A dynamic, easily managed proprietary routing protocol that can add reliability and scalability to medium-size to large Cisco internetworks, Enhanced Interior Gateway Routing Protocol (EIGRP) uses a combination of distance vector and link-state routing algorithms. EIGRP also offers support for IP, IPX, and AppleTalk routing.
EIGRP is a robust routing protocol offering many features. Benefits include the following:
- Equal-cost load balancing—routes traffic equally over multiple links
- Classless routing—support for subnetting and Variable Length Subnet Mask (VLSM)
- Neighbor discovery—establishes peer relationships via the Hello protocol
- Incremental updates—sends only updates to the route table instead of sending the entire route table
- Multiprotocol routing—supports IP, IPX, and AppleTalk
How EIGRP works
EIGRP builds routing databases using the Hello protocol. When EIGRP is implemented on a router, the router sends out small Hello packets via the multicast address 22.214.171.124. When the router’s neighbors (i.e. routers on the same subnet) receive the Hello packets, they respond with Hello packets of their own.
What happens next? The router uses the Hello packets it has received to build three tables for each protocol being routed through the network. For example, if a network were running IP, IPX, and AppleTalk, a total of nine tables would be created, three for each protocol.
The first table created is the Neighbor table. The neighbor table contains address, interface, uptime, and other information about neighboring routers.
The next table created is the Topology table. This is the repository for all of the valid routes throughout the network.
The last table is the Routing table. It is built from the topology table using the Diffusing Update Algorithm (DUAL). The routing table is essentially a listing of the best routes through the internetwork. The metrics used to determine the best route through the internetwork include bandwidth, delay, reliability, load, and Maximum Transmission Unit (MTU).
Benefits of using EIGRP
EIGRP is a dynamic, easily managed proprietary routing protocol that can add reliability and scalability to medium-size to large Cisco internetworks. For more information on implementing and maintaining EIGRP, check out EIGRP Network Design Solutions from Cisco Press.
Warren Heaton CCDA, CCNA, MCSE+I, is the Cisco Program Manager for A Technological Advantage in Louisville, KY.
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