Specifications

mpls mtu
MP-273
Cisco IOS Multiprotocol Label Switching Command Reference
January 2010
Usage Guidelines Usage Guidelines for Cisco IOS Release 12.2(25)S
Caution Although you can set the MPLS MTU to a value greater than the interface MTU, set the MPLS MTU
less than or equal to the interface MTU to prevent data corruption, dropped packets, and high CPU
conditions. A best practice is to set the interface MTU of the core-facing interface to a value greater than
either the IP MTU or interface MTU of the edge-facing interface.
If the interface MTU is less than 1524 bytes, you can set the maximum MPLS MTU to 24 bytes more
than the interface MTU. For example, if the interface MTU is set to 1510 bytes, then you can set the
maximum MPLS MTU to 1534 bytes (1510 + 24).
If the interface MTU is greater than or equal to 1524 bytes, then you can set the maximum MPLS MTU
a
s high as the interface MTU. For example, if the interface MTU is set to 1600 bytes, then you can set
the MPLS MTU to a maximum of 1600 bytes. If you set the MPLS MTU higher than the interface MTU,
traffic is dropped.
For interfaces that do not allow you to configure the interface MTU value and the interface MTU is
1500 bytes, the MPLS MTU range is from 64 to 1524 bytes.
If you upgrade to Cisco IOS Release 12.2(25)S from an earlier release and you have an MPLS MTU
set
ting that does not conform to these guidelines, the system does not accept the MPLS MTU setting. If
this happens, reconfigure the MPLS MTU setting to conform to the guidelines.
Usage Guidelines for Cisco IOS Release 12.2(27)SBC, 12.2(33)SRA, 12.4(11)T, 12.2(33)SXH, and Later Releases
In Cisco IOS Release 12.2(27)SBC, 12.2(33)SRA, 12.4(11)T, 12.2(33)SXH, and later releases, you
cannot set the MPLS MTU value larger than the interface MTU value. This is to prevent conditions such
as dropped packets, data corruption, and high CPU rates.
• If you attempt to set the MPLS MTU value higher than the interface MTU value, the software
displays the following error, which reminds you to set the interface MTU to a higher value before
you set the MPLS MTU value:
% Please increase interface mtu to xxxx and then set mpls mtu
• If you have an interface with a default interface MTU value of 1580 or less (such as an Ethernet
interface), the mpls mtu command provides the override keyword, which allows you to set the
MPLS MTU value higher than the interface MTU value. The override keyword is not available for
interface types that do not have a default interface MTU value of 1580 or less.
Note The override keyword is supported in 12.2(27)SBC, 12.2(33)SRA, 12.4(11)T, 12.2(33)SXH,
and later releases.
• If you have configuration files with MPLS MTU values that are larger than the interface MTU values
and you upgrade to Cisco IOS Release 2.2(27)SBC, 12.2(33)SRA, 12.4(11)T, 12.2(33)SXH, or a
later release, the software does not change the MPLS MTU value. When you reboot the router, the
software accepts whatever values are set for the MPLS MTU and the interface MTU. The following
error message is displayed during system initialization:
Setting the mpls mtu to xxxx on interface x/x, which is higher than the interface MTU
xxxx. This could lead to packet forwarding problems including packet drops.
Set the MPLS MTU values lower than the interface MTU values.