Question

Exam 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.

Cisco Certified Design Expert CCDE v3.0 400-007 certification exam practice question and answer (Q&A) dump with detail explanation and reference available free, helpful to pass the Cisco Certified Design Expert CCDE v3.0 400-007 exam and earn Cisco Certified Design Expert CCDE v3.0 400-007 certification.

Correct Answer

A. Limit the query domain by use of summarization.

Explanation 1

To design EIGRP topologies to converge as fast as possible in the event of a point-to-point link failure, we can limit the query domain by use of summarization. This means that when a link failure occurs, the query messages to recompute routes will have a smaller domain to traverse, leading to faster convergence.

One way to limit the query domain is by building neighbor adjacencies in a squared fashion ¹. In this topology, each router is directly connected to its neighbors, forming a square or grid-like topology. This topology is also known as a mesh topology.

Therefore, the correct answer is A. Limit the query domain by use of summarization.

Explanation 2

Explanation 3

The question is about how to design EIGRP topologies to converge as fast as possible in the event of a point-to-point link failure. According to one source, EIGRP convergence is the process of finding a new route path when a link or node failure occurs. The convergence time depends on how fast the failure is detected and how fast a new path is computed and updated.

One way to reduce the convergence time is to limit the query domain, which is the set of routers that need to be involved in the recomputation of a new path. This can be done by using summarization, default routes, or distribute lists. Summarization reduces the number of prefixes that need to be advertised and updated, default routes provide a backup path for unreachable destinations, and distribute lists filter the routing updates that are sent or received.

Another way to reduce the convergence time is to build neighbor adjacencies in a triangulated fashion, which means that each router has at least two EIGRP neighbors on different links. This provides redundancy and faster convergence in case of a link failure, as the router can switch to an alternate path without waiting for queries and replies.

Therefore, based on the information from the web, the correct answer to the question is E. Build neighbor adjacencies in a triangulated fashion.

Explanation 4

Explanation 5

In order to design EIGRP topologies that converge as fast as possible in the event of a point-to-point link failure, we can limit the query domain by use of summarization or default routes.

Summarization is a technique used to reduce the size of routing tables by aggregating multiple routes into a single route. This can be done by summarizing routes at the boundary between different network areas. By limiting the query domain through summarization, EIGRP can converge faster in the event of a point-to-point link failure.

Default routes are another way to limit the query domain. A default route is a route that is used when there is no other specific route available for a particular destination. By using default routes, EIGRP can limit the scope of its queries and converge faster in the event of a point-to-point link failure.

Therefore, option A and B are correct.

Option C and E are incorrect because building neighbor adjacencies in a squared or triangulated fashion does not affect how fast EIGRP topologies converge in the event of a point-to-point link failure.

Option D is incorrect because distribute lists are used to filter routing updates and do not affect how fast EIGRP topologies converge in the event of a point-to-point link failure.

Explanation 6

Explanation 7

The correct answer is E. Build neighbor adjacencies in a triangulated fashion. Building neighbor adjacencies in a triangulated fashion means that each router has at least two EIGRP neighbors that can provide backup paths for each destination prefix. This way, if a point-to-point link fails, the router can immediately switch to the backup path without waiting for EIGRP to send queries or updates. This can achieve fast convergence and minimize packet loss. Building neighbor adjacencies in a triangulated fashion also reduces the query domain and prevents query storms, which are situations where EIGRP queries propagate throughout the network and cause high CPU utilization and network instability. Building neighbor adjacencies in a squared fashion means that each router has only one EIGRP neighbor that can provide a backup path for each destination prefix. This way, if a point-to-point link fails, the router has to wait for EIGRP to send queries or updates to find a new path. This can cause slow convergence and packet loss. Building neighbor adjacencies in a squared fashion also increases the query domain and makes query storms more likely.

Explanation 8

The correct answer to the question is C. Build neighbor adjacencies in a squared fashion.

EIGRP (Enhanced Interior Gateway Routing Protocol) is an advanced distance-vector routing protocol used in Cisco networks. Convergence refers to the process of network devices reaching a consistent and stable state after a topology change or network event.

To design EIGRP topologies to converge as fast as possible in the event of a point-to-point link failure, building neighbor adjacencies in a squared fashion is recommended. Here’s a comprehensive explanation of this approach:

1. Neighbor Adjacencies: In EIGRP, routers establish neighbor adjacencies to exchange routing information and perform efficient route calculations. Neighbor adjacencies are formed with directly connected routers.
2. Squared Fashion: Building neighbor adjacencies in a squared fashion means connecting routers in a meshed or redundant manner. This is achieved by connecting each router to multiple neighbors, creating multiple point-to-point links.
3. Faster Convergence: When a point-to-point link failure occurs, the routers can quickly reroute traffic to alternate paths through other neighbor adjacencies. The redundant connections enable faster convergence because there are multiple paths available to reach destinations, allowing for quicker detection and rerouting around the failed link.
4. Reduced Query Domain: By building neighbor adjacencies in a squared fashion, the query domain for EIGRP can be limited. The query domain is the area in the network where routers generate queries to find alternate paths during convergence. By limiting the query domain, the impact of link failures is contained within a smaller area, resulting in faster convergence.

The other options listed in the question are not directly related to designing EIGRP topologies for faster convergence:

• Limiting the query domain by use of summarization (A) involves summarizing routes to reduce the size of the routing table and the number of updates exchanged between routers. While summarization can help optimize routing protocols, it does not specifically address faster convergence in the event of a point-to-point link failure.
• Limiting the query domain by use of default routes (B) involves using default routes to advertise a single route to a remote network instead of multiple specific routes. While default routes can help simplify routing tables and reduce the number of updates, they do not directly contribute to faster convergence in the event of a link failure.
• Limiting the query domain by use of distribute lists (D) involves using access control lists (ACLs) to filter or control the distribution of routing updates. While distribute lists can influence routing behavior, they are not designed specifically for faster convergence in the event of a link failure.
• Building neighbor adjacencies in a triangulated fashion (E) is not a recommended approach for EIGRP convergence. Triangulated connections can introduce unnecessary complexity and can potentially result in suboptimal routing decisions during convergence.

In summary, to design EIGRP topologies for fast convergence in the event of a point-to-point link failure, it is recommended to build neighbor adjacencies in a squared fashion. This approach provides redundancy, multiple paths, and a limited query domain, allowing for quick rerouting and faster convergence.

Explanation 9

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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