In present day and future routing environments, Enhanced Interior Gateway Routing Protocol(EIGRP) offers benefits and features over historial distance-vector routing protocols like Routing Information Protocol version1(RIPv1) and Interior Gateway Routing Protocol(IGRP). These benefits include rapid convergence, lower bandwidth utilization, and multiple routed protocol support(IP, Internet Packet Exchange(IPX), and Appletalk).
EIGRP uses three databases in the path selection process: the EIGRP neighbor table, the EIGRP topology table, and the IP routing table. The neighbor table contains a list of directly connected EIGRP routers that have established an adjacency with a given router. The topology table includes route entries for all destinations that the router has learned. EIGRP chooses the best successor routers to a destination from the topology table and places these routers in the routing table.
EIGRP supports five packet type: hello, update, query, reply, and acknowledgement(ACK). Fields in the hello packet build adjacencies between EIGRP neighbors. The CISCO ISO software debug and show commands are used to troubleshoot EIGRP neighbor adjacencies problems.
The EIGRP Diffusing Update Algorithm(DUAL) finite state machine embodies the decision process for all route computations. DUAL tracks all routers advertised by all EIGRP neighbors, then uses a formula to calculate the best route.
A basic EIGRP configuration contains a default network and uses the wildcard option for the network statement. The Cisco IOS show commands can be used to troubleshoot EIGRP configuration problems. Advanced configuration options for EIGRP include manual route summarization, unequal path-cost load balancing, and limiting EIGRP bandwidth utilization on WAN links.
EIGRP is scaled in large and growing internetworks by using two methods to perform query scoping, with limits the range of EIGRP queries. These two methods are: effective route summarization and the EIGRP stub command.
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