Content Overview Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco-proprietary routing protocol based on Interior Gateway Routing Protocol (IGRP). Unlike IGRP, which is a classful routing protocol, EIGRP supports classless interdomain routing (CIDR), allowing network designers to maximize address space by using CIDR and variable-length subnet mask (VLSM). Compared to IGRP, EIGRP boasts faster convergence times, improved scalability, and superior handling of routing loops. Furthermore, EIGRP can replace Novell Routing Information Protocol (RIP) and AppleTalk Routing Table Maintenance Protocol (RTMP), serving both IPX and AppleTalk networks with powerful efficiency. EIGRP is often described as a hybrid routing protocol, offering the best of distance vector and link-state algorithms. EIGRP is an advanced routing protocol that relies on features commonly associated with link-state protocols. Some of the best features of OSPF, such as partial updates and neighbor discovery, are similarly put to use by EIGRP. However, EIGRP is easier to configure than OSPF. EIGRP is an ideal choice for large, multi-protocol networks built primarily on Cisco routers. This module covers common EIGRP configuration tasks. Particular attention is paid to the ways in which EIGRP establishes relationships with adjacent routers, calculates primary and backup routes, and when necessary, responds to failures in known routes to a particular destination. A network is made up of many devices, protocols, and media that allow data communication to happen. When one piece of the network does not work properly, one or two users may be unable to communicate, or the entire network may fail. In either case, the network administrator must quickly identify and troubleshoot problems when they arise. Network problems commonly result from the following: A network administrator should approach troubleshooting in a methodical manner, using a general problem-solving model. It is often useful to check for physical layer problems first and then move up the layers in an organized manner. Although this module will focus on troubleshooting the operation of routing protocols, which work at Layer 3, it is important to eliminate any problems that may exist at lower layers. Students completing this module should be able to:
Content 3.1 EIGRP Concepts 3.1.1 Comparing EIGRP with IGRP Cisco released EIGRP in 1994 as a scalable, improved version of its proprietary distance vector routing protocol, IGRP. The same distance vector technology found in IGRP is used in EIGRP, and the underlying distance information remains the same. EIGRP improves the convergence properties and the operating efficiency significantly over IGRP. This allows for an improved architecture while retaining the existing investment in IGRP. Comparisons between EIGRP and IGRP fall into the following major categories: IGRP and EIGRP are compatible with each other. This compatibility provides seamless interoperability with IGRP routers. This is important so users can take advantage of the benefits of both protocols. EIGRP offers multiprotocol support, but IGRP does not. EIGRP and IGRP use different metric calculations. EIGRP scales the metric of IGRP by a factor of 256. That is because EIGRP uses a metric that is 32 bits long, and IGRP uses a 24-bit metric. By multiplying or dividing by 256, EIGRP can easily exchange information with IGRP. IGRP has a maximum hop count of 255. EIGRP has a maximum hop count limit of 224. This is more than adequate to support the largest, properly designed internetworks. Enabling dissimilar routing protocols such as OSPF and RIP to share information requires advanced configuration. Redistribution, the sharing of routes, is automatic between IGRP and EIGRP as long as both processes use the same autonomous system (AS) number. In Figure , RTB automatically redistributes EIGRP-learned routes to the IGRP AS, and vice versa. EIGRP will tag routes learned from IGRP or any outside source as external because they did not originate from EIGRP routers. IGRP cannot differentiate between internal and external routes. Notice that in the show ip route command output for the routers in Figure , EIGRP routes are flagged with D, and external routes are denoted by EX. RTA identifies the difference between the network learned via EIGRP (172.16.0.0) and the network that was redistributed from IGRP (192.168.1.0). In the RTC table, the IGRP protocol makes no such distinction. RTC, which is running IGRP only, just sees IGRP routes, despite the fact that both 10.1.1.0 and 172.16.0.0 were redistributed from EIGRP. Interactive Media Activity Checkbox: IGRP and EIGRP Comparison When the student has completed this activity, the student will be able to identify the difference between IGRP and EIGRP.
Content 3.1 EIGRP Concepts 3.1.2 EIGRP concepts and terminology EIGRP routers keep route and topology information readily available in RAM, so they can react quickly to changes. Like OSPF, EIGRP saves this information in several tables and databases. EIGRP saves routes that are learned in specific ways. Routes are given a particular status and can be tagged to provide additional useful information. EIGRP maintains three tables: 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. 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 within the hold time, then the hold time expires. When the hold time expires, the Diffusing Update Algorithm (DUAL), which is the EIGRP distance vector algorithm, is informed of the topology change and must recalculate the new topology. The topology table is made up of all the EIGRP routing tables in the autonomous system. DUAL takes the information supplied in the neighbor table and the topology table and calculates the lowest cost routes to each destination. By tracking this information, EIGRP routers can identify and switch to alternate routes quickly. The information that the router learns from the DUAL is used to determine the successor route, which is the term used to identify the primary or best route. A copy is also placed in the topology table. Every EIGRP router maintains a topology table for each configured network protocol. All learned routes