Segment Routing—IS-IS v4 node SID

The Segment Routing—ISIS v4 node SID feature provides support for segment routing on Cisco Intermediate System-to-Intermediate System (IS-IS) networks.

Information About Segment Routing IS-IS v4 Node SID

Segment Routing IS-IS v4 Node SID

Segment Routing relies on a small number of extensions to Cisco Intermediate System-to-Intermediate System (IS-IS) and Open Shortest Path First (OSPF) protocols. There are two levels of configuration required to enable segment routing for a routing protocol instance. The top level segment routing configuration which is managed by segment routing infrastructure component enables segment routing, whereas, segment routing configuration at the router level enables segment routing for a specific address-family of a routing protocol instance. There are three segment routing states:

  • SR_NOT_CONFIGURED
  • SR_DISABLED
  • SR_ENABLED

Segment routing configuration under the IGPs is allowed only if the SR state is either SR_DISABLED or SR_ENABLED. The SR_ENABLED state indicates that there is at least a valid SRGB range reserved through the MFI successfully. You can enable segment routing for IGPs under the router configuration sub mode, through commands. However, IGP segment routing are enabled only after the global SR is configured.


Note
IS-IS protocol SR command is based on per topology (IPv4 address family).

The SR_ENABLED is a necessary state for any protocol to enable SR, however, it is not a sufficient for enabling SR for a protocol instance. The reason being that the IS-IS still does not have any information about segment routing global block (SRGB) information. When the request to receive information about the SRGB is processed successfully, the IS-IS SR operational state is enabled.

Segment Routing requires each router to advertise its segment routing data-plane capability and the range of MPLS label values that are used for segment routing in the case where global SIDs are allocated. Data-plane capabilities and label ranges are advertised using the SR-capabilities sub-TLV inserted into the IS-IS Router Capability TLV-242 that is defined in RFC4971.

ISIS SR-capabilities sub TLV includes all reserved SRGB ranges. However, the Cisco implementation supports only one SRGB range. The supported IPv4 prefix-SID sub TLV are TLV-135 and TLV-235.

How to Configure Segment Routing —IS-IS v4 Node SID

Configuring Segment Routing

Before You Begin

Before configuring IS-IS to support segment routing you must first configure the segment routing feature in global configuration mode.

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    segment-routing mpls

    4.    connected-prefix-sid-map

    5.    address-family ipv4

    6.    1.1.1.1/32 index 100 range 1

    7.    exit-address-family


DETAILED STEPS
     Command or ActionPurpose
    Step 1enable


    Example: 
    Device# enable
     

    Enables privileged EXEC mode.

    • Enter your password if prompted.
     
    Step 2configure terminal


    Example: 
    Device# configure terminal
     

    Enters global configuration mode.

     
    Step 3 segment-routing mpls

    Example: 
    Device(config-sr)# segment-routing mpls
     

    Enables the segment feature using the mpls data plane.

     
    Step 4 connected-prefix-sid-map

    Example: 
    Device(config-srmpls)# connected-prefix-sid-map
     

    Enters a sub-mode where you can configure address-family specific mappings for local prefixes and SIDs.

     
    Step 5 address-family ipv4

    Example: 
    Device(config-srmpls-conn)# address-family ipv4
     

    Specifies IPv4 address prefixes.

     
    Step 61.1.1.1/32 index 100 range 1

    Example: 
    Device(config-srmpls-conn-af)# 1.1.1.1/32 100 range 1
     

    Associates SID 100 with the address 1.1.1.1/32.

     
    Step 7exit-address-family

    Example: 
    Device(config-srmpls-conn-af)# exit-address-family
     

    Exits the address family.

     

    Configuring Segment Routing on IS-IS Network

    Before You Begin

    Before you configure segment routing on IS-IS network, IS-IS must be enabled on your network.

