In medium to large-sized networks which consist of many remote sites with long distance places linked together using the networking router, communication between sites can take place using routing protocols run on each routers. Cisco routers play main role in communication between remote sites to happened, and for the same Cisco routers the dynamic routing protocols used is typically the EIGRP Protocol. See also WAN technologies.
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. See also basic knowledge about routing guide.
Link-state and balanced hybrid protocols such as EIGRP protocol and OSPF routing protocols 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.
EIGRP 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. See also basic static route.
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 – 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.
The following figure shows the way EIGRP protocol figures out which routes can be used after a route fails without causing loops.
In the figure, Router E learns three routes to Subnet 1, from Routers B, C, and D. After calculating each route’s metric based on bandwidth and delay information received in the routing update, Router E finds that the route through Router D has the lowest metric, so Router E adds that route to its routing table, as shown.
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 when the currently-best route through Router D failed. 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. In the above Figure, the route through Router C would not cause a loop, so Router E lists the route through Router C as a feasible successor. Router E thinks that using the route through Router B could cause a loop, so that route is not listed as a feasible successor.
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. For example, Router E computes a metric of 14,000 on its best route (through Router D). Router C’s computed metric is lower than 14,000 (it’s 13,000), so Router E believes that if the existing route failed using the route to Subnet 1, through Router C, it would not cause a loop. As a result, Router E adds a route through Router C to the topology table as a feasible successor route. Conversely, Router B’s computed metric is 15,000, which is larger than Router E’s computed metric of 14,000, so Router E does not consider the route through Router B a feasible successor. If the route to Subnet 1 through Router D fails, Router E can immediately put the route through Router C into the routing table, without fear of creating a loop. Convergence occurs almost instantly in this case.
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.
See also:
- Computer network repair and troubleshooting
- The best wireless routers – the best for your need
- Business firewalls
- Sample risk assessment – a case study
- Understanding STP protocols
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