Enhanced Interior Gateway Routing Protocol (EIGRP)

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In most networking environments, RIP is not the preferred choice of routing protocol, as its time to converge and scalability are poor compared to EIGRP, OSPF, or IS-IS. However, it is simple to configure because, unlike other protocols, RIP does not require any parameters.

The convergence time of a group of routers is a measure of how quickly they reach the state of convergence. It is a major design goal and an important performance indicator for routing protocols, which should implement a mechanism that allows all routers running the protocol to converge quickly and reliably. Of course, the size of the network is also important. A larger network will take longer to converge than a smaller one.

RIP is a routing protocol that converges so slowly that even a network of a few routers can take a couple of minutes to converge. Triggered updates can speed up RIP’s convergence when a new route is advertised, but flushing an existing route takes longer due to the use of holddown timers.

You may think that having implemented RIP has resolved all of your routing issues. They are, for the most part. However, there is a problem with getting data to Turris Medius.

Here’s the output from the traceroute command:

Unable to reach destination address

Analyze the output shown above. What do you believe the issue is? What could you check on your router to help you solve this problem?

The problem is that there are far too many hops.

The Hop-Count is the number of routers that a packet must “hop” through in order to reach a specific IP network.

The number of hops, or hop count, is used by RIP to determine the best possible route to a host or network. The term hop count is also known as the metric. A hop count of 16 in RIP indicates infinity, or the impossibility of reaching the destination. This limits the longest path in the network that can be managed by RIP to 15 gateways.

It takes more than 15 hops to get to Turris Medius. Hence the issue.

Q: RIP always selects the route with the fewest hops. But what about the line's speed? Doesn't that make a difference?

To find the best route, RIP can only use the hop count. RIP doesn’t know the speed of a particular network line, so it treats all lines equally. This means that if there are two routes to a specific network, RIP will choose the one with the fewest hops, even if the other route is much faster.

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So what do we do? We can’t simply redesign the RIP protocol. Is there another routing protocol that we could use instead?

The answer is yes. Earlier in the tutorial, we had you use the RIP protocol, which works fine for a network with a small number of routers and paths, but it just does not scale. There are several other advanced IP routing protocols available. These include OSPF, IGRP, EIRGP, and BGP.

Cisco’s proprietary routing protocols are IGRP and EIGRP. The IGRP protocol is now considered obsolete. It was replaced by EIGRP, but you may come across it here and there. These protocols are only supported by Cisco routers. OSPF and BGP are standards-based routing protocols that are supported by a wide range of router manufacturers.

But, which one do you choose?

If you have all Cisco routers on your network, EIGRP is probably the best option. But if you have to interact with routers from other manufactures, then OSPF (Open Shortest Path First) and BGP (Border Gateway Protocol) are best. Also, if you are connecting a router to an existing network, you must use the same routing protocol as the other routers.

The scope of this tutorial does not allow for a detailed explanation of these protocols. Consult the RFCs for more information on the aforementioned protocols.

IGRP – Interior Gateway Routing Protocol
EIGRP – Cisco’s Enhanced Interior Gateway Routing Protocol (EIGRP)
OSPF – OSPF Version 2
BGP – Multiprotocol Extensions for BGP-4

You can find more information on RIP by looking at RFC 1058.

We’d like to demonstrate the differences between routing protocols. Let us investigate.

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OSPF is an open routing standard that is supported by the majority of routers. It is fast to converge to boot. EIGRP is similar to OSPF. The fact that EIGRP only runs on Cisco hardware is most likely the biggest disadvantage.

What Exactly Makes EIGRP So Special?

EIGRP knows its neighbors

A router using the EIGRP protocol is aware of its neighbors and shares routes with them. Neighbors can be statically entered (a good security measure) or discovered by a router using EIGRP HELLO packets.

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1 If our EIGRP router does not have static neighbors, it will send out multicast HELLO packets on all of its connected interfaces when it boots up.
2 When a neighboring connected router receives a HELLO packet, it returns a reply packet containing all the routes it knows if the ASN and subnet from the sending router match.
3 Our router will return a short ACK packet to let the neighbor know it received the routes.

To be considered neighbors, routers must share an IP subnet and an Autonomous System Number (ASN).

EIGRP uses the Diffusing Update Algorithm

The Diffusing Update Algorithm (DUAL) is a way of calculating routes when there are changes to a network topology. It aids in keeping routes free of loops.

- Enhanced Interior Gateway Routing Protocol (EIGRP)

A router loop is where packets would just go from one router to the next and never get anywhere. They just go around the circle.

EIGRP uses the Reliable Transport Protocol to send its information

RTP ensures that EIGRP route information sent by a router reaches its neighboring routers in the correct order and without errors.

EIGRP sends route updates only when there are changes

A router only updates other routers when the network topology to which it is connected changes. This allows for very fast convergence of a network.

A network is said to “converge” when all the routers have the correct routing information for the network.

How Do We Set Up EIGRP?

Setting up EIGRP on a router is fairly simple. You enter the config mode, then enter the router EIGRP configuration. When you add some networks in here, the router turns on EIGRP. You must add the neighbor routers with whom this router will exchange EIGRP route information. When you exit command mode, remember to save your changes with the write memory command.

Router#config t
Router(config)#router eigrp ASN
Router(config-router)#network network SUBNET [WILCARD_MASK]
Router(config-router)#neighbor ip_address

The explanation of these commands, as well as detailed instructions on how to configure EIGRP using Cisco Packet Tracer, are provided in the next tutorial.

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