Enhanced Interior Gateway Routing Protocol ( EIGRP) is a Cisco proprietary routing protocol. Because it is proprietary, you can use it only 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. So if you are unfamiliar with EIGRP in general, or if you need more detail on how the protocol works, we recommend reading the relevant sections from IP Routing by Ravi Malhotra (O'Reilly).
Like RIP, EIGRP is based on a distance vector algorithm to determine the best path to a destination. But EIGRP uses a more complex metric than RIP's simple hop count. The EIGRP metric, which is based on the minimum bandwidth and net delay along each possible path, means that EIGRP can accommodate larger networks than RIP. But it also means that EIGRP needs a different algorithm for loop removal. This is because in EIGRP it isn't possible to simply increment the hop count to infinity to eliminate a loop, as RIP does. EIGRP uses a more sophisticated algorithm is 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. It 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.
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 that are used 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 by using multicast packets, which helps to limit bandwidth usage on segments that hold many routers. EIGRP uses a multicast address 224.0.0.10, sending packets as raw IP packets by 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.
EIGRP includes many of the features that are needed in larger networks, such as Classless Interdomain Routing (CIDR) and Variable Length Subnet Masks (VLSM). But we suspect that this protocol owes most of its popularity to the fact that it is considerably easier to configure in a medium-sized to large network than other protocols with similar capabilities, such as Open Shortest Path First (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 by Kevin Dooley (O'Reilly) for information about efficient EIGRP architectures.