Cisco IOS Multiprotocol Label Switching Configuration Guide, Release 12.2SR
MPLS MTU Command Changes
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Table Of Contents

MPLS MTU Command Changes

Contents

Information About MPLS MTU Command Changes

MPLS MTU Values During Upgrade

Guidelines for Setting MPLS MTU and Interface MTU Values

MPLS MTU Values for Ethernet Interfaces

How to Configure MPLS MTU Values

Setting the Interface MTU and MPLS MTU Values

Setting the MPLS MTU Value on an Ethernet Interface

Configuration Examples for Setting the MPLS MTU Values

Setting the Interface MTU and MPLS MTU: Example

Setting the MPLS MTU Value on an Ethernet Interface: Example

Feature Information for MPLS MTU Command Changes


MPLS MTU Command Changes

First Published: August 11, 2004
Last Updated: December 8, 2009

This document explains the change in behavior of the mpls mtu command for the following Cisco IOS releases:

•12.2(27)SBC and later

•12.2(33)SRA and later

•12.4(11)T and later

•12.2(33)SXH and later

You cannot set the MPLS MTU value larger than the interface MTU value. This eliminates problems, such as dropped packets, data corruption, and high CPU rates from occurring when MPLS MTU value settings are larger than interface MTU values. Cisco IOS software allows the MPLS MTU value to be higher than the interface MTU value only for interfaces that have a default interface MTU value of 1580 or less.

Note In Cisco IOS Release 15.0(1)M1, you can configure the interface MTU on PA-1FE and PA-2FE port adapters (PAs). The range of values is 1500 -1530. Before this enhancement, the MTU of those interfaces was not configurable. When you attempted to configure the interface MTU, the following message was displayed:
% Interface {Interface Name} does not support user settable mtu.

Finding Feature Information in This Module

Your Cisco IOS software release may not support all of the features documented in this module. To reach links to specific feature documentation in this module and to see a list of the releases in which each feature is supported, use the "Feature Information for MPLS MTU Command Changes" section.

Finding Support Information for Platforms and Cisco IOS Software Images

Use Cisco Feature Navigator to find information about platform support and Cisco IOS software image support. Access Cisco Feature Navigator at http://www.cisco.com/go/fn. You must have an account on Cisco.com. If you do not have an account or have forgotten your username or password, click Cancel at the login dialog box and follow the instructions that appear.

Contents

•Information About MPLS MTU Command Changes

•How to Configure MPLS MTU Values

•Configuration Examples for Setting the MPLS MTU Values

•Feature Information for MPLS MTU Command Changes

Information About MPLS MTU Command Changes

Before configuring the interface or MPLS MTU values, you should understand the following concepts:

•MPLS MTU Values During Upgrade

•Guidelines for Setting MPLS MTU and Interface MTU Values

•MPLS MTU Values for Ethernet Interfaces

MPLS MTU Values During Upgrade

If you have configuration files with MPLS MTU values that are larger than the interface MTU values and you upgrade to Cisco IOS Release 12.2(27)SBC, 12.2(33)SRA, 12.4(11)T, 12.2(33)SXH or later releases, the software does not change the MPLS MTU value. When you reboot the router, the software accepts the values that 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. 


You must set the MPLS MTU values equal to or lower than the interface MTU values.
Caution If you do not set the MPLS MTU less than or equal to the interface MTU, data corruption, dropped packets, and high CPU conditions can occur.

Guidelines for Setting MPLS MTU and Interface MTU Values

When configuring the network to use MPLS, set the core-facing interface MTU values greater than the edge-facing interface MTU values, using one of the following methods:

•Set the interface MTU values on the core-facing interfaces to a higher value than the interface MTU values on the customer-facing interfaces to accommodate any packet labels, such as MPLS labels, that an interface might encounter. Make sure that the interface MTUs on the remote end interfaces have the same interface MTU values. The interface MTU values on both ends of the link must match.

•Set the interface MTU values on the customer-facing interfaces to a lower value than the interface MTU on the core-facing interfaces to accommodate any packet labels, such as MPLS labels, than an interface might encounter. When you set the interface MTU on the edge interfaces, ensure that the interface MTUs on the remote end interfaces have the same values. The interface MTU values on both ends of the link must match.

