Features and Characteristics of EIGRP Protocol

Before you start

Objectives : You will be able to understand EIGRP and its feature after reading this blog.

Prerequisites: You have to know what is routing protocols and different types of routing protocols

Tags : Routing Protocols, Router, EIGRP, unequal cost load balancing, EIGRP configuration .

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Definition

Enhance interior Gateway routing protocol (EIGRP) is a hybrid protocol which have both types of features of distance vector protocols and link-state protocols.

EIGRP Features and Operations

EIGRP is a classless, distance-vector protocol that uses the concept of an autonomous system to describe a set of contiguous routers that run the same routing protocol and share routing information, which also includes the subnet mask in its route updates. This is a very big deal because by advertising subnet information, this robust protocol enables us to use VLSM and permits summarization to be included within the design of EIGRP networks.

EIGRP is sometimes referred to as a hybrid routing protocol or an advanced distance-vector protocol because it has characteristics of both distance-vector and some link-state protocols. For example, EIGRP doesn’t send link-state packets like OSPF does. Instead, it sends traditional distance-vector updates that include information about networks plus the cost of reaching them from the perspective of the advertising router.

EIGRP has a default hop count of 100, with a maximum of 255, but don’t let this confuse you because EIGRP doesn’t rely on hop count as a metric like RIP does. In EIGRP-speak, hop count refers to how many routers an EIGRP route update packet can go through before it will be discarded, which limits the size of the autonomous system (AS). So don’t forget that this isn’t how metrics are calculated with EIGRP!.

Tables used in EIGRP

Neighbor Table: Neighbor table Each router keeps state information about adjacent neighbors. When a newly discovered neighbor is found, its address and interface are recorded and the information is held in the neighbor table, stored in RAM. Sequence numbers are used to match acknowledgments with update packets. The last sequence number received from the neighbor is recorded so that out-of-order packets can be detected. We’ll get into this more, later in the chapter, when we look at the neighbor table and find out how it’s useful for troubleshooting links between neighbor routers.

Topology Table : The topology table is populated by the neighbor table and the Diffusing Update Algorithm (DUAL) calculates the best loop-free path to each remote network. It contains all destinations advertised by neighboring routers, holding each destination address and a list of neighbors that have advertised the destination. For each neighbor, the advertised metric (distance), which comes only from the neighbor’s routing table, is recorded, as well as, the FD. The best path to each remote network is copied and placed in the routing table and then IP will use this route to forward traffic to the remote network. The path copied to the routing table is called a successor router—think “successful” to help you remember. The path with a good, but less desirable, cost will be entered in the topology table as a backup link and called the feasible successor. Let’s talk more about these terms now.