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In medium to large-sized networks consisted of many remote sites with long distance places linked together using router, communication  between sites can take place using routing protocols run on each routers. Cisco routers play main role in communication between remote sites that happens, and for the same Cisco routers the dynamic routing protocols used is typically the EIGRP Protocol.

Routing protocols learn routes – the current best routes – and put those subnets in the IP routing table which is typically  accomplished by using the distance vector protocol. Distance vector protocols were  designed to advertise just the basic routing information across the  network to save bandwidth, and take only little processing and memory. Besides distance vector protocol, couple of routing protocols i.e. link-state and balanced hybrid routing protocols can also be used. EIGRP protocol is Cisco proprietary balanced hybrid types routing protocol  which converges very fast compared to  other routing protocols. Unfortunately this protocol can only work in Cisco routers.

Link-state and balanced hybrid protocols such as EIGRP protocol and OSPF were designed under the assumptions of faster links and more  processing power in the routers, but they can gain some important advantages over distance vector protocols – mainly, faster convergence.

Protocol Concept

Besides support two distance vector IP routing protocols—RIP and IGRP, Cisco routers support two link-state IP routing protocols – OSPF and  Intermediate System-to-Intermediate System (IS-IS). Furthermore, Cisco  supports a single balanced hybrid IP routing protocol – EIGRP protocol. Why  Cisco uses balanced hybrids? Because EIGRP has some features that act like distance vector protocols and some that act like link-state protocols.

The following figure shows the typical sequence used by two EIGRP protocol routers that connect to the same subnet. They discover each other as neighbors, and they reliably exchange full routing information. The process is different from OSPF, but the same  goal of reliably ensuring that all neighbors  receive all routing information is achieved. EIGRP protocol sends and receives EIGRP hello packets to ensure that the neighbor is still up and working (find more)– like OSPF, but with a different Hello  packet than OSPF.  When link status changes or new subnets are discovered, reliable routing updates are sent, but only with the new information—again, like OSPF.

IGRP and EIGRP protocol have the same  formula based bandwidth and delay to calculate the metric associated with a route but EIGRP multiply  the number by 256 to accommodate calculations when very high bandwidth values are used.

EIGRP Protocol Loop Avoidance

Most dynamic protocols have the same difficult problem, the loop avoidance. Unlike link-state protocols which have each router keep a full topology of the network to avoid the loop, EIGRP protocol avoids loops by keeping some basic topological information but not full information.

EIGRP protocol runs a simple algorithm to identify which routes could be used immediately after a route failure, without causing a loop. EIGRP then keeps  these loop-free backup routes in its topology table and uses them if the currently best route fails.To learn more visit point out website and inform yourself about routing technologies.

EIGRP protocol builds a topology table that includes the currently-best route plus the alternative routes that would not cause loops if they  were used. EIGRP calls the best route  (the route with the lowest metric) the successor. Any backup routes that could be used without causing a loop are called feasible successors.

EIGRP decides if a route can be a feasible successor if the computed metric for that route on the neighbor is less than its own computed metric.When a route  fails and the route has no feasible successor, EIGRP uses a distributed algorithm called Diffusing Update Algorithm (DUAL). DUAL sends queries looking  for a loop-free route to the subnet in question. When the new route is found, DUAL  adds it to the routing table.

Pros:

• EIGRP protocol  converges much more quickly than do distance vector protocols, mainly because EIGRP does not need the loop-avoidance features that slow down  distance vector convergence.
• EIGRP takes less memory and processing than link-state protocols.

Cons:

• EIGRP protocol is Cisco  proprietary protocols which  can only work on Cisco routers. If  you have multiple vendor routers  within the site, use OSPF instead.

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