MPLS Layer 3 VPNs Inter-AS and CSC Configuration Guide, Cisco IOS XE Release 3S
MPLS VPN Explicit Null Label Support with BGP IPv4 Label Session
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MPLS VPN Explicit Null Label Support with BGP IPv4 Label Session

Contents

MPLS VPN Explicit Null Label Support with BGP IPv4 Label Session

The MPLS VPN Explicit Null Label Support with BGP IPv4 Label Session feature provides a method to advertise explicit null in a Border Gateway Protocol (BGP) label session for a carrier supporting carrier (CSC) customer edge (CE) device.

Prerequisites for MPLS VPN Explicit Null Label Support with BGP IPv4 Label Session

  • You must configure your network for Multiprotocol Label Switching (MPLS) Virtual Private Network (VPN).
  • You must configure the Border Gateway Protocol (BGP) to distribute labels between the carrier supporting carrier (CSC) customer edge (CE) device (CSC-CE) and CSC-provider edge (PE) devices.

Restrictions for MPLS VPN Explicit Null Label Support with BGP IPv4 Label Session

  • Configure an explicit null label only in a carrier supporting carrier (CSC) customer edge (CE) device (CSC-CE) topology.
  • Configure an explicit null label only on a per-neighbor basis.

Information About MPLS VPN Explicit Null Label Support with BGP IPv4 Label Session

Feature Design of MPLS VPN Explicit Null Label Support with BGP IPv4 Label Session

On a carrier supporting carrier (CSC) customer edge (CE) device (CSC-CE) with Border Gateway Protocol (BGP) IPv4 label distribution, BGP advertises an implicit null label for directly connected routes. This causes the previous hop (penultimate) device to do penultimate hop popping (PHP).

The MPLS VPN Explicit Null Label Support BGP IPv4 Label Session feature makes the penultimate device swap the incoming label for (or impose) the explicit null label. This action forces the egress device to process the explicit null label by popping it and inspecting the packet that remains.

Benefits of MPLS VPN Explicit Null Label Support BGP IPv4 Label Session

The explicit null label helps to preserve quality of service (QoS) bits from one Service Level Agreement (SLA) to another until the packets reach their carrier supporting carrier (CSC) customer edge (CE) destination.

How to Configure MPLS VPN Explicit Null Label Support with BGP IPv4 Label Session

Configuring CSC with BGP

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    router bgp autonomous-system-number

    4.    address-family ipv4 [unicast]

    5.    neighbor ip-address send-label explicit-null

    6.    neighbor {ip-address | peer-group-name} activate

    7.    end


DETAILED STEPS
      Command or Action Purpose
    Step 1 enable


    Example:
    Device> enable
     

    Enables privileged EXEC mode.

    • Enter your password if prompted.
     
    Step 2 configure terminal


    Example:
    Device# configure terminal
     

    Enters global configuration mode.

     
    Step 3 router bgp autonomous-system-number


    Example:
    Device(config)# router bgp 100 
    
     

    Enters router configuration mode and configures the device to run a Border Gateway Protocol (BGP) process.

     
    Step 4 address-family ipv4 [unicast]


    Example:
    Device(config-router)# address-family ipv4 
    
     

    Enters address family configuration mode for the IPv4 address family from which you can configure routing sessions that use standard IPv4 address prefixes.

     
    Step 5 neighbor ip-address send-label explicit-null


    Example:
    Device(config-router-af)# neighbor 10.0.0.2 send-label explicit-null 
    
     

    Advertises the capability of a device to send Multiprotocol Label Switching (MPLS) labels with BGP routes.

    • The explicit-null keyword allows a carrier supporting carrier (CSC) customer edge (CE) device to send labels with a value of 0 to its neighbor.
     
    Step 6 neighbor {ip-address | peer-group-name} activate


    Example:
    Device(config-router-af)# neighbor 192.168.99.70 activate 
    
     

    Enables the neighbor to exchange prefixes for the IPv4 address family with the local device.

     
    Step 7 end


    Example:
    Device(config-router-af)# end
     

    Returns to privileged EXEC mode.

