EIGRP thoughts

EIGRP (Enhanced Interior Gateway Protocol) is a Cisco proprietary routing protocol. It is a hybrid between distance-vector and link-state protocols, incorporating the best from both worlds.

This protocol is able to route multiple protocols, such as IP, IPX, Appletalk (should you ever need to route Appletalk...), but the biggest difference in how EIGRP computes the metric. It uses a combination of bandwidth and delay, each multiplied by 255 to give a 32-bit metric to determine the best path to any given destination. You can also add in other factors, such as load, MTU and reliability to further enhance the routing decisions. It is also capable of unequal load balancing, splitting up traffic according to available bandwidth, over six paths compared to OSPF and RIP's equal load balancing, and it has a maximum hop count of 244 compared to RIP's 15. Also, EIGRP is classfull by default but can be configured as classless (like RIPv2) by issuing the "no auto-summary" command.

It discovers neighbors using multicast packets to 224.0.0.10, and like OSPF stores the data in a topology table. Once the topolgy tables are in sync, they send out hello packets to keep their dead timers from expiring. Like OSPF, the hello and dead timers differ based on the topology of the network. Multi-access broadcast and point-to-point networks have a hello timer of 5 seconds and a dead timer of 15 seconds, while non-broadcast multi-access have a 60 and 180 hello and dead timer, respectively. The dead timer is 3 times the hello timer, which helps me remember for some reason. Worth noting as well is that it uses the Diffusing Update Algorithm (or DUAL) to compute topology changes in a fractions of a second.

The reason EIGRP can converge so quickly when the topolgy changes is that it chooses a successor route and also a feasible successor. The feasible successor is a backup route in case the successor goes down. It does this by tracking the metric from each neighbor router to all routes advertised by its neighbor. That is the advertised distance. It then adds its composite metric to the advertised distance to come up with the feasible distance. The lowest feasible distance is then added to the route table as the successor. To become the feasible successor, a route must have a lower advertised distance than the feasible distance of the successor route.

Along with calculating the feasible successor route to speed convergence in the case of a link going down, EIGRP has another efficiency over OSPF. In the case of multiple routes to a given network, but no advertised distance is lower than the feasible distance (thereby causing EIGRP to have no feasible successor), EIGRP handles the down link differently than OSPF. OSPF would flood the link state to all routers in the area, consuming considerable amounts of bandwidth and processing power. EIGRP simply queries its neighbors for a route to that path and the router with the down link and then computes a successor path based on the information it receives in return. Of course, it must not create a loop and has to wait out the "Stuck in Active" timer of 180 seconds (which also would exceed the dead timer in a NBMA network) to make sure it gets every possible reply.

Like OSPF, EIGRP also utilizes stub networks to allow route summarization where there is only one path in and out of a network. If the only link to a network goes down, it doesn't do any good to start querying neighbors for another route.

Configuring EIGRP is quite simple in its most basic form. Like RIP, you simply initialize the protocol using the "router" command. The major difference here is that EIGRP requires an Autonomous System number the must be the same on all EIGRP routers or they will not get the updates from one another. Contrast this with OSPF's process ID, because they come in the same location of the router command:

EIGRP
RouterA(config)#router eigrp 100

100 is the randomly assigned AS number. It is randomly assigned by the planner of the network, by the way.

OSPF
RouterA(config)#router ospf 100

In this case, 100 is the process ID which does not need to match anything else. Any network advertised in a given area will update with OSPF.

From here, it is imperative to configure the bandwidth of serial links correctly, since Cisco IOS assumes all serial links are T1's. Ethernet interfaces report their bandwidth accurately. It is also important to remember that, if configuring a network as a stub network, you need to turn off the auto-summarization or manually configure a summary route. Also for contrast, to veiw OSPF's topolgy table you enter "show ip ospf database" where in EIGRP you enter "show ip eigrp topology" which just makes more sense to me. I believe in "calling it what it is"...

With all that said, there are only two drawbacks to EIGRP. One is that you cannot use multiple vendors' equipment. EIGRP is Cisco-only. The other thing is that it consumes a lot fo processing and memory because it maintains a topolgy, neighbor and routing table for each protocol being routed. While that isn't a big deal now that the world is run on IP, it could be a consideration.

EIGRP is a nice protocol, efficient and easy to configure. If you only have Cisco equipment, it only makes sense. Unless you are like me and really loathe using proprietary protocols to base your infrastructure upon. I just like having options.