EIGRP is tolerant of topologies that are not hierarchical for small and medium networks. If the network is large, say more than 1000 routers, there are two possible solutions:
- Use a hierarchical topology with route summarization: As the scale of the network increases, risk of instability or long convergence times becomes greater. For example, if a network has reached the point where it includes 500 routers, EIGRP may stop working well without a structured hierarchy.
- Use multiple EIGRP autonomous systems: EIGRP autonomous systems are sometimes used as a scaling technique. The usual rationale is to reduce the volume of EIGRP queries by limiting them to one EIGRP autonomous system.
EIGRP Stub Routers
EIGRP allows for the configuration of stub routers for remote branches. It is used to reduce EIGRP query traffic between hub routers and remote branch routers that are connected over WAN links. EIGRP stub routing conserves memory and CPU resources and improves network stability. When the stub routing feature is enabled on the spoke router, the router advertises only specified routes to the hub router. The router does not advertise routes received from other EIGRP neighbors to the hub router. The only disadvantage is that the stub router cannot be used as a backup path between two hub sites.
Figure 3-9 shows an example of EIGRP stub router operation. If the LAN 10.10.10.0/24 goes down, the Hub1 router sends query packets everywhere; however, there is no need to send query packets to stub branches because no alternate routes exist there. Once you configure the branch routers as EIGRP stub routers, the query is sent only to the Hub2 router.
Figure 3-9 EIGRP Stub Routers
There are a few options when configuring the EIGRP stub routers:
- Receive-only: The stub router does not advertise the network.
- Connected: The stub router can advertise directly connected networks.
- Static: The stub router can advertise static routes.
- Summary: The stub router can advertise summary routes.
- Redistribute: The stub router can advertise redistributed routes.
EIGRP Variance Command
EIGRP allows unequal-cost routing with the use of the variance # command. If you have an active route with a metric of 10 and have feasible successors of 15, 25, and 55, you can adjust the variance number to make those routes active. If you use variance 2, then the active metric of 10 gets multiplied by 2, which equals 20. Any feasible successor less than 20 gets added as an active route. The route with a metric of 15 is added, so you have two active routes.
If you use variance 3, the routes with metrics of 10, 15, and 25 become active (3 × 10 = 30). Note that for this example, using a variance of 4 or 5 does not add the route with a metric of 55. You need to use a variance of 6 to add the route with a metric of 55 (6 × 10 = 60).