    SUMMARY STEPS

      1.    router isis

      2.    net network-entity-title

      3.    metric-style wide

      4.    segment-routing mpls

      5.    exit

      6.    show isis segment-routing


    DETAILED STEPS
       Command or ActionPurpose
      Step 1 router isis

      Example: 
      Device(config-router)# router isis
       

      Enables the IS-IS routing protocol and enters router configuration mode.

       
      Step 2 net network-entity-title

      Example: 
      Device(config-router)# net 49.0000.0000.0003.00
       

      Configures network entity titles (NETs) for the routing instance.

       
      Step 3 metric-style wide

      Example: 
      Device(config-router)# metric-style wide
       

      Configures the device to generate and accept only wide link

      metrics.

       
      Step 4segment-routing mpls

      Example: 
      Device(config-router)# segment-routing mpls
       

      Configures segment routing operation state.

       
      Step 5 exit

      Example: 
      Device(config-router)# exit
       

      Exits segment routing mode and returns to the configuration terminal mode.

       
      Step 6 show isis segment-routing

      Example: 
      Device# show is-is segment-routing
       

      Displays the current state of the IS-IS segment routing.

       

      The following example displays output from the show isis segment-routing state command for the segment routing under IS-IS: 

      Device# show isis segment-routing

ISIS protocol is registered with MFI
ISIS MFI Client ID:0x63
Tag 1 - Segment-Routing:
   SR State:SR_ENABLED
   Number of SRGB:1
   SRGB Start:16000, Range:8000, srgb_handle:0x4500AED0, srgb_state: created
   Address-family IPv4 unicast SR is configured
     Operational state:Enabled

      Configuring Prefix-SID for IS-IS

      This task explains how to configure prefix segment identifier (SID) index under each interface.

      Before You Begin

      Segment routing must be enabled on the corresponding address family.

      SUMMARY STEPS

        1.    enable

        2.    configure terminal

        3.    segment-routing mpls

        4.    connected-prefix-sid-map

        5.    address-family ipv4

        6.    1.1.1.1/32 index 100 range 1

        7.    exit


      DETAILED STEPS
         Command or ActionPurpose
        Step 1 enable

        Example: 
        Device# enable
         

        Enables privileged EXEC mode.

         
        Step 2 configure terminal

        Example: 
        Device# configure terminal
         

        Enters global configuration mode.

         
        Step 3segment-routing mpls


        Example: 
        Device(config)# segment-routing mpls
         

        Configures segment routing mpls mode.

         
        Step 4connected-prefix-sid-map


        Example: 
        Device(config-srmpls)# connected-prefix-sid-map
         

        Enters a sub-mode where you can configure address-family specific mappings for local prefixes and SIDs.

         
        Step 5address-family ipv4


        Example: 
        Device(config-srmpls-conn)# address-family ipv4
         

        Specifies the IPv4 address family and enters router address family configuration mode.

         
        Step 61.1.1.1/32 index 100 range 1

        Example: 
        Device(config-srmpls-conn-af)# 1.1.1.1/32 100 range 1
         

        Associates SID 100 with the address 1.1.1.1/32.

         
        Step 7 exit

        Example: 
        Device(config-router)# exit
         

        Exits segment routing mode and returns to the configuration terminal mode.

         

        Configuring Prefix Attribute N-flag-clear

        By default, a flag called N-flag is set by IS-IS when advertising a SID which is associated with a loopback address. If you wish to clear this flag add explicit configuration.

        SUMMARY STEPS

          1.    enable

          2.    configure terminal

          3.    interface loopback3

          4.    isis prefix n-flag-clear


        DETAILED STEPS
           Command or ActionPurpose
          Step 1enable


          Example: 
          Device# enable
           

          Enables privileged EXEC mode.

          • Enter your password if prompted.
           
          Step 2configure terminal


          Example: 
          Device# configure terminal
           

          Enters global configuration mode.

           
          Step 3 interface loopback3

          Example: 
          Device(config)# interface loopback3
           

          Specifies the interface loopback.

