IP Routing: EIGRP Configuration Guide, Cisco IOS Release 15M&T
Add Path Support in EIGRP
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Add Path Support in EIGRP

Add Path Support in EIGRP

The Add Path Support in EIGRP feature enables hubs in a single Dynamic Multipoint VPN (DMVPN) domain to advertise multiple best paths to connected spokes when the Enhanced Interior Gateway Routing Protocol (EIGRP) is the routing protocol between the hubs and the spokes. This module provides information about the Add Path Support in EIGRP feature and explains how to configure it.

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Prerequisites for Add Path Support in EIGRP

All interfaces in an Enhanced Interior Gateway Routing Protocol (EIGRP) topology are by default configured with the next-hop-self command. This command enables EIGRP to set the local outbound interface as the next-hop value while advertising a route to a peer, even when advertising routes out of the interface on which the routes were learned. This default EIGRP behavior may interfere with the add-pathscommand that helps configure the Add Path Support in EIGRP feature. Therefore, before you configure this feature on a hub device in a Dynamic Multipoint VPN (DMVPN) domain, you must disable the next-hop-self command that is configured on the hub interface that connects to spokes in the DMVPN domain.

Restrictions for Add Path Support in EIGRP

  • The Add Path Support in EIGRP feature can be enabled only in Enhanced Interior Gateway Routing Protocol (EIGRP) named mode configurations.
  • The variance command should not be configured when the Add Path Support in EIGRP feature is enabled. The variance command alters the metrics of routes in an EIGRP topology, thereby enabling EIGRP to balance traffic among desired paths. Therefore, if you configure the variance command on a hub device, the command may interfere with the configuration of this feature.

Information About Add Path Support in EIGRP

EIGRP Add Path Support Overview

In most Dynamic Multipoint VPN (DMVPN) domains, two or more spokes are connected to the same LAN segment. These spokes connect to more than one hub (for hub redundancy) through different service providers (for service-provider redundancy). In a single DMVPN domain, a hub connects to all spokes through one tunnel interface. In Enhanced Interior Gateway Routing Protocol (EIGRP) topologies, when a hub has more than one path (with the same metric but through different spokes) to reach the same network, both paths are chosen as best paths. However, by default, EIGRP advertises only one path as the best path to connected spokes. With the implementation of the Add Path Support in EIGRP feature, hubs in an EIGRP-DMVPN domain can advertise up to four additional best paths to connected spokes, thereby allowing load balancing and path redundancy. This feature supports both IPv4 and IPv6 configurations.

How Add Path Support in EIGRP Works

A typical single Dynamic Multipoint VPN (DMVPN) domain consists of dual hubs (for hub redundancy) connected to more than one service provider (for service-provider redundancy). In the figure below, two hub devices—Hub-1 and Hub-2—are connected through tunnel interfaces to a DMVPN domain.
Figure 1. Single DMVPN Domain

The DMVPN domain is in turn connected to two service providers—Service-Provider 1 and Service-Provider 2. Four spoke devices in this DMVPN domain—Spoke-1, Spoke-2, Spoke-3, and Spoke-4. Spoke-1 and Spoke-3 are connected to Service-Provider 1, and Spoke-2 and Spoke-4 are connected to Service-Provider 2. The Enhanced Interior Gateway Routing Protocol (EIGRP) is the routing protocol between the hubs and the spokes over the tunnel interfaces.

Spoke-1 and Spoke-2 are connected to a LAN with the network address 192.168.1.0/24. Both these spokes are connected to both the hubs through two different service providers, and hence, these spokes advertise the same LAN network to both hubs. Typically, spokes on the same LAN advertise the same metric; therefore, based on the metric, Hub-1 and Hub-2 have dual Equal-Cost Multipath (ECMP) routes to reach network 192.168.1.0/24. However, because EIGRP is a distance vector protocol, it advertises only one best path to the destination. Therefore, in this EIGRP-DMVPN domain, the hubs advertise only one route (for example, through Spoke-1) to reach network 192.168.1.0/24. When clients in subnet 192.168.2.0/24 communicate with clients in subnet 192.168.1.0/24, all traffic is directed to Spoke-1. Because of this default EIGRP behavior, there is no load balancing on Spoke-3 and Spoke-4. Additionally, if Spoke-1 fails or if the network of Service-Provider 1 goes down, EIGRP must reconverge to provide connectivity to 192.168.1.0/24.

The Add Path Support in EIGRP feature enables EIGRP to advertise up to four additional paths to connected spokes in a single DMVPN domain. If you configure this feature in the example topology discussed above, both Spoke-1 and Spoke-2 will be advertised to Spoke-3 and Spoke-4 as best paths to network 192.168.1.0, thereby allowing load balancing among all spokes in this DMVPN domain.

How to Configure Add Path Support in EIGRP

Configuring IPv4 Add Path Support on a Hub

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    router eigrp virtual-name

    4.    address-family ipv4 autonomous-system as-number

    5.    af-interface {default | interface-type interface-number}

    6.    no next-hop-self [no-ecmp-mode]

    7.    add-paths number

    8.    end

    9.    show running-config


DETAILED STEPS
     Command or ActionPurpose
    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 eigrp virtual-name


    Example:
    Device(config)# router eigrp name
     

    Configures an Enhanced Interior Gateway Routing Protocol (EIGRP) routing process and enters router configuration mode.

