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Configuring the MTU for Layer 2 Interfaces

By default, the MTU used to advertise a Layer 2 pseudowire is determined by taking the interface MTU for the associated physical interface and subtracting the encapsulation overhead for sending IP packets based on the encapsulation. However, encapsulations that support multiple logical interfaces (and multiple Layer 2 pseudowires) rely on the same interface MTU (since they are all associated with the same physical interface). This can prove to be a limitation for VLAN Layer 2 pseudowires using the same Ethernet interface or for Layer 2 pseudowire DLCIs using the same Frame Relay interface.

This can also affect multivendor environments. For example, if you have three PE devices supplied by different vendors and one of the devices only supports an MTU of 1500, even if the other devices support larger MTUs you must configure the MTU as 1500 (the smallest MTU of the three PE devices).

You can explicitly configure which MTU is advertised for a Layer 2 pseudowire, even if the Layer 2 pseudowire is sharing a physical interface with other Layer pseudowires. When you explicitly configure an MTU for a Layer 2 pseudowire, be aware of the following:

  • For BGP-based applications such as l2vpn, the advertised MTU will be zero unless an MTU value is explicitly set at the [edit routing-instances routing-instance-name protocols (l2vpn ) site site-name] hierarchy level.

  • An explicitly configured MTU is signaled to the remote PE device. The configured MTU is also compared to the MTU received from the remote PE device. If there is a conflict, the Layer 2 pseudowire is taken down.

  • If you configure an MTU for an ATM cell relay interface on an ATM II PIC, the configured MTU is used to compute the cell bundle size advertised for that Layer 2 pseudowire, instead of the default interface MTU.

  • A configured MTU is used only in the control plane. It is not enforced in the data plane. You need to ensure that the CE device for a given Layer 2 pseudowire uses the correct MTU for data transmission.

The following procedure describes how to configure the MTU for the Layer 2 interface. This information applies to the following Layer 2 technologies:

  • Layer 2 VPNs

  • Layer 2 Circuits

  1. To configure the MTU for a Layer 2 circuit, include the mtu statement:

    For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.

  2. To allow a Layer 2 pseudowire to be established even though the MTU configured on the local PE router does not match the MTU configured on the remote PE router, include the ignore-mtu-mismatch statement:

    For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.

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