     

    Verifying the Explicit Null Configuration

    SUMMARY STEPS

      1.    enable

      2.    show ip bgp neighbors [ip-address [advertised-routes | dampened-routes | flap-statistics | paths [regexp] | received prefix-filter | received-routes | routes]]


    DETAILED STEPS
        Command or Action Purpose
      Step 1 enable


      Example:
      Device> enable
       

      Enables privileged EXEC mode.

      • Enter your password if prompted.
       
      Step 2 show ip bgp neighbors [ip-address [advertised-routes | dampened-routes | flap-statistics | paths [regexp] | received prefix-filter | received-routes | routes]]


      Example:
      Device# show ip bgp neighbors
       

      Displays information about the TCP and Border Gateway Protocol (BGP) connections to neighbors including explicit null.

      • The optional ip-address argument displays the IP address of the neighbor whose routes you have learned If you omit this argument, all neighbors are displayed.
      • The optional advertised-routes keyword displays all the routes the device has advertised to the neighbor.
      • The optional dampened-routes keyword displays the dampened routes to the neighbor at the IP address specified.
      • The optional flap-statistics keyword displays the flap statistics of the routes learned from the specified neighbor (external BGP [eBGP] peers only).
      • The optional path regexp keyword and argument displays autonomous system paths learned from the specified neighbor. An optional regular expression can be used to filter the output.
      • The optional received prefix-filter keyword displays the configured prefix list filter for the specified IP address.
      • The optional received-routes keyword displays all received routes (both accepted and rejected) from the specified neighbor.
      • The optional routes keyword displays all routes that are received and accepted. This is a subset of the output from the received-routes keyword.
       
      What to Do Next

      Configuration Examples for MPLS VPN Explicit Null Label Support with BGP IPv4 Label Session

      Example: Configuring CSC-CE with BGP

      In the following example, the carrier supporting carrier (CSC) is configured with the Border Gateway Protocol (BGP) to distribute labels and to advertise explicit null for all its connected routes:

      neighbor 10.0.0.0 send-label explicit-null
      router bgp 100
       bgp log-neighbor-changes
       neighbor 10.0.0.0 remote-as 200
       !
      address-family ipv4
       neighbor 10.0.0.0 activate
       neighbor 10.0.0.0 send-label explicit-null
       no auto-summary
       no synchronization
       exit-address-family 

      Example: Verifying the Explicit Null Configuration

      In this example, the show ip bgp neighbors command displays information about connected Border Gateway Protocol (BGP) neighbors, including IP addresses, version numbers, neighbor capabilities, message statistics, and address family statistics that show if explicit null is configured:

      Device# show ip bgp neighbors 
      
      BGP neighbor is 10.0.0.2,  remote AS 300, external link
        BGP version 4, remote router ID 10.0.0.20
        BGP state = Established, up for 00:45:16
        Last read 00:00:16, hold time is 180, keepalive interval is 60 seconds
        Neighbor capabilities:
          Route refresh: advertised and received(new)
          Address family IPv4 Unicast: advertised and received
          ipv4 MPLS Label capability: advertised and received
        Message statistics:
          InQ depth is 0
          OutQ depth is 0
                               Sent       Rcvd
          Opens:                  1          1
          Notifications:          0          0
          Updates:                1          2
          Keepalives:            47         47
          Route Refresh:          0          0
          Total:                 49         50
        Default minimum time between advertisement runs is 30 seconds
      
       For address family: IPv4 Unicast
        BGP table version 9, neighbor version 9/0
        Output queue sizes : 0 self, 0 replicated
        Index 1, Offset 0, Mask 0x2
        Member of update-group 1
        My AS number is allowed for 3 number of times
        AF-dependant capabilities:
          Outbound Route Filter (ORF) type (128) Prefix-list:
        Sending Prefix & Label(advertise explicit-null set)       !Explicit null is configured
                                       Sent       Rcvd
        Prefix activity:               ----       ----
          Prefixes Current:               3          3 (Consumes 144 bytes)
          Prefixes Total:                 3          6
          Implicit Withdraw:              0          3
          Explicit Withdraw:              0          0
          .........	
          .........

