to a destination are maintained in the topology table. The topology table includes the following fields: The EIGRP routing table holds the best routes to a destination. This information is retrieved from the topology table. Each EIGRP router maintains a routing table for each network protocol. A successor is a route selected as the primary route to use to reach a destination. DUAL identifies this route from the information contained in the neighbor and topology tables and places it in the routing table. There can be up to four successor routes for any particular route. These can be of equal or unequal cost and are identified as the best loop-free paths to a given destination. A copy of the successor routes is also placed in the topology table. A feasible successor (FS) is a backup route. These routes are identified at the same time the successors are identified, but they are only kept in the topology table. Multiple feasible successors for a destination can be retained in the topology table although it is not mandatory. A router views its feasible successors as neighbors downstream, or closer to the destination than it is. Feasible successor cost is computed by the advertised cost of the neighbor router to the destination. If a successor route goes down, the router will look for an identified feasible successor. This route will be promoted to successor status. A feasible successor must have a lower advertised cost than the existing successor cost to the destination. If a feasible successor is not identified from the existing information, the router places an Active status on a route and sends out query packets to all neighbors in order to recompute the current topology. The router can identify any new successor or feasible successor routes from the new data that is received from the reply packets that answer the query requests. The router will then place a Passive status on the route. The topology table can record additional information about each route. EIGRP classifies routes as either internal or external. EIGRP adds a route tag to each route to identify this classification. Internal routes originate from within the EIGRP autonomous system (AS). External routes originate outside the EIGRP AS. Routes learned or redistributed from other routing protocols, such as Routing Information Protocol (RIP), OSPF, and IGRP, are external. Static routes originating outside the EIGRP AS are external. The tag can be configured to a number between 0-255 to customize the tag.
Content 3.1 EIGRP Concepts 3.1.3 EIGRP design features EIGRP operates quite differently from IGRP. EIGRP is an advanced distance vector routing protocol and acts as a link-state protocol when updating neighbors and maintaining routing information. The advantages of EIGRP over simple distance vector protocols include the following: EIGRP routers converge quickly because they rely on DUAL. DUAL guarantees loop-free operation at every instant throughout a route computation allowing all routers involved in a topology change to synchronize at the same time. EIGRP makes efficient use of bandwidth by sending partial, bounded updates and its minimal consumption of bandwidth when the network is stable. EIGRP routers make partial, incremental updates rather than sending their complete tables. This is similar to OSPF operation, but unlike OSPF routers, EIGRP routers send these partial updates only to the routers that need the information, not to all routers in an area. For this reason, they are called bounded updates. Instead of using timed routing updates, EIGRP routers keep in touch with each other using small hello packets. Though exchanged regularly, hello packets do not use up a significant amount of bandwidth. EIGRP supports IP, IPX, and AppleTalk through protocol-dependent modules (PDMs). EIGRP can redistribute IPX RIP and SAP information to improve overall performance. In effect, EIGRP can take over for these two protocols. An EIGRP router will receive routing and service updates, updating other routers only when changes in the SAP or routing tables occur. Routing updates occur as they would in any EIGRP network, using partial updates. EIGRP can also take over for the AppleTalk Routing Table Maintenance Protocol (RTMP). As a distance vector routing protocol, RTMP relies on periodic and complete exchanges of routing information. To reduce overhead, EIGRP redistributes AppleTalk routing information using event-driven updates. EIGRP also uses a configurable composite metric to determine the best route to an AppleTalk network. RTMP uses hop count, which can result in suboptimal routing. AppleTalk clients expect RTMP information from local routers, so EIGRP for AppleTalk should be run only on a clientless network, such as a wide-area network (WAN) link.
Content 3.1 EIGRP Concepts 3.1.4 EIGRP technologies EIGRP includes many new technologies, each of which represents an improvement in operating efficiency, speed of convergence, or functionality relative to IGRP and other routing protocols. These technologies fall into one of the following four categories: 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, much the same way that OSPF routers do. EIGRP routers establish adjacencies as described in Figure . 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: Reliable Transport Protocol (RTP) is a transport-layer protocol that can guarantee ordered delivery of EIGRP packets to all neighbors. On an IP network, hosts use TCP to sequence packets and ensure their timely