Changing the interface MTU can also modify the IP MTU, Connectionless Network Service (CLNS) MTU, and other MTU values, because they depend on the value of the interface MTU. The Open Shortest Path First (OSPF) routing protocol requires that the IP MTU values match on both ends of the link. Similarly, the Intermediate System-to-Intermediate System (IS-IS) routing protocol requires that the CLNS MTU values match on both ends of the link. If the values on both ends of the link do not match, IS-IS or OSPF cannot complete its initialization.

If the configuration of the adjacent router does not include the mpls mtu and mtu commands, add these commands to the router.

Note The MPLS MTU setting is displayed only in the show running-config output if the MPLS MTU value is different from the interface MTU value. If the values match, only the interface MTU value is displayed.

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

Note In Cisco IOS Release 15.0(1)M1, you can configure the interface MTU on PA-1FE and PA-2FE port adapters. The range of values is 1500 -1530. Before this enhancement, the MTU of those interfaces was not configurable.

MPLS MTU Values for Ethernet Interfaces

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 an 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. For configuration details, see the "Setting the MPLS MTU Value on an Ethernet Interface" section.

Setting the MPLS MTU value higher than the Ethernet interface MTU value can lead to dropped packets, data corruption, or high CPU rates. When you set the MPLS MTU value higher than the Ethernet interface MTU value, the software displays the following message: 

%MFI-30-MPLS_MTU_SET: Setting the mpls mtu to xxxx on Ethernet x/x, which is higher than 
the interface MTU xxxx. This could lead to packet forwarding problems including packet 
drops. 

Most drivers will be able to support baby giants and will gracefully drop packets that are 
too large. Certain drivers will have packet forwarding problems including data corruption. 

Setting the mpls mtu higher than the interface mtu can lead to packet forwarding problems 
and may be blocked in a future release.

Note The override keyword is supported in Cisco IOS Release 12.2(27)SBC, 12.2(33)SRA, 12.4(11)T, 12.2(33)SXH, and later releases, but may not be supported in a future release.

How to Configure MPLS MTU Values

The following sections explain how to configure MPLS MTU and interface MTU values:

•Setting the Interface MTU and MPLS MTU Values

•Setting the MPLS MTU Value on an Ethernet Interface

Setting the Interface MTU and MPLS MTU Values

Use the following steps to set the interface MTU and the MPLS MTU.

Note In Cisco IOS Release 15.0(1)M1, you can configure the interface MTU on PA-1FE and PA-2FE port adapters. The range of values is 1500 -1530.Before this enhancement, the MTU of those interfaces was not configurable.

SUMMARY STEPS

1. enable

2. configure terminal

3. interface type slot/port

4. mtu bytes

5. mpls mtu bytes

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

interface type slot/port

Example:

Router(config)# interface Serial 1/0

Enters interface configuration mode to configure the interface.

Step 4 

mtu bytes

Example:

Router(config-if) mtu 1520

Sets the interface MTU size.

Step 5 

mpls mtu bytes

Example:

Router(config-if) mpls mtu 1520

Sets the MPLS MTU to match the interface MTU.

Setting the MPLS MTU Value on an Ethernet Interface

Use the following steps to set the MPLS MTU value on an Ethernet interface.

SUMMARY STEPS

1. enable

2. configure terminal

3. interface type slot/port

4. mpls mtu override bytes

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

interface type slot/port

Example:

Router(config)# interface ethernet 1/0

Enters interface configuration mode to configure the Ethernet interface.

Step 4 

mpls mtu override bytes

Example:

Router(config-if) mpls mtu override 1510

Sets the MPLS MTU to a value higher than the interface MTU value.

Caution Setting the MPLS MTU value higher than the Ethernet interface MTU value can lead to dropped packets, data corruption, or high CPU rates.

Configuration Examples for Setting the MPLS MTU Values

This section includes the following examples:

•Setting the Interface MTU and MPLS MTU: Example

•Setting the MPLS MTU Value on an Ethernet Interface: Example

Setting the Interface MTU and MPLS MTU: Example

The following example shows how to set the interface and MPLS MTU values. The serial interface in the following configuration examples is at the default interface MTU value. The MPLS MTU value is not set. The interface settings are as follows: 

interface Serial 4/0

 ip unnumbered Loopback0

 mpls traffic-eng tunnels

 mpls ip

 serial restart-delay 0

 ip rsvp bandwidth 2000 2000

end

The following example attempts to set the MPLS MTU value to 1520. This returns an error, because the MPLS MTU value cannot be set to a greater value than the interface MTU. 

