Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco proprietary routing protocol. You can only use it in an all-Cisco network, but EIGRP more than makes up for this deficiency by being easy to configure, fast, and reliable. A detailed discussion of the protocol's theory and operation is out of the scope of this book. If you are unfamiliar with EIGRP in general, or need more detail on how the protocol works, we recommend reading the relevant sections of IP Routing (O'Reilly).
Like RIP, EIGRP is based on a distance vector algorithm that determines the best path to a destination. But EIGRP uses a more complex metric than RIP's simple hop count. The EIGRP metric is based on the minimum bandwidth and net delay along each possible path, which means that EIGRP can accommodate larger networks than RIP. It also means that EIGRP needs a different algorithm for loop removal, because EIGRP can't simply increment the hop count to infinity to eliminate a loop, as RIP does. EIGRP uses a more sophisticated algorithm called Diffusing Update Algorithm (DUAL).
The DUAL algorithm ensures that every router can individually make sure that its routing table is always free from loops. EIGRP also allows the router to take advantage of several different possible paths, if they all have the same metric. This facilitates load sharing among equal cost links. Further, the EIGRP topology database on each router keeps track of higher cost candidates for the same destinations. This helps routing tables throughout the network to reconverge quickly after a topology change such as a link or router failure.
It is the sophisticated DUAL algorithm that distinguishes EIGRP from the earlier Cisco proprietary protocol, called Interior Gateway Routing Protocol (IGRP). IGRP is rarely used anymore, except for backward compatibility with older networks. Rather than implementing a new network with IGRP, we recommend using either EIGRP or OSPF. In fact, Cisco includes many useful features such as automatic two-way redistribution that make the migration from IGRP to EIGRP relatively straightforward.
EIGRP operates very efficiently over large networks. It achieves this efficiency in part by sending non-periodic updates. This means that, unlike RIP, EIGRP only distributes information about routes that have changed, and only when there is a change to report. The rest of the time, routers only exchange small "Hello" packets to verify that routing peers are still available. So, in a relatively stable network, EIGRP uses very little bandwidth. This is especially useful in WAN configurations.
It is also extremely efficient over LAN portions of a network. On each network segment, routers exchange routing information using multicast packets, which helps to limit bandwidth usage on segments that hold many routers. EIGRP uses multicast address 224.0.0.10, sending packets as raw IP packets using protocol number 88. These multicast packets are always sent with a TTL value of 1 to ensure that locally relevant routing information doesn't leak off the local segment and confuse routers elsewhere in the network.
Every router in an EIGRP network includes a topology table, which is a central feature of the DUAL algorithm. Every time a router receives a new piece of routing information from one of its neighbors, it updates the topology table. This helps to give it a reliable and up-to-date image of all of the connections in the network that are currently in use. Every destination subnet known to EIGRP appears in the topology table.
EIGRP includes many of the features such as Classless Inter-Domain Routing (CIDR) and Variable Length Subnet Masks (VLSM) that are needed in larger networks. But we suspect that this protocol owes most of its popularity to the fact that it is considerably easier to configure in medium-sized to large networks than other protocols with similar capabilities (such as OSPF).
Much of this chapter will discuss special features that Cisco has built into this protocol to help improve scalability. A detailed discussion of design guidelines for building scalable and reliable EIGRP networks is out of the scope of this book. Please refer to Designing Large-Scale LANs (O'Reilly) for information about efficient EIGRP architectures.
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