What is IGRP and EIGRP Routing Protocols

What is IGRP and EIGRP Routing Protocols

The routing protocols IGRP and EIGRP are summarized in great detail to help you master network technology and enter the IT world. Today we will review the routing protocols IGRP and EIGRP. From their names alone, we can guess that there are many associations between them.


IGRP is an internal gateway routing protocol designed by Cisco in the mid-1980s. Use composite user configuration metrics, including latency, bandwidth, reliability, and load. It has a high span in the same autonomous system and is suitable for complex networks. Cisco IOS allows the router administrator to set the weight of IGRP’s network bandwidth, delay, reliability and load to affect the calculation of metrics.

IGRP is a Cisco source proprietary routing protocol that provides routing functions in an autonomous system. In the mid-1980s, the most commonly used internal routing protocol was Routing Information Protocol (RIP). Although RIP is very useful for the routing of small or medium-sized interconnection networks of the same model, with the continuous development of the network, its limitations are becoming more and more obvious. The practicality of Cisco routers and the powerful functionality of IGRP make many small Internet organizations use IGRP to replace RIP.

As early as the 1990s, Cisco introduced enhanced IGRP, which further improved the operating efficiency of IGRP. For greater flexibility, IGRP supports multi-path routing services. In the Round Robin mode, two lines with the same bandwidth can run a single communication flow. If one line fails to transmit, the system will automatically switch to the other line. Multipath can be multi-path lines with different standards but still work.

IGRP maintains a set of timers and variables with time intervals. It includes update timer, expiration timer, hold timer and clear timer. The update timer specifies the frequency at which route update messages should be sent. The default value in IGRP is 90 seconds. The expiration timer specifies how long the router should wait before declaring that the route is invalid when there is no route update message for a specific route. This value in IGRP defaults to three times the update cycle. The holding time variable specifies a hold down period. This value in IGRP defaults to three times the update period plus 10 seconds, that is, 280 seconds. Finally, the clearing timer specifies the waiting time before the router empties the route table. The default value of IGRP is seven times the route update cycle.


EIGRP: Enhanced Interior Gateway Routing Protocol. Enhanced internal gateway routing protocol. EIGRP is a private agreement of Cisco (it was publicly owned in 2013). EIGRP combines the Cisco special protocol of link state and distance vector routing protocol, and uses the diffusion correction algorithm (DUAL) to achieve rapid convergence. It can not send regular route update information to reduce the bandwidth occupation.

EIGRP uses DUAL to achieve fast convergence. The router running EIGRP stores the neighbor’s routing table, so it can quickly adapt to changes in the network. If there is no suitable route in the local route table and no suitable alternative route in the topology table, EIGRP will query the neighbors to find alternative routes. The query will continue to propagate until an alternative route is found or it is determined that there is no alternative route.

Furthermore, EIGRP sends partial updates instead of regular updates, and only sends when the routing path or metric changes. Only the changed link information is included in the update, not the entire routing table, which can reduce the bandwidth consumption. In addition, it also automatically limits the propagation of these partial updates and only transmits them to the required routers. Therefore, EIGRP consumes much less bandwidth than IGRP. This behavior is also different from the link state routing protocol, which sends updates to all routers in the region.

EIGRP uses a variety of parameters to calculate metric values to the target network, including bandwidth, delay, reliability, loading, and MTU. These five parameters are represented by K values, namely K1, K2, K3, K4, and K5. If the five K values between two EIGRP routers are different, it means that the two parties have different methods to calculate metric values; Whether EIGRP or other protocols, when it is necessary to use bandwidth to calculate metric, only the bandwidth in the outgoing direction of the interface is calculated, while the bandwidth in the incoming direction of the interface is not included. That is, on a link, only the bandwidth of one outgoing interface is calculated, while the bandwidth of the incoming interface is ignored.

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