Oct 27

EIGRP Explained, Simple and Easy


Enhanced Interior Gateway Routing Protocol is a routing protocol supported the principles of the inside Gateway Routing Protocol. Both the protocols are operated by Cisco on their devices.

EIGRP uses a DUAL algorithm to avoid loops and send occasional hello packets to check the status of neighbor routers and always selects the most optimal route for efficiency.

There are three table that is maintained by EIGRP:

Neighbor Table: Shares routing information only with neighbors to know who the neighbors are. When a new neighbor is discovered, EIGRP would add its address and interface on which neighbor is connected in neighbor table. EIGRP uses separate neighbor table for each routed protocol.

Topology Table: It contains a list of all destinations and routes advertised by neighboring routers. EIGRP selects single best route for every destination from this list. That route goes in routing table. Remaining routes are marked as backup routes. EIGRP refers selected route as Successor and backup route as Feasible Successor. EIGRP uses separate topology table for every routed protocol.

Routing Table: Stores single best route for each destination in this table. Router uses this table to forward the packet. There is a separate routing table for every routed protocol.

EIGRP is a hybrid protocol, so it has both link state and distance vector protocol. EIGRP uses composite metric calculation formula to select the best route for destination and sends partial or full update only when something is change in network.

EIGRP operates very efficiently over large networks. It achieves this efficiency partially by sending non-periodic updates. The rest of the time, routers only exchange small “Hello” packets to verify that routing peers are still available. It is very efficient over LAN. On each network segment, routers exchange routing information using multicast packets, which helps to limit bandwidth usage on segments that hold many routers.

Following are the features of EIGRP:

EIGRP does not make periodic updates. Instead, it sends partial updates when path or the metric changes for that route. When path information changes, DUAL sends an update about only that link instead of about the whole table.

As it is a classless routing protocol, it shows a routing mask for each destination network. The routing mask feature enables EIGRP to support discontinuous subnetworks and variable-length subnet masks.

EIGRP uses multicast and unicast instead of broadcast. As a result, end stations are unaffected by routing updates and requests for topology information.

EIGRP supports unequal metric load balancing, which allows administrators to raised distribute traffic flow in their networks.

EIGRP uses the Diffusing Update Algorithm to attain rapid convergence. A router that uses EIGRP stores all available backup routes for destinations so it can quickly adapt to alternate routes. If no appropriate route or backup route exists within the local routing table, EIGRP queries its neighbors to find an alternate route.

EIGRP enables administrators to create summary routes anywhere within the network rather than consider the traditional distance vector approach of performing classful route summarization only at major network boundaries.

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