           
          Step 4isis prefix n-flag-clear

          Example: 
          Device(config-if)# isis prefix n-flag-clear
           

          Clears the prefix N-flag.

           

          Configuring Explicit Null Attribute

          To disable penultimate-hop-popping (PHP) and add explicit-Null label, explicit-null option needs to be specified. Once the option is given, IS-IS sets the E flag in the prefix-SID sub TLV.

          By default, a flag called E-flag (Explicit-Null flag) is set to 0 by ISIS when advertising a Prefix SID which is associated with a loopback address. If you wish to set this flag add explicit configuration.

          SUMMARY STEPS

            1.    enable

            2.    configure terminal

            3.    segment-routing mpls

            4.    set-attributes

            5.    address-family ipv4

            6.    explicit-null

            7.    exit-address-family


          DETAILED STEPS
             Command or ActionPurpose
            Step 1enable


            Example: 
            Device# enable
             

            Enables privileged EXEC mode.

            • Enter your password if prompted.
             
            Step 2configure terminal


            Example: 
            Device# configure terminal
             

            Enters global configuration mode.

             
            Step 3segment-routing mpls

            Example: 
            Device(config)# segment-routing mpls
             

            Configures segment routing mpls mode.

             
            Step 4set-attributes

            Example: 
            Device(config-srmpls)# set-attributes
             

            Sets the attribute.

             
            Step 5address-family ipv4


            Example: 
            Device(config-srmpls-attr)# address-family ipv4
             

            Specifies the IPv4 address family and enters router address family configuration mode.

             
            Step 6explicit-null


            Example: 
            Device(config-srmpls-attr-af)# explicit-null
             

            Specifies the explicit-null.

             
            Step 7exit-address-family


            Example: 
            Device(config-srmpls-attr-af)# exit-address-family
             

            Exits the address family.

             

            Configuration Examples for Segment Routing —IS-IS v4 Node SID

            Example: Configuring Segment Routing on IS-IS Network

            The following example shows how to configure prefix segment identifier (SID) index under each interface: 

            Device(config)#segment-routing mpls
 Device(config-srmpls)#connected-prefix-sid-map
  Device(config-srmpls-conn)#address-family ipv4
   Device(config-srmpls-conn-af)#10.1.2.2/32 index 2 range 1
  Device(config-srmpls-conn-af)#exit-address-family
 Device(config-srmpls-conn-af)#end

            Example: Configuring Explicit Null Attribute

            The following is an example for configuring explicit null attribute: 

            Device(config)# segment-routing mpls
Device(config-srmpls)# set-attributes
 Device(config-srmpls-attr)# address-family ipv4
  Device(config-srmpls-attr-af)# explicit-null
 Device (config-srmpls-attr-af)# exit-address-family

            Additional References for Segment Routing-IS-IS v4 Node SID

            Related Documents

            Related Topic

            Document Title

            Cisco IOS commands

            Cisco IOS Master Command List, All Releases http:/​/​www.cisco.com/​c/​en/​us/​td/​docs/​ios-xml/​ios/​mcl/​allreleasemcl/​all-book.html

            IP Routing ISIS commands

            Cisco IOS IP Routing ISIS commands http:/​/​www.cisco.com/​c/​en/​us/​td/​docs/​ios-xml/​ios/​mcl/​allreleasemcl/​all-book.html

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            Feature Information for Segment Routing—IS-IS v4 Node SID

            Table 1 Feature Information for Segment Routing—IS-IS v4 Node SID

            Feature Name

            Releases

            Feature Information

            Segment Routing—IS-IS v4 Node SID

            Cisco IOS XE Release 3.16S

            The Segment Routing—ISIS v4 node SID feature provides support for segment routing on IS-IS networks.

            The following commands were introduced or modified: connected-prefix-sid-map, show isis segment-routing, isis prefix n-flag-clear, explicit-null