Q in Q MUT size?

As we know , when deploying Q in Q in the network , it may increasing the Ethernet frame size , assume that the L3 packet (L2 payload) is 1500 byte , after adding L2 information (18byte ) , VLAN tag , metro tag. , we actually get a 1526 byte frame . ( I am not sure for this )

My question is what system MTU size should I config?

here is a cisco document :


I confused about the "system MTU"  ....

Thanks in advance!


  • Hi

    System MTU can be used to increase the MTU size for interfaces at 10/100 (applicable in ME3400, ME3400E, ME3750 series for example). For Gigabit interfaces MTU is increased using the system mtu jumbo command (again having the same series of switches as example). In routed interfaces the MTU is increased with system mtu routing command. For the afforementioned series of switches all commands are applied in global configuration mode and its not possible to configure indivudual settings on ports.Other series of switches such as Catalyst 4948 provide up to 32 (for L2 and L3 types) different values for MTU systemwide. On Catalyst 4948 the system mtu sets the global system baby giant support for certain interfaces (allowing up to 1552 bytes ethernet payload) while permiting per interface settings (eg on TenGig types) for specific MTU values. Per interface mtu values take precedence over global command settings. The MTU values vary on different catalyst switches but in general the maximum allowable system MTU is up to 1998 bytes for Fastethernet types and up to 9000 for Gigabitethernet.

    p { margin-bottom: 0.08in; }

    Because the 802.1Q
    tunneling feature increases the frame size by 4 bytes when the metro
    tag is added, you must configure all switches in the service-provider
    network to be able to process maximum frames by increasing the switch
    system MTU size to at least 1504 bytes.

    Check also the following link:






  • Thanks for your information ![:D]

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