MTU size in a MPLS network

MTU size never crossed my mind until MPLS designed network came to the picture. After adding a switch to the network, OSPF got up, but BGP was going up and down and up and down… That led to the MTU mismatch because bigger frames are needed in a MPLS network.

The default MTU frame size is 1500 bytes, but in a MPLS design where you use labels, with every label you need a little more room. For the one label design you need to increase the MTU size to 1504 bytes. MPLS VPN, which is the most common design, uses two labels and thus you have to increase the MTU size to 1508 bytes or by eight bytes. Each label increased the size of the original IP packet to a value bigger than the original MTU size can handle. In a default MTU size, configuration packets that are bigger get fragmented and when such packets are sent with the Do-Not-Fragment (DF) bit set, packets get dropped by the router.

MTU size is configured on the MPLS interface of the device but first you can check the current MTU size that is configured with the show mpls interfaces “interface_name” detail command. To change the size input next command:

1. configure terminal

2. interface “your mpls interface”

3. mpls mtu 1508 or the maximum of 9000 to be sure

Then check your configuration again with the show mpls interfaces “interface_name” detail command.

 

An Example:

london#show mpls interfaces fastEthernet 2/6 detail
Interface FastEthernet2/6:
        IP labeling enabled
        LSP Tunnel labeling not enabled
        BGP labeling not enabled
        MPLS not operational
        MTU = 1500
london#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
london(config)#interface FastEthernet2/6
london(config-if)#mpls mtu 1508
london(config-if)#^Z
london#
london#show mpls interfaces fastEthernet 2/6 detail
Interface FastEthernet2/6:
        IP labeling enabled
        LSP Tunnel labeling not enabled
        BGP labeling not enabled
        MPLS not operational
        MTU = 1508
 

Join the Conversation

1 Comment

  1. Just a side note…

    Cisco 3750 series switch supports jumbo frames on the Gigabit ports. The MTU can only be changed globaly but not ona an individual interface. Do not forget to reload the switch for the MTU to take affect.

    Here is an example:

    3750(config)#system mtu jumbo 9000

    Note: The default MTU size is 1500 bytes. If a value is entered that is outside the allowed range for the specific type of interface, the value is not accepted.

    Jumbo frame support is available in Cisco IOS Software Release 12.1(13)EW and later for the Supervisor III and IV. On the 4000/4500 platform, jumbo frames are supported only on non-blocking Gigabit Ethernet ports on certain blade models.

    These are the Gigabit modules and specific ports that support jumbo frames:

    Both supervisor uplink ports on Supervisor III (WS-X4013) and Supervisor IV (WS-X4014)
    WS-X4306-GB
    WS-X4232-GB-RJ, ports 1-2 only
    WS-X4418-GB, ports 1-2 only
    WS-X4412-2GB-TX, ports 13-14 only

    Configuration has to be made on each port for the feature to take affect.

    Read this for more INFO: https://supportforums.cisco.com/docs/DOC-2013

Leave a comment

Your email address will not be published. Required fields are marked *

Visit Us On LinkedinVisit Us On Google PlusCheck Our Feed