# EIGRP - Enhanced Interior Gateway Routing Protocol ## What is EIGRP EIGRP stands for Enhanced Interior Gateway Routing Protocol. It is an advanced distance vector routing protocol developed by Cisco. EIGRP combines features of both distance vector and link state protocols, making it sometimes called a hybrid or advanced distance vector protocol. EIGRP was originally proprietary to Cisco but was released as an open standard in 2013. It is widely used in enterprise networks that use Cisco equipment. EIGRP provides fast convergence, low bandwidth usage, and support for VLSM and CIDR. ## EIGRP Metric EIGRP uses a composite metric that by default considers bandwidth and delay. The metric calculation uses the minimum bandwidth along the path and the cumulative delay of all links in the path. EIGRP can also optionally include reliability and load in its metric calculation, but these are disabled by default because they change frequently. The composite metric formula produces a number that represents the overall cost of using a particular path. ## DUAL Algorithm EIGRP uses the Diffusing Update Algorithm or DUAL to calculate routes and ensure loop-free paths. DUAL maintains multiple paths to each destination and can instantly switch to a backup path if the primary path fails without needing to recalculate routes from scratch. Each router maintains a topology table containing all routes advertised by EIGRP neighbors. For each destination the router identifies a successor which is the primary next-hop with the lowest metric and a feasible successor which is a backup next-hop that is guaranteed to be loop-free. ## EIGRP Neighbor Relationships EIGRP routers discover neighbors by sending Hello packets on each EIGRP-enabled interface. When two EIGRP routers discover each other they exchange their complete routing tables. After the initial exchange only changes are sent, reducing bandwidth usage compared to protocols that send complete updates periodically. EIGRP neighbors must be in the same autonomous system and must agree on the K-values used in the metric calculation. ## Partial and Bounded Updates Unlike RIP which sends complete routing table updates every 30 seconds, EIGRP sends updates only when there is a change in the network. When a change occurs EIGRP sends updates only to routers that are affected by the change rather than to all neighbors. This reduces bandwidth usage and CPU processing significantly in large stable networks. ## EIGRP Tables EIGRP maintains three tables. The neighbor table lists all EIGRP neighbors and their status. The topology table contains all routes advertised by all EIGRP neighbors along with metrics. The routing table contains the best routes selected from the topology table. Only routes that are successors or feasible successors are placed in the routing table. ## Advantages of EIGRP EIGRP converges very quickly because feasible successors are already calculated and can be used immediately when a primary path fails. It uses bandwidth efficiently by sending only incremental updates. It supports VLSM, CIDR, and discontinuous networks. It can load balance across unequal cost paths, a feature not available in most other routing protocols.