     
    Step 4 address-family ipv4 autonomous-system as-number


    Example:
    Device(config-router)# address-family ipv4 autonomous-system 3
     

    Enters address family configuration mode and configures an EIGRP routing instance.

     
    Step 5 af-interface {default | interface-type interface-number}


    Example:
    Device(config-router-af)# af-interface tunnel 0 
     

    Enters address family interface configuration mode and configures interface-specific EIGRP commands.

     
    Step 6no next-hop-self [no-ecmp-mode]


    Example:
    Device(config-router-af-interface)# no next-hop-self no-ecmp-mode
     

    Instructs EIGRP to use the received next hop and not the local outbound interface address as the next hop to be advertised to neighboring devices.

     
    Step 7 add-paths number


    Example:
    Device(config-router-af-interface)# add-paths 4
     

    Enables EIGRP to advertise multiple paths as best paths to connected spokes in a single Dynamic Multipoint VPN (DMVPN) domain.

     
    Step 8 end


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

    Exits address family interface configuration mode and returns to privileged EXEC mode.

     
    Step 9 show running-config


    Example:
    Device# show running-config | section eigrp
     

    Displays contents of the current running configuration file.

    • Use the output modifier “|” to display the EIGRP section of the running configuration, and to verify whether the add-paths command is enabled in the configuration.
     

    Configuring IPv6 Add Path Support on a Hub

    SUMMARY STEPS

      1.    enable

      2.    configure terminal

      3.    ipv6 unicast-routing

      4.    router eigrp virtual-name

      5.    address-family ipv6 autonomous-system as-number

      6.    af-interface {default | interface-type interface-number}

      7.    no next-hop-self [no-ecmp-mode]

      8.    add-paths number

      9.    end

      10.    show running-config


    DETAILED STEPS
       Command or ActionPurpose
      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 ipv6 unicast-routing


      Example:
      Device(config)# ipv6 unicast-routing
       

      Enables the forwarding of IPv6 unicast datagrams.

       
      Step 4 router eigrp virtual-name


      Example:
      Device(config)# router eigrp name
       

      Configures an Enhanced Interior Gateway Routing Protocol (EIGRP) routing process and enters router configuration mode.

       
      Step 5 address-family ipv6 autonomous-system as-number


      Example:
      Device(config-router)# address-family ipv6 autonomous-system 3
       

      Enters address family configuration mode and configures an EIGRP routing instance.

       
      Step 6 af-interface {default | interface-type interface-number}


      Example:
      Device(config-router-af)# af-interface tunnel 0 
       

      Enters address family interface configuration mode and configures interface-specific EIGRP commands.

       
      Step 7no next-hop-self [no-ecmp-mode]


      Example:
      Device(config-router-af-interface)# no next-hop-self no-ecmp-mode
       

      Instructs EIGRP to use the received next-hop address and not the local outbound interface address as the next hop to be advertised to neighboring devices.

       
      Step 8 add-paths number


      Example:
      Device(config-router-af-interface)# add-paths 4
       

      Enables EIGRP to advertise multiple paths as best paths to connected spokes in a single Dynamic Multipoint VPN (DMVPN) domain.

       
      Step 9 end


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

      Exits address family interface configuration mode and returns to privileged EXEC mode.

       
      Step 10 show running-config


      Example:
      Device# show running-config | section eigrp
       

      Displays contents of the current running configuration file.

      • Use the output modifier “|” to display the EIGRP section of the running configuration, and to verify whether the add-paths command is enabled in the configuration.
       

      Configuration Examples for Add Path Support in EIGRP

      Example: Configuring IPv4 Add Path Support on a Hub

      Device(config)# router eigrp name
      Device(config-router)# address-family ipv4 autonomous-system 10
      Device(config-router-af)# af-interface tunnel 0
      Device(config-router-af-interface)# no next-hop-self no-ecmp-mode
      Device(config-router-af-interface)# add-paths 4
      Device(config-router-af-interface)# end

      Example: Configuring IPv6 Add Path Support on a Hub

      Device(config)# ipv6 unicast-routing
      Device(config)# router eigrp name
      Device(config-router)# address-family ipv6 autonomous-system 10
      Device(config-router-af)# af-interface tunnel 0
      Device(config-router-af-interface)# no next-hop-self no-ecmp-mode
      Device(config-router-af-interface)# add-paths 4
      Device(config-router-af-interface)# end

      Additional References for Add Path Support in EIGRP

      Related Documents

      Technical Assistance

      Description

      Link

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

      Feature Information for Add Path Support in EIGRP

      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 Add Path Support in EIGRP

      Feature Name

      Releases

      Feature Information

      Add Path Support in EIGRP

      Cisco IOS XE Release 3.8S

      15.2(1)E

      The Add Path Support in EIGRP feature enables a hub in a single Dynamic Multipoint VPN (DMVPN) domain to advertise multiple paths to connected spokes when the Enhanced Interior Gateway Routing Protocol (EIGRP) is the routing protocol between the hub and the spokes.

      The following command was introduced or modified: add-paths.