1.3d EIGRP Load Balancing

1.3d i Equal-cost

EIGRP will load balance 4 equal cost paths into the routing table by default.  Traffic is shared equally between each path.  The number of paths is configurable using the command: max-paths <[1-6]>.

Configuration

router eigrp [NAME]
 address-family ipv4 unicast autonomous-system [AS-Number]
  topology base
-> maximum-paths 6
  exit-af-topology
  network x.x.x.x
 exit-address-family

Verification

R1#show ip protocols | inc Maximum 
      Maximum path: 6

1.3d ii Unequal-cost

EIGRP allows for unequal cost load balancing using the variance command where variance is a multiplier with a value between 1 and 128.  1 is equal-cost. 

Configuration

router eigrp [NAME]
 address-family ipv4 unicast autonomous-system [AS-Number]
  topology base
-> variance 128
  exit-af-topology
  network x.x.x.x
 exit-address-family

Verification

R1#show ip protocols | inc variance
      Maximum metric variance 128

1.3d iii Add-path 

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.

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.

Source: How Add Path Support in EIGRP Works

Configuration

router eigrp [NAME]
 address-family ipv4 unicast autonomous-system [AS-Number]
  af-interface Tunnel0
-> no next-hop-self
-> add-paths 4
  exit-af-interface
  !
  topology base
   maximum-paths 6
   variance 128
  exit-af-topology
  network x.x.x.x
 exit-address-family