IP Routing: EIGRP Configuration Guide, Cisco IOS XE Gibraltar 16.12.x
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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.
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.
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
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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 .
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