Enhanced Interior Gateway Routing Protocol (EIGRP) Configurations Theory

Enhanced Interior Gateway Routing Protocol Configurations Theory

EIGRP is the advance version of Cisco's earlier version IGRP. Before you learn more about EIGRP let be familiar with IGRP.

Interior Gateway Routing Protocol (IGRP)

The Interior Gateway Routing Protocol (IGRP) is a Cisco-proprietary routing protocol for IP. it is a distance vector protocol.

• It uses a sophisticated metric based on bandwidth and delay.
• It uses triggered updates to speed-up convergence.
• It supports unequal-cost load balancing to a single destination.

IGRP is Cisco proprietary uses bandwidth, delay, reliability, load, and MTU as its metrics (bandwidth and delay be default).

IGRP's routing update period is every 90 seconds. Its hold-down period is 280 seconds, and its flush period is 630 seconds.

It also supports triggered updates and load balancing across unequal-cost paths.

IGRP requires an AS number in its router command; plus, when entering network numbers for the network command, they are entered as the classful network number, as they are for RIP.

IGRP supports both equal- and unequal-cost paths for load balancing to single destination Equal-cost paths are enabled by default, where IGRP supports up to six equal-cost paths (four by default) to a single destination in the IP routing table. IGRP, however, also supports unequal-cost paths, but this feature is disabled by default.

Enhanced Interior Gateway Routing Protocol

The Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco-proprietary routing protocol for IP. These characteristics include:

• Fast convergence
• Loop-free topology
• VLSM and route summarization
• Multicast and incremental updates
• Routes for multiple routed protocols

Here is a brief comparison of EIGRP and IGRP:

• Both offer load balancing across six paths (equal or unequal).
• They have similar metric structures.
• EIGRP has faster convergence (triggered updates and saving a neighbor's routing table locally).
• EIGRP has less network overhead, since it uses incremental updates.

Interesting point about these protocols is that if you have some routers in your network running IGRP and others running EIGRP and both sets have the same autonomous system number, routing information will automatically be shared between the two.

• EIGRP uses a 32-bit metric, while IGRP uses a 24-bit metric.
• EIGRP uses the Diffusing Update Algorithm (DUAL) to update the routing table.
• One really unique feature of EIGRP is that it supports three routed protocols: IP, IPX, and AppleTalk
• Hello packets are generated every five seconds on LAN interfaces as multicasts (224.0.0.10).

For EIGRP routers to become neighbors, the following information must match:

• The AS number
• The K-values (these enable/disable the different metric components)

When two routers determine whether they will become neighbors, they go through the following process:

• The first router generates a Hello with configuration information.
• If the configuration information matches, the second router responds with an Update message with topology information.
• The first router responds with an ACK message, acknowledging the receipt of the second's ACK.
• The first router sends its topology to the second router via an Update message.
• The second router responds back with an ACK.

You must specify the AS number when configure EIGRP. Even though EIGRP is classless, you must configure it as a classful protocol when specifying your network numbers with the network command.

EIGRP Terms

Term	Definition
Successor	The best path to reach a destination within the topology table.
Feasible successor	The best backup path to reach a destination within the topology table—multiple successors can be feasible for a particular destination.
Routing table	This is all of the successor routes from the topology table. There is a separate routing table for each routed protocol.
Advertised distance	The distance (metric) that a neighboring router is advertising for a specific route.
Feasible distance	The distance (metric) that your router has computed to reach a specific route: the advertised distance from the neighboring router plus the local router's interface metric.
Neighbor table	Contains a list of the EIGRP neighbors and is similar to the adjacencies that are built in OSPF between the designated router/backup DR and the other routers on a segment. Each routed protocol (IP, IPX, and AppleTalk) for EIGRP has its own neighbor table.
Topology table	Similar to OSPF's database, contains a list of all destinations and paths the EIGRP router learned—it is basically a compilation of the neighboring routers' routing tables. A separate topology table exists for each routed protocol.