EIGRP:
EIGRP (Enhanced Interior Gateway Routing Protocol) is an advanced distance-vector routing protocol that permits routers to exchange Path information in a more efficient way than with previous network protocols. EIGRP grew out of IGRP (Interior Gateway Routing Protocol) EIGRP or IGRP can interoperate because the metric used with one protocol can be translated into the metrics of the other protocol.
EIGRP, a router maintains a copy of its neighbor’s routing tables. If it cannot locate the route to a destination in one of these tables, it queries its neighbors for a route and they in turn query their neighbors until a route is found. When a routing table entry changes in one of the routers, it notifies its neighbors of the only the changes unlike previous protocols that require sending the entire routing table. To keep all routers aware of the state of neighbors, each router sends out a periodic “hello” packet. A router from which no “hello” packet has been received in a certain period of time is assumed to be inoperative.
EIGRP uses the Diffusing-Update Algorithm (DUAL) to determine the most efficient route to a destination based on cost. A DUAL finite state machine contains decision information used by the algorithm to determine the least-cost route.
Autonomous System:
Each EIGRP process is identified by an autonomous system (AS) number, just like IGRP processes. Routers with the same AS numbers will exchange routing information with each other, resulting in a routing domain. Routers with dissimilar AS numbers will not exchange any routing information by default. However, routes from one routing domain can be leaked into another domain through the redistribution commands.
Route Summarization:
The default behavior of EIGRP is to summarize on network-number boundaries. This is similar to RIP and IGRP and is a prudent way for a routing protocol to reduce the number of routes that are propagated between routers.
Load Balancing:
In general, load balancing is the capability of a router to distribute traffic over all the router network ports that are the same distance from the destination address. Load balancing increases the utilization of network segments, and so increases effective network bandwidth. There are two types of load balancing:
Equal cost path – Applicable when different paths to a destination network report the same routing metric value. The maximum-paths command determines the maximum number of routes that the routing protocol can use.
Unequal cost path – Applicable when different paths to a destination network report are of different routing metric values. The variance command determines which of these routes is used by the router.
Every routing protocol supports equal cost path load balancing. In addition, EIGRP also support unequal cost path load balancing. Use the variance n command in order to instruct the router to include routes with a metric of less than n times the minimum metric route for that destination. The variable n can take a value between 1 and 128. The default is 1, which means equal cost load balancing. Traffic is also distributed among the links with unequal costs, proportionately, with respect to the metric.
EGIRP Metric:
EIGRP uses the minimum bandwidth on the path to a destination network and the total delay to compute routing metrics. Although you can configure other metrics, we do not recommend it, as it can cause routing loops in your network. The bandwidth and delay metrics are determined from values configured on the interfaces of routers in the path to the destination network.
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