You might be wondering how EIGRP neighbors talk to each other. How do they know their neighbor is down? How do they know about recent updates on the network? How do they get information about their neighbors? How do they talk back to their neighbor? And how do they accept the message from their neighbor?
EIGRP like any other routing protocols has their own message types or packet types in order to communicate and synchronize within their network. These messages can be sent either in unicast or multicast and reliably or unreliable.
Let’s start with the “HELLO” message. Routing protocols are not snobbish like a human being (just kidding!). They greeted each other to know if they’re still doing fine. EIGRP’s Hello message is used for neighbor discovery, recovery, and to maintain adjacencies. It is sent to the multicast group address 184.108.40.206. In addition, it is sent with unreliable delivery which means that they do not require acknowledgement from the other device to know that it was received. Five seconds (5) is default hello interval for high bandwidth links like higher than T1 links, PPP or HDLC leased circuits, Frame Relay point-to-point subinterfaces, and etc. Sixty (60) seconds is the default for slower than T1 links.
EIGRP already has what it takes to be the best IGP. However, there is a need of open standard protocol with fast convergence and supports large enterprise networks. Here comes the Open Shortest Path First (OSPF). OSPF is a link-state routing protocol and uses the Dijkstra’s algorithm (SPF) in finding the shortest path in the network. The OSPF process is a bit complex and understanding the algorithm itself is very intricate. I don’t want to elaborate it piece by piece as I’m afraid if I’m going to be serious about understanding the formula of the algorithm, I might discover a new one which would replace OSPF. No, I’m just joking. (more…)
EIGRP like OSPF has three tables: Neighbor, Topology and Routing tables. However, don’t be confused because not all EIGRP tables have the same built like OSPF.
The neighbor, topology, and routing tables are very important in implementing and troubleshooting EIGRP. I often neglect the topology and routing table before because what is important to me is that all neighbors are up. But this is not the case most especially if you want to progress your learning to higher Cisco certification level.
Let’s start with the neighbor table.
If you want to know whether you have established adjacency with neighboring EIGRP router and also to know the uptime, then you go to the neighbor table.
EIGRP really hates the querying process so it always preferred to have a backup route in case the link to a destination is unreachable. And, that backup route is the feasible successor. A feasible successor is a backup path used in the event the successor route disappears. It can be seen in the EIGRP topology table and because of this, the router doesn’t need to recalculate the metric. It automatically chooses the feasible successor when successor route fails.
Now that you know the feasible successor, automatically you know what the successor is. The successor is the best route to the destination. Unlike feasible successor that can only be seen in the topology table, it is seen in the routing table. Of course, the routing table has the best routes.
So what is reported distance? Is it the same with advertised distance? Actually, they are just the same. Reported distance / advertised distance is the cost from the next hop router to the destination. (more…)
The “no auto-summary” command is one of the most important commands that you shouldn’t neglect in configuring EIGRP. Honestly, during my first exam relating EIGRP in the Cisco Networking Academy, I just put this command to all routers under the EIGRP process. Well, it works! Without delving into the importance, I got a passing score. Yeah!
But just putting the command without the “ifs”, it made me realize that there is something I need to understand about this.
The “no auto-summary” command is configured under the EIGRP process. It prevents auto-summarization of networks. Without enabling this command, the routes from its interfaces will be advertised as classful A, B or C networks to its neighbors.
Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco proprietary protocol. It uses It is a hybrid protocol because it has features of the Link State and Distance Vector routing protocols. Confused? If you’re new with IGP, it is natural. You are not alone. There are millions to billions of people around the world who doesn’t know what the heck is EIGRP.
EIGRP is Cisco’s baby. It works on all Cisco devices. It is an advanced distance vector routing protocol that has some link state features. Like RIP, it has the hop count feature. Routes that reached the maximum hop cunt will be tagged as unreachable. Although it is not used as its metric, it limits the EIGRP AS when routing to a remote network. The default hop count is 100 and the value can vary between 1 – 255. And, like OSPF it does not send the whole routing table when there is a routing change. (more…)
When I first heard about Interior Gateway Protocol (IGP) and Exterior Gateway Protocol (EGP), I was just like, “what on bits and bytes are these?” If you are new to networks and you suddenly heard these on your first day, you might ignore the meaning and also the importance. However, you don’t have to memorize the meaning because you will know it by heart as days go by. (more…)