch
    supports large mtu.
     
    , Mar 5, 2009
    #4
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