CCDE 400-007: How can EIGRP topologies be designed to converge as fast as possible in the event of point-to-point link failure?

Question

How can EIGRP topologies be designed to converge as fast as possible in the event of a point-to-point link failure?

A. Limit the query domain by use of summarization.
B. Limit the query domain by use of default routes.
C. Build neighbor adjacencies in a squared fashion.
D. Limit the query domain by use of distribute lists.
E. Build neighbor adjacencies in a triangulated fashion.

Answer

A. Limit the query domain by use of summarization.

Explanation

Answer: A

EIGRP is a distance vector routing protocol that uses a Diffusing Update Algorithm (DUAL) to compute the shortest path to a destination network. EIGRP maintains a topology table that contains all the routes learned from its neighbors, and selects the best route (called the successor) based on the lowest feasible distance (FD). The FD is calculated from the reported distance (RD) advertised by the neighbor plus the cost of the link to that neighbor.

When a point-to-point link fails, EIGRP sends a query message to all its neighbors asking for an alternate route to the destination network. The neighbors reply with either a feasible successor (a backup route that has a lower RD than the FD of the original route) or an unreachable message. If a feasible successor is found, EIGRP updates its routing table and sends an update message to its neighbors. If no feasible successor is found, EIGRP goes into active mode and continues to query its neighbors until it receives a reply or reaches a maximum number of retries (by default, 16).

The query process can be slow and consume a lot of bandwidth and CPU resources, especially in large networks with many routers and routes. To speed up convergence and reduce overhead, EIGRP can limit the query domain by use of summarization. Summarization means aggregating multiple routes into a single route with a less specific prefix length. For example, instead of advertising four routes with /24 prefix length (such as 10.1.1.0/24, 10.1.2.0/24, 10.1.3.0/24, and 10.1.4.0/24), EIGRP can advertise one route with /22 prefix length (such as 10.1.0.0/22).

Summarization has several benefits for EIGRP convergence:

• It reduces the size of the routing table and the topology table, which saves memory and CPU resources.
• It reduces the number of updates and queries that need to be sent and processed, which saves bandwidth and CPU resources.
• It creates a boundary for queries, which prevents them from propagating beyond the summarized area.

To enable summarization in EIGRP, you can use either manual summarization or automatic summarization.

Manual summarization allows you to configure a summary route on any interface of an EIGRP router using the ip summary-address eigrp command. For example, if you want to summarize four routes with /24 prefix length (such as 10.1.1.0/24, 10.1.2.0/24, 10.1.3.0/24, and 10.1.4.0/24) into one route with /22 prefix length (such as 10.1.0.0/22) on interface Ethernet0/0 of router R1, you can use this command:

R1(config)#interface Ethernet0/0
R1(config-if)#ip summary-address eigrp 1 10.1.0.0 255.255.252.0

This command tells R1 to advertise the summary route 10.1.0.0/22 to its neighbors on interface Ethernet0/0 with an administrative distance of 5 and a metric equal to the lowest metric of any component route.

Manual summarization gives you more control over where and how to summarize routes, but it requires careful planning and configuration.

Automatic summarization allows EIGRP to automatically create summary routes at network boundaries based on classful addressing rules. For example, if EIGRP learns two routes with /24 prefix length (such as 10.1

Reference

• Configuration Notes for the Implementation of EIGRP over Frame Relay and Low Speed Links – Cisco
• Why EIGRP converge faster than OSPF? (cisco.com)
• EIGRP Timers, Failure, and Convergence Explained – Study CCNP (study-ccnp.com)
• Strategies for Optimizing Network Performance – Cisco Community
• Troubleshoot EIGRP Common Issues – Cisco

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