Contents

MPLS MTU Command Changes

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

  • 12.2(27)SBC and later
  • 12.2(33)SRA and later
  • 12.2(33)SXH and later
  • 12.4(11)T and later
  • 15.0(1)M1
  • 15.1(2)S

You cannot set the Multiprotocol Label Switching (MPLS) maximum transmission unit (MTU) to a value larger than the interface MTU value. This eliminates problems such as dropped packets, data corruption, and high CPU rates from occurring when the MPLS MTU value settings are larger than the 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 from 1500 to 1530. Before this enhancement, the MTU of those interfaces was not configurable, and any attempt to configure the interface MTU displayed the following message: %Interface{InterfaceName}doesnotsupportusersettablemtu.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

Information About MPLS MTU Command Changes

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, that 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 initialization.

If the configuration of the adjacent router does not include the mplsmtuand 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 message, which prompts 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 15.1(2)S, the mplsmtu command was modified. This command was made available in L3VPN encapsulation configuration mode. The maximum keyword was replaced with the max keyword. The override keyword and the bytes argument were removed from the GRE tunnel interface. To set MPLS MTU to the maximum MTU on L3VPN profiles, use the mplsmtu command in L3VPN encapsulation configuration mode.

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 mplsmtu command provides an overridekeyword, which allows you to set the MPLS MTU to a 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.

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

Theoverride 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

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 from 1500 to 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

    6.    end


DETAILED STEPS
     Command or ActionPurpose
    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.

     
    Step 6 end


    Example:
    Router(config-if)# end
     

    Exits interface configuration mode and returns to privileged EXEC mode.

     

    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

      5.    end


    DETAILED STEPS
       Command or ActionPurpose
      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 to a value higher than the Ethernet interface MTU value can lead to dropped packets, data corruption, or high CPU rates.

       
      Step 5 end


      Example:
      Router(config-if)# end
       

      Exits interface configuration mode and returns to privileged EXEC mode.

       

      Setting the MPLS MTU Value to the Maximum on L3VPN Profiles

      Use the following steps to set the MPLS MTU value to the maximum on L3VPN profiles.

      SUMMARY STEPS

        1.    enable

        2.    configure terminal

        3.    l3vpn encapsulation ip profile

        4.    mpls mtu max

        5.    end


      DETAILED STEPS
         Command or ActionPurpose
        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 l3vpn encapsulation ip profile


        Example:
        Router(config)# l3vpn encapsulation ip profile1
         

        Configures an L3VPN encapsulation profile and enters the L3VPN encapsulation configuration mode.

         
        Step 4 mpls mtu max


        Example:
        Router(config-l3vpn-encap-ip)# mpls mtu max
         

        Sets the MPLS MTU value to the maximum MTU on the L3VPN profile.

         
        Step 5 end


        Example:
        Router(config-l3vpn-encap-ip)# end
         

        Exits L3VPN encapsulation configuration mode and returns to privileged EXEC mode.

         

        Configuration Examples for Setting the MPLS MTU Values

        Example Setting the Interface MTU and MPLS MTU

        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 MPLS MTU cannot be set to a value greater than the value of 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

        Example Setting the MPLS MTU Value on an Ethernet Interface


        Caution

        Setting the MPLS MTU to a 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

        Example Setting the MPLS MTU Value to the Maximum MTU on L3VPN profiles

        The following example shows how to set the MPLS MTU value to the maximum MTU on L3VPN profiles:

        Router# configure terminal
Router(config)# l3vpn encapsulation ip profile1
Router(config-l3vpn-encap-ip)# mpls mtu max

        Additional References

        Related Documents

        Related Topic

        Document Title

        Cisco IOS commands

        Cisco IOS Master Commands List, All Releases

        MPLS commands

        MIBs

        MIBs

        MIBs Link

        None.

        To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:

        http:/​/​www.cisco.com/​go/​mibs

        Technical Assistance

        Description

        Link

        The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

        http:/​/​www.cisco.com/​cisco/​web/​support/​index.html

        Feature Information for MPLS MTU Command Changes

        The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

        Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

        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 12.2(33)SXH 12.4(11)T 15.0(1)M1 15.1(2)S

        This document explains the changes to the mplsmtu 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.

        In 15.1(2)S, the mplsmtu command was made available in L3VPN encapsulation configuration mode. The maximum keyword was replaced with the max keyword. The override keyword and the bytes argument were removed from the GRE tunnel interface.

