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-paths command 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 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 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 6

no 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 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

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 7

no 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

Related Topic

Document Title

Cisco IOS commands

Cisco IOS Master Command List, All Releases

EIGRP commands

Cisco IOS IP Routing: EIGRP Command Reference

EIGRP FAQ

EIGRP Frequently Asked Questions

EIGRP technology white papers

Enhanced Interior Gateway Routing Protocol

Technical Assistance

Description

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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

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 .