Hi, I'm just starting to study for my CCNP and am confused by the way that the EIGRP cost calculation is described in the online material. On slide 13 of the Quick Learming Module (http://www.cisco.com/E-Learning/bulk/public/ccnp/EIGRP_QLM/player.html) it shows a chain of 4 routers and says that the EIGRP cost is the sum of the delay terms and the bandwidth term for the slowest link. In earlier slides though it says that the cost is the sum of the cost to the next hop router plus the AD of the next hop router. But surely this will use the bandwidth term of that link which may not be the slowest link in the path - if I'm wrong how does the router know the slowest bandwidth which may be on a link several hops away (2 on slide 13). Thanks for any help you may be able to give me with this. Regards, Jim
I am not in a position to say anything particular about the content of the slides. But I will say that if a slide says that the EIGRP metric (technically it is not a cost - but referring to it as a cost can work for our discussion here) calculation includes AD then there is a mistake. AD is used in deciding which route to put into the routing table if there are candidate routes from different protocols but AD is not part of the EIGRP metric calculation.
There is a fairly simple method through which the EIGRP router knows what is the slowest link along the path. When an EIGRP router receives an advertisement it includes what the neighbor knows including the lowest bandwidth from that neighbor toward the destination. The router knows what kind of link it is connected on. If the local link is lower bandwidth than the advertised bandwidth the router puts the local bandwidth into the topology table entry and advertises that lower bandwidth when it advertises to other neighbors. But if the bandwidth in the advertisement is lower than the local link the router does not do anything with the local link bandwidth and uses the advertised bandwidth.
Perhaps an example might help. If you are an EIGRP router and you have a neighbor on your left and a neighbor on your right. The neighbor on your left sends you an advertisement for a prefix and the advertisement has the bandwidth as 10Mb. You are connected by T1. So you put T1 into the topology table and you advertise the prefix to the neighbor on your right. The neighbor on your right is connected by 100Mb Ethernet but it does not use the 100 Mb bandwidth and uses the T1 bandwidth in calculating the metric.