Router# configure terminal


Enter configuration commands, one per line.  End with CNTL/Z.

Router(config)# interface serial 4/0


Router(config-if)# mpls mtu 1520


% Please increase interface mtu to 1520 and then set mpls mtu

The following example first sets the interface MTU to 1520 and then sets the MPLS MTU to 1520: 

Router(config-if)# mtu 1520

Router(config-if)# mpls mtu 1520

The following example shows the new interface MTU value. The MPLS MTU value is not displayed, because it is equal to the interface value. 

Router# show running-config interface serial 4/0


Building configuration...

interface Serial4/0

 mtu 1520

 ip unnumbered Loopback0

 mpls traffic-eng tunnels

 mpls ip

 serial restart-delay 0

 ip rsvp bandwidth 2000 2000

end

The following example sets the MPLS MTU value to 1510: 

Router(config-if)# mpls mtu 1510

The following example shows the new interface MTU value. The MPLS MTU value is displayed, because it is different than the interface MTU value. 

Router# show running-config interface serial 4/0


Building configuration...

interface Serial4/0

 mtu 1520

 ip unnumbered Loopback0

 mpls mtu 1510

 mpls traffic-eng tunnels

 mpls ip

 serial restart-delay 0

 ip rsvp bandwidth 2000 2000

end

Setting the MPLS MTU Value on an Ethernet Interface: Example

Caution Setting the MPLS MTU value higher than the Ethernet interface MTU value can lead to dropped packets, data corruption, or high CPU rates.

The following example shows how to set the MPLS MTU values on an Ethernet interface. The Ethernet interface in the following configuration examples is at the default interface MTU value. The MPLS MTU value is not set. The interface settings are as follows: 

interface Ethernet 2/0

 ip unnumbered Loopback0

 mpls traffic-eng tunnels

 mpls ip

 serial restart-delay 0

 ip rsvp bandwidth 2000 2000

end

The following example uses the override keyword to set the MPLS MTU to 1520, which is higher than the Ethernet interface's MTU value: 

Router(config-if)# mpls mtu override 1520


%MFI-30-MPLS_MTU_SET: Setting the mpls mtu to 1520 on Ethernet2/0, which is higher than 
the interface MTU 1500. This could lead to packet forwarding problems including packet 
drops.

The following example shows the new MPLS MTU value: 

Router# show running-config interface ethernet 2/0


Building configuration...

interface Ethernet 2/0

 mtu 1500

 ip unnumbered Loopback0

 mpls mtu 1520

 mpls traffic-eng tunnels

 mpls ip

 serial restart-delay 0

 ip rsvp bandwidth 2000 2000

end

Feature Information for MPLS MTU Command Changes

Table 1 lists the release history for this feature.

Not all commands may be available in your Cisco IOS software release. For release information about a specific command, see the command reference documentation.

Cisco IOS software images are specific to a Cisco IOS software release, a feature set, and a platform. Use Cisco Feature Navigator to find information about platform support and Cisco IOS software image support. Access Cisco Feature Navigator at http://www.cisco.com/go/fn. You must have an account on Cisco.com. If you do not have an account or have forgotten your username or password, click Cancel at the login dialog box and follow the instructions that appear.

Note Table 1 lists only the Cisco IOS software release that introduced support for a given feature in a given Cisco IOS software release train. Unless noted otherwise, subsequent releases of that Cisco IOS software release train also support that feature.

Table 1 Feature Information for MPLS MTU Command Changes 

Feature Name
Releases
Feature Information

MPLS MTU Command Changes

12.2(27)SBC
12.2(28)SB
12.2(33)SRA
12.4(11)T
12.2(33)SXH
15.0(1)M1

This document explains the changes to the mpls mtu command. You cannot set the MPLS MTU value larger than the interface MTU value, except for Ethernet interfaces.

In 12.2(28)SB, support was added for the Cisco 10000 router.

In 12.2(33)SRA, support was added for the Cisco 7600 series router.

This feature was integrated into Cisco IOS Release 12.4(11)T.

This feature was integrated into Cisco IOS Release 12.2(33)SXH.

In 15.0(1)M1, you can configure the interface MTU on PA-1FE and PA-2FE port adapters.

The following sections provide information about this feature:

•Information About MPLS MTU Command Changes

•How to Configure MPLS MTU Values

•Configuration Examples for Setting the MPLS MTU Values


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