This chapter introduces you to Enhanced Interior Gateway Routing Protocol (EIGRP). This chapter covers the following topics:
- EIGRP Overview
- EIGRP Terminology and Operation
- Configuring and Verifying EIGRP
- Configuring EIGRP Authentication
- Using EIGRP in an Enterprise Network
- Verifying EIGRP Operation
In present-day and future routing environments, Enhanced Interior Gateway Routing Protocol (EIGRP) offers benefits and features over historic distance vector routing protocols, such as Routing Information Protocol Version 1 (RIPv1) and Interior Gateway Routing Protocol (IGRP). These benefits include rapid convergence, lower bandwidth utilization, and multiple-routed protocol support.
This chapter introduces EIGRP terminology and concepts and EIGRP configuration, verification, and troubleshooting. The chapter also explores topics such as route summarization, load balancing, bandwidth usage, and authentication. The chapter concludes with a discussion of EIGRP design and configuration techniques to implement an effective enterprise network.
This section introduces EIGRP and describes its four underlying technologies.
EIGRP Capabilities and Attributes
EIGRP is a Cisco-proprietary protocol that combines the advantages of link-state and distance vector routing protocols. EIGRP has its roots as a distance vector routing protocol and is predictable in its behavior. Like its predecessor IGRP, EIGRP is easy to configure and is adaptable to a wide variety of network topologies. What makes EIGRP an advanced distance vector protocol is the addition of several link-state features, such as dynamic neighbor discovery. EIGRP is an enhanced IGRP because of its rapid convergence and the guarantee of a loop-free topology at all times. Features of this hybrid protocol include the following:
- Fast convergence—EIGRP uses the Diffusing Update Algorithm (DUAL) to achieve rapid convergence. A router running EIGRP stores its neighbors' routing tables so that it can quickly adapt to changes in the network. If no appropriate route or backup route exists in the local routing table, EIGRP queries its neighbors to discover an alternative route. These queries are propagated until an alternative route is found, or it is determined that no alternative route exists.
- Variable-length subnet masking (VLSM) support—EIGRP is a classless routing protocol, which means that it advertises a subnet mask for each destination network; this enables EIGRP to support discontinuous subnetworks and VLSM.
- Partial updates—EIGRP sends partial triggered updates instead of periodic updates. These updates are sent only when the path or the metric for a route changes; they contain information about only that changed link rather than the entire routing table. Propagation of these partial updates is automatically bounded so that only those routers that require the information are updated. As a result, EIGRP consumes significantly less bandwidth than IGRP. This behavior is also different than link-state protocol operation, which sends a change update to all routers within an area.
- Multiple network layer support—EIGRP supports IP, AppleTalk, and Novell NetWare Internetwork Packet Exchange (IPX) using protocol-dependent modules that are responsible for protocol requirements specific to the network layer. EIGRP's rapid convergence and sophisticated metric offer superior performance and stability when implemented in IP, IPX, and AppleTalk networks.
Other EIGRP features include the following:
- Seamless connectivity across all data link layer protocols and topologies—EIGRP does not require special configuration to work across any Layer 2 protocols. Other routing protocols, such as Open Shortest Path First (OSPF), require different configurations for different Layer 2 protocols, such as Ethernet and Frame Relay (as you will see in Chapter 4, "Configuring the Open Shortest Path First Protocol"). EIGRP was designed to operate effectively in both local-area network (LAN) and wide-area network (WAN) environments. In multiaccess topologies, such as Ethernet, neighbor relationships are formed and maintained using reliable multicasting. EIGRP supports all WAN topologies: dedicated links, point-to-point links, and nonbroadcast multiaccess (NBMA) topologies. EIGRP accommodates differences in media types and speeds when neighbor adjacencies form across WAN links. The amount of bandwidth that EIGRP uses on WAN links can be limited.
- Sophisticated metric—EIGRP uses the same algorithm for metric calculation as IGRP, but represents values in a 32-bit format, rather than IGRP's 24-bit format, to give additional granularity (thus, the EIGRP metric is the IGRP metric multiplied by 256). A significant advantage of EIGRP (and IGRP) over other protocols is its support for unequal metric load balancing that allows administrators to better distribute traffic flow in their networks.
- Use of multicast and unicast—EIGRP uses multicast and unicast for communication between routers, rather than broadcast. As a result, end stations are unaffected by routing updates or queries. The multicast address used for EIGRP is 184.108.40.206.
Like most IP routing protocols, EIGRP relies on IP packets to deliver routing information (Integrated Intermediate System-to-Intermediate System [IS-IS] is the exception, as you will see in Chapter 6, "Configuring the Integrated Intermediate System-to-Intermediate System Protocol"). The EIGRP routing process is a transport layer function of the Open System Interconnection (OSI) reference model. IP packets carrying EIGRP information have protocol number 88 in their IP header, as illustrated in Figure 3-1.
Figure 3-1 EIGRP Is a Transport Layer Function
Figure 3-2 illustrates how EIGRP performs automatic route summarization at major network boundaries. Administrators can also configure manual summarization on arbitrary bit boundaries on any router interface (as long as a more-specific route exists in the routing table) to shrink the size of the routing table. EIGRP also supports the creation of supernets or aggregated blocks of addresses (networks).
Figure 3-2 EIGRP Performs Route Summarization by Default
EIGRP supports both hierarchical and nonhierarchical IP addressing.
Underlying Processes and Technologies
EIGRP uses the following four key technologies that combine to differentiate it from other routing technologies:
- Neighbor discovery/recovery mechanism—EIGRP's neighbor discovery mechanism enables routers to dynamically learn about other routers on their directly attached networks. Routers also must discover when their neighbors become unreachable or inoperative. This process is achieved with low overhead by periodically sending small hello packets. As long as a router receives hello packets from a neighboring router, it assumes that the neighbor is functioning, and the two can exchange routing information.
Reliable Transport Protocol (RTP)—RTP is responsible for guaranteed, ordered delivery of EIGRP packets to all neighbors. RTP supports intermixed transmission of multicast or unicast packets. For efficiency, only certain EIGRP packets are transmitted reliably.
For example, on a multiaccess network that has multicast capabilities, such as Ethernet, it is not necessary to send hello packets reliably to all neighbors individually, so EIGRP sends a single multicast hello packet containing an indicator that informs the receivers that the packet need not be acknowledged. Other types of packets, such as updates, indicate in the packet that acknowledgment is required. RTP contains a provision for sending multicast packets quickly even when unacknowledged packets are pending, which helps ensure that convergence time remains low in the presence of varying speed links.
- DUAL finite-state machine—DUAL embodies the decision process for all route computations. DUAL tracks all routes advertised by all neighbors and uses distance information, known as a metric or cost, to select efficient, loop-free paths to all destinations.
- Protocol-dependent modules—EIGRP's protocol-dependent modules are responsible for network layer protocol-specific requirements. EIGRP supports IP, AppleTalk, and Novell NetWare; each protocol has its own EIGRP module and operates independently from any of the others that might be running. The IP-EIGRP module, for example, is responsible for sending and receiving EIGRP packets that are encapsulated in IP. Likewise, IP-EIGRP is also responsible for parsing EIGRP packets and informing DUAL of the new information that has been received. IP-EIGRP asks DUAL to make routing decisions, the results of which are stored in the IP routing table. IP-EIGRP is also responsible for redistributing routes learned by other IP routing protocols.