EIGRP… I don’t know what it is, but for some reason it took me a good three days to finish a 28 page chapter on EIGRP. In contrast it took me a couple of hours to get a good idea what OSPF is all about – not to say that I can renumerate the bullet points. I’ll have to read it again, of course. There goes the plan to skim over an 80-page discussion of EIGRP from Jeff Doyle’s Routing TCP/IP, Volume 1, 2nd Ed. I just don’t have time to do it if I have to keep in line with my written study schedule. I’ll have my chance to read it later when I’m reviewing, and also when I tackle CCNP.

I don’t know, somehow I got stuck reading all about feasible distance, reported distance, successor & feasible successor, and all kinds of metric calculations and i just I couldn’t move forward. It seemd as if everytime I read them, I wonder off to some far off place and just never got a grip of the concept.

I eventually got the gist. I have to put that on the burner, let it simmer and move on to cutting some vegetables before coming back and revisiting it for reinforcement.

Here’s the skinny on EIGRP:

• Cisco-proprietary
• Three general steps for EIGRP to add routes to the IP routing table:
  1. Neighbor Discovery
     • through Hello messages
     • Hello messages always sent to 224.0.0.10
     • Must pass the authentication process
     • Must use the same configured AS number
     • The source IP address of a neigbors Hello must be in the same subnet
  2. Topology Exchange
     • Full topology exchange between neighbors, and partial exchange thereafter.
     • Uses Update Messages sent to 224.0.0.10 multicast if needed or to the unicast address of the neighbor.
     • Update messages uses Reliable Transport Protocol (RTP). RTP resends lost routing updates. RTP also helps avoid loops.
  3. Choosing Routes
     • chooses the lowest metric as best route to put on routing table.
• Bandwidth and Delay  affect the calculation fo the EIGRP metric
  • metric = (( 10^7 / least-bandwidth) + cumulative-delay) x 256
  • bandwidth uses the unit of kilobits per second (e.g. 10Mbps = 10,000 kbps)
  • cumulative-delay is the sum of all the links in the route. Use units of “tens of microseconds”
  • can also use interface load and interface reliability to calculate metric.
• Feasible Distance is the calculated metric on a router to find the best route, among several different routes, to reach a subnet. 
  • Successor – the term used to define the best route. This is what is added to the IP routing table
• Reported Distance is the metric of a route that is reported by the next door neighbor. this value is used to determine if the route can become a feasible successor route.
  • feasible successor – basically a backup route. The book defines the feasiblility condition as: a nonsuccessor route’s Reported Distance (RD) that is less than the Feasible Distance (FD).
• Diffusing Update Algorithm (DUAL)  is the algorithm that EIGRP uses to send queries that look for loop-free route when a route fails. 
• Important verification commands:
  • show ip eigrp neighbors
  • show ip eigrp topology
  • show ip route
  • show ip eigrp interfaces
  • show ip protocols