Configuring a Label Switching MTU

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Router(config-if)# mpls mtu bytes

• Label switching increases the maximum MTU requirements on an interface, because of additional label header.

• Interface MTU is automatically increased on WAN interfaces; IP MTU is automatically decreased on LAN interfaces.

• Label-switching MTU can be increased on LAN interfaces (resulting in jumbo frames) to prevent IP fragmentation.

• The jumbo frames are not supported by all LAN switches.

• 2003, Cisco Systems, Inc. All right* reserved. MPLS v2.0-3-9

mpls mtu

To set the per-interface MTU for labeled packets, use the mpls mtu interface configuration command:

Syntax Description




The minimum MTU is 128 bytes. The maximum depends on the type of interface medium.

Configuring Label-Switching MTU (Cont.) Example

One way of preventing labeled packets from exceeding the maximum size (and being fragmented as a result) is to increase the MTU size of labeled packets for all segments in the label switched path (LSP) tunnel. The problem will typically occur on LAN switches, where it is more likely that a device does not support oversized packets (also called jumbo frames or, sometimes, giants or baby giants). Some devices support jumbo frames, and some need to be configured to support them.

The MPLS MTU size is increased automatically on WAN interfaces and needs to be increased manually on LAN interfaces.

The MPLS MTU size has to be increased on all LSRs attached to a LAN segment. Additionally, the LAN switches used to implement switched LAN segments need to be configured to support jumbo frames. No additional configuration is necessary for shared LAN segments implemented with hubs.

A different approach is needed if a LAN switch does not support jumbo frames. The problem may be even worse for networks that do not allow ICMP MTU discovery messages to be forwarded to sources of packets and if the Don't Fragment (DF) bit is strictly used. This situation can be encountered where firewalls are used.

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