Rather than dive straight in to configuring EIGRP, I thought it would be nice to cover some of the terminology and some of the basic commands for setting it up with a little explication on what exactly they do. I have not decided if I will actually at any point include my labs on this site, as from experience I know there are some great labs around already. But what ever happens, I will either add my own or make sure there are links to some of the good ones I have come across in the course materials.
DUAL, This is the EIGRP algorthem that runs to determine what routes are used by traffic.
Autonomous System (AS), all EIGRP routers that are to share the same routing topology, must be running in the same AS, we will cover setting this up later in the post.
Neighbour, a neighbour is a router that can be reached through a network segment that is running the same EIGRP AS and is directly sharing routing information with the local router.
Successor, the neighbouring router with the best cost path too a destination network, will be marked as the successor for that network and will be used as the next hop to forward traffic to it.
Feasible Successor, Other routers that also have a path to the same destination as the successor will be marked as Feasible Successors and used in the event of the primary successor failing. For a router to be considered a feasible successor the advertised distance to the destination, must be less than the feasible distance of the successor. (don’t worry I will come back to this with a digram later)
Feasible distance, this is the total cost to reach a destination network, it includes the cost of the link to the neighbour who is advertising the link.
Advertised Distances, this is the cost of reaching a destination network as advertised by the neighbouring router, or to look at it another way, it is the feasible distance minus the cost of the link to the neighbour.
Neighbour Table, contains a list of the neighbouring routers and information about them.
Topology Table, contains a list of all the known destination networks along with what neighbours are advertising them and there costs among other information.
Routing Table, once DUAL has run against the Topology table, it picks the best routes and adds them in to the routing table. It is this table that is used for actual routing discussions for data packets.
Metric, the metric is used when determining the best path to a destination network. It is worked out from a formula that included, Delay, Bandwidth, Reliability and Load.
Metric = 256*([K1*Bw + K2*Bw/(256-Load) + K3*Delay]*[K5/(Reliability + K4)])
with default settings K1 and K3 are set to = 1 and K2, K4 and K5 = 0 and this reduces the formmular to metric = 256*(Bw + Delay), where Bw is 10^7 divided by the bandwidth in kb/s (bandwidth is taken as the minimum along the path), and delay is the sum of the delay on each hop of the path in 10’s of microseconds, be-aware delay on show interface is shown in microseconds so needs to be divided by 10.
Note that the MTU is also sent as part of the metric, however it is not used for the purpose of finding the best route but is tracked so the routers know end to end the max MTU that can be used.
OK this post is getting longer than I expected, so rather than carry on with the configuration, I think I will get together a digram to help visualise some of these terms. So look out for part 6 coming your way in the near future