      Additional References for MPLS VPN Explicit Null Label with BGP IPv4 Label Session

      Related Documents

      Related Topic

      Document Title

      Cisco IOS commands

      Cisco IOS Master Commands List, All Releases

      MPLS commands

      Cisco IOS Multiprotocol Label Switching Command Reference

      BGP configuration tasks

      IP Routing: BGP Configuration Guide

      BGP commands

      Cisco IOS IP Routing: BGP Command Reference

      RFCs

      RFC

      Title

      RFC 1163

      A Border Gateway Protocol

      RFC 1164

      Application of the Border Gateway Protocol in the Internet

      RFC 2283

      Multiprotocol Extensions for BGP-4

      RFC 2547

      BGP/MPLS VPNs

      RFC 3107

      Carrying Label Information in BGP-4

      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 VPN Explicit Null Label Support with BGP IPv4 Label Session

      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 VPN Explicit Null Label Support with BGP IPv4 Label Session

      Feature Name

      Releases

      Feature Information

      MPLS VPN Explicit Null Label Support with BGP IPv4 Label Session

      12.0(27)S

      12.0(27)S1

      12.2(27)SBA

      12.2(33)SRA

      12.2(33)SXH

      Cisco IOS XE Release 2.3

      The MPLS VPN Explicit Null Label Support with BGP IPv4 Label Session feature provides a method to advertise explicit null in a BGP label session for a carrier supporting carrier (CSC) customer edge (CE) device.

      In 12.0(27)S, this feature was introduced.

      In 12.0(27)S1, support was added for the Cisco 12000 series Internet routers.

      In 12.2(27)SBA, support was added for the Cisco 10000 series router.

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

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

      In Cisco IOS XE Release 2.3, support was added for the Cisco ASR 1000 Series Routers.

      The following commands were introduced or modified: debug ip bgp, neighbor send-label explicit-null, show ip bgp neighbors, show ip bgp vpnv4, show mpls forwarding-table

      Glossary

      BGP

      Border Gateway Protocol. The exterior Border Gateway Protocol used to exchange routing information between devices in separate autonomous systems. BGP uses TCP. Because TCP is a reliable protocol, BGP does not experience problems with dropped or fragmented data packets.

      CE device

      customer edge device. A device on the border between a VPN provider and a VPN customer that belongs to the customer.

      eBGP

      external Border Gateway Protocol. A BGP session between devices in different autonomous systems. When a pair of devices in different autonomous systems are more than one IP hop away from each other, an external BGP session between those two devices is called multihop external BGP.

      label

      A short, fixed-length data identifier that tells switching nodes how to forward data (packets or cells).

      label distribution

      The techniques and processes used to cause routed traffic to travel through the network on a path other than the one that would have been chosen if standard routing methods had been used.

      LDP

      Label Distribution Protocol. The protocol that supports MPLS hop-by-hop forwarding by distributing bindings between labels and network prefixes. The Cisco proprietary version of this protocol is the Tag Distribution Protocol (TDP).

      LSP

      label switched path. A configured connection between two devices, in which MPLS is used to carry packets. A path created by the concatenation of one or more label switched hops, allowing a packet to be forwarded by swapping labels from an MPLS node to another MPLS node.

      MPLS

      Multiprotocol Label Switching. A method for directing packets primarily through Layer 2 switching rather than Layer 3 routing. In MPLS, packets are assigned short, fixed-length labels at the ingress to an MPLS cloud by using the concept of forwarding equivalence classes. Within the MPLS domain, the labels are used to make forwarding decisions mostly without recourse to the original packet headers; formerly known as tag switching.

      NLRI

      Network Layer Reachability Information. BGP sends routing update messages containing NLRI, which describes the route. In this context, an NLRI is a prefix. A BGP update message carries one or more NLRI prefixes and the attributes of a route for the NLRI prefixes. The route attributes include a BGP next hop gateway address, community values, and other information.

      PE device

      provider edge device. A device on the border between a VPN provider and a VPN customer that belongs to the provider.

      QoS

      quality of service. A measure of performance for a transmission system that reflects its transmission quality and service availability.

      router

      A network layer device that uses one or more metrics to determine the optimal path along which network traffic should be forwarded. Routers forward packets from one network to another based on network layer information.

      VPN

      Virtual Private Network. A secure IP-based network that shares resources on one or more physical networks. A VPN contains geographically dispersed sites that can communicate securely over a shared backbone.