![]() Delay refers to the router’s perception of the time it takes to send a frame over the link. Bandwidth refers to the perceived speed of each link. The words bandwidth and delay have specific meaning with EIGRP. The cumulative delay for all links in the routeĪs a result, EIGRP defines the concept of the best route based on the constraining bandwidth (speed) of the links in the route, plus the total delay in the route.The slowest link in the end-to-end route.More importantly, that function uses two input variables by default: EIGRP uses a math function to calculate the metric. One of the most compelling reasons to consider using EIGRP instead of other IGPs is the strength of the EIGRP metric. EIGRP Topology and the Metric Calculation R1 notices that 15 seconds pass without receiving a hello from R2, so then R1 can choose new routes that do not use R2 as a next-hop router. If R2 does fail, R2 no longer sends hello messages. Under normal conditions, R1 receives hellos from R2 every 5 seconds, well within R1’s hold interval (15 seconds) before R1 would consider R2 to have failed. Routers also must receive a hello from a neighbor with a time called the hold interval, with a default setting of three times the hello interval.įor instance, imagine both R1 and R2 use default settings of 5 and 15 for their hello and hold intervals. For instance, routers R1 and R2 do not have to send their hellos at the same time. The routers use their own independent hello interval, which defines the time period between each EIGRP hello. The EIGRP hello message and protocol defines that each router should send a periodic hello message on each interface, so that all EIGRP routers know that the router is still working. Instead, EIGRP defines the concept of a neighbor relationship, using EIGRP hello messages to monitor that relationship. The fact that EIGRP does not send routing information on a short periodic timed basis greatly reduces EIGRP overhead traffic, but it also means that EIGRP cannot rely on these updates to monitor the state of neighboring routers. Then, over time, as facts change, the router simply reacts, sending partial updates with the new information. When a router first comes up, it advertises known routing information. Unlike RIP, EIGRP does not send full or partial update messages based on a periodic timer. EIGRP Maintains Neighbor Status Using Hello Regardless, at the end of the day, once you enable EIGRP on all your routers and Layer 3 switches, the devices will learn good routes for all the subnets in the network. Note that this section does not attempt to mention all the features of EIGRP, but instead to give some highlights that point out some of EIGRP’s unique features compared to other IGPs. This third of four sections of the chapter discusses some of those similarities and differences, so you get a sense of how EIGRP works. Of course, EIGRP works differently in several ways as compared to RIPv2. Routers that use EIGRP send messages so that other routers learn routes, they listen for messages to learn routes, they choose the best route among multiple routes to the same subnet based on a metric, and they react and converge when the network topology changes. Regardless of what label you put on EIGRP, the protocol uses several features that work either like basic DV protocols such as RIP, or they work similarly enough. Frankly, over the years, different Cisco documents and different books (mine included) have characterized EIGRP as either its own category, called a balanced hybrid routing protocol, or as some kind of advanced DV protocol. Instead of naming the original IGRP Version 1, and the new one IGRP Version 2, Cisco named the new version Enhanced IGRP (EIGRP).ĮIGRP acts a little like a DV protocol, and a little like no other routing protocol. Cisco has already created the Interior Gateway Routing Protocol (IGRP) in the 1980s, and the same needs that drove people to create RIPv2 and OSPF drove Cisco to improve IGRP as well. ![]() ![]() Enhanced Interior Gateway Routing Protocol (EIGRP) went through a similar creation process as compared to RIP, but with the work happening inside Cisco. ![]()
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