various show commands. Figure lists the key EIGRP show commands and briefly discusses their functions. The Cisco IOS debug feature also provides useful EIGRP monitoring commands. Lab Activity Lab Exercise: Verifying Basic EIGRP Configuration This lab is to setup an IP addressing scheme for the network and to verify EIGRP configuration. Lab Activity e-Lab Activity: Verifying Basic EIGRP In this lab, the student will configure and verify EIGRP Routing.

Content 3.2 EIGRP Configuration 3.2.4 Building neighbor tables Simple distance vector routers do not establish any relationship with their neighbors. RIP and IGRP routers merely broadcast or multicast updates on configured interfaces. In contrast, EIGRP routers actively establish relationships with their neighbors as do OSPF routers. The neighbor table is the most important table in EIGRP. Each EIGRP router maintains a neighbor table that lists adjacent routers. This table is comparable to the adjacency database used by OSPF. There is a neighbor table for each protocol that EIGRP supports. EIGRP routers establish adjacencies with neighbor routers by using small hello packets. Hellos are sent by default every five seconds. An EIGRP router assumes that, as long as it is receiving hello packets from known neighbors, those neighbors and their routes remain viable or passive. By forming adjacencies, EIGRP routers do the following:

Dynamically learn of new routes that join their network
Identify routers that become either unreachable or inoperable
Rediscover routers that had previously been unreachable

The following fields are found in a neighbor table:

Neighbor address – This is the network layer address of the neighbor router.
Hold time – This is the interval to wait without receiving anything from a neighbor before considering the link unavailable. Originally, the expected packet was a hello packet, but in current Cisco IOS software releases, any EIGRP packets received after the first hello will reset the timer.
Smooth Round-Trip Timer (SRTT) – This is the average time that it takes to send and receive packets from a neighbor. This timer is used to determine the retransmit interval (RTO).
Queue count (Q Cnt) – This is the number of packets waiting in a queue to be sent. If this value is constantly higher than zero, there may be a congestion problem at the router. A zero means that there are no EIGRP packets in the queue.
Sequence Number (Seq No) – This is the number of the last packet received from that neighbor. EIGRP uses this field to acknowledge a transmission of a neighbor and to identify packets that are out of sequence. The neighbor table is used to support reliable, sequenced delivery of packets and can be regarded as analogous to the TCP protocol used in the reliable delivery of IP packets.

Interactive Media Activity Crossword Puzzle: EIGRP Concepts and Terminology When the student has completed this activity, the student will understand the different EIGRP concepts and terminology.

Content 3.2 EIGRP Configuration 3.2.5 Discover routes EIGRP routers keep route and topology information available in RAM, so changes can be reacted to quickly. Like OSPF, EIGRP keeps this information in several tables or databases. The EIGRP distance vector algorithm, DUAL, uses the information gathered in the neighbor and topology tables and calculates the lowest cost route to the destination. The primary route is called the successor route. When calculated, DUAL places the successor route in the routing table and a copy in the topology table. DUAL also attempts to calculate a backup route in case the successor route fails. This is called the feasible successor route. When calculated, DUAL places the feasible route in the topology table. This route can be called upon if the successor route to a destination becomes unreachable or unreliable.

Content 3.2 EIGRP Configuration 3.2.6 Select routes If a link goes down, DUAL looks for an alternative route path, or feasible successor, in the topology table. If a feasible successor is not found, the route is flagged as Active, or unusable at present. Query packets are sent to neighboring routers requesting topology information. DUAL uses this information to recalculate successor and feasible successor routes to the destination. Once DUAL has completed these calculations, the successor route is placed in the routing table. Then both the successor route and feasible successor route are placed in the topology table. The route to the final destination will now pass from an Active status to a Passive status. This means that the route is now operational and reliable. The sophisticated algorithm of DUAL results in EIGRP having exceptionally fast convergence. To better understand convergence using DUAL, consider the example in Figure . All routers have built a topology table that contains information about how to route to destination network Z. Each table identifies the following:

The routing protocol or EIGRP
The lowest cost of the route or Feasible Distance (FD)
The cost of the route as advertised by the neighboring router or Reported Distance (RD)

The Topology heading identifies the preferred primary route, which is called the successor route (Successor). If it is identified, the Topology heading will also identify the backup route, which is called the feasible successor (FS). Note that it is not necessary to have an identified feasible successor.

Content 3.2 EIGRP Configuration 3.2.7 Maintaining routing tables DUAL tracks all routes advertised by neighbors using the composite metric of each route to compare them. DUAL also guarantees that each path is loop-free. Lowest-cost paths are then inserted by the DUAL algorithm into the routing table. These primary routes are known as successor routes. A copy of the successor paths is placed in the topology table. EIGRP keeps important route and topology information readily available in a neighbor table and a topology table. These tables supply DUAL with comprehensive route information in case of network disruption. DUAL selects alternate routes quickly by using the information in these tables. If a link goes down, DUAL looks for an alternative route path, or feasible successor, in the topology table. If a feasible successor is not found, the route is flagged as active, or unusable at present. Query packets are sent to neighboring routers requesting topology information. DUAL uses this information to recalculate successor and feasible successor routes to the destination. Once DUAL has completed these calculations, the successor route is placed in the routing table. Then both the successor route and feasible successor route are placed in the topology table. The route to the final destination will now pass from an active status to a passive status. This means that the route is now operational and reliable. EIGRP routers establish and maintain adjacencies with neighbor routers by using small hello packets. Hellos are sent by default every five seconds. An EIGRP router assumes that, as long as it is receiving hello packets from known neighbors, those neighbors and their routes remain viable, or passive. When newly discovered neighbors are learned, the address and interface of the neighbor is recorded. This information is stored in the neighbor data structure. When a neighbor sends a hello packet, it advertises a hold time. The hold time is the amount of time a router treats a neighbor as reachable and operational.In other words, if a hello packet is not heard from within the hold time, the hold time expires. When the hold time expires, DUAL is informed of the topology change, and must recalculate the new topology. In the example in Figures - , DUAL must reconstruct the topology following the discovery of a broken link between router D and router B. The new successor routes will be placed in the updated routing