        MPLS MTU Command Changes

        MPLS MTU Command Changes

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

        • 12.2(27)SBC and later
        • 12.2(33)SRA and later
        • 12.2(33)SXH and later
        • 12.4(11)T and later
        • 15.0(1)M1
        • 15.1(2)S

        You cannot set the Multiprotocol Label Switching (MPLS) maximum transmission unit (MTU) to a value larger than the interface MTU value. This eliminates problems such as dropped packets, data corruption, and high CPU rates from occurring when the MPLS MTU value settings are larger than the 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 from 1500 to 1530. Before this enhancement, the MTU of those interfaces was not configurable, and any attempt to configure the interface MTU displayed the following message: %Interface{InterfaceName}doesnotsupportusersettablemtu.

        Finding Feature Information

        Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module.

        Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

        Information About MPLS MTU Command Changes

        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, that 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 initialization.

        If the configuration of the adjacent router does not include the mplsmtuand 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 message, which prompts 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 15.1(2)S, the mplsmtu command was modified. This command was made available in L3VPN encapsulation configuration mode. The maximum keyword was replaced with the max keyword. The override keyword and the bytes argument were removed from the GRE tunnel interface. To set MPLS MTU to the maximum MTU on L3VPN profiles, use the mplsmtu command in L3VPN encapsulation configuration mode.

        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 mplsmtu command provides an overridekeyword, which allows you to set the MPLS MTU to a 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.

        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

        Theoverride 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

        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 from 1500 to 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

          6.    end


        DETAILED STEPS
           Command or ActionPurpose
          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.

           
          Step 6 end


          Example:
          Router(config-if)# end
           

          Exits interface configuration mode and returns to privileged EXEC mode.

           

          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

            5.    end


          DETAILED STEPS
             Command or ActionPurpose
            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 to a value higher than the Ethernet interface MTU value can lead to dropped packets, data corruption, or high CPU rates.

             
            Step 5 end


            Example:
            Router(config-if)# end
             

            Exits interface configuration mode and returns to privileged EXEC mode.

             

            Setting the MPLS MTU Value to the Maximum on L3VPN Profiles

            Use the following steps to set the MPLS MTU value to the maximum on L3VPN profiles.

            SUMMARY STEPS

              1.    enable

              2.    configure terminal

              3.    l3vpn encapsulation ip profile

              4.    mpls mtu max

              5.    end


            DETAILED STEPS
               Command or ActionPurpose
              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 l3vpn encapsulation ip profile


              Example:
              Router(config)# l3vpn encapsulation ip profile1
               

              Configures an L3VPN encapsulation profile and enters the L3VPN encapsulation configuration mode.

               
              Step 4 mpls mtu max


              Example:
              Router(config-l3vpn-encap-ip)# mpls mtu max
               

              Sets the MPLS MTU value to the maximum MTU on the L3VPN profile.

               
              Step 5 end


              Example:
              Router(config-l3vpn-encap-ip)# end
               

              Exits L3VPN encapsulation configuration mode and returns to privileged EXEC mode.

               

              Configuration Examples for Setting the MPLS MTU Values

              Example Setting the Interface MTU and MPLS MTU

              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 MPLS MTU cannot be set to a value greater than the value of 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

              Example Setting the MPLS MTU Value on an Ethernet Interface


              Caution

              Setting the MPLS MTU to a 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

              Example Setting the MPLS MTU Value to the Maximum MTU on L3VPN profiles

              The following example shows how to set the MPLS MTU value to the maximum MTU on L3VPN profiles:

              Router# configure terminal
Router(config)# l3vpn encapsulation ip profile1
Router(config-l3vpn-encap-ip)# mpls mtu max

              Additional References

              Related Documents

              Related Topic

              Document Title

              Cisco IOS commands

              Cisco IOS Master Commands List, All Releases

              MPLS commands

              MIBs

              MIBs

              MIBs Link

              None.

              To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:

              http:/​/​www.cisco.com/​go/​mibs

              Technical Assistance

              Description

              Link

              The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

              http:/​/​www.cisco.com/​cisco/​web/​support/​index.html

              Feature Information for MPLS MTU Command Changes

              The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

              Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

              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 12.2(33)SXH 12.4(11)T 15.0(1)M1 15.1(2)S

              This document explains the changes to the mplsmtu 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.

              In 15.1(2)S, the mplsmtu command was made available in L3VPN encapsulation configuration mode. The maximum keyword was replaced with the max keyword. The override keyword and the bytes argument were removed from the GRE tunnel interface.