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EIGRP Cost Calculations

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

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Hall of Fame Super Silver

Re: EIGRP Cost Calculations

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.

HTH

Rick

3 REPLIES
Hall of Fame Super Silver

Re: EIGRP Cost Calculations

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.

HTH

Rick

New Member

Re: EIGRP Cost Calculations

Rick,

Many thanks for your very clear and helpful response. The papers I have read have not made it clear that the advertisements received from neighbors will include the lowest bandwidth toward the destination. Knowing that everything now falls into place.

Best Regards

Jim

Hall of Fame Super Silver

Re: EIGRP Cost Calculations

Jim

I am glad that my explanation has helped things fall into place. Thank you for using the rating system to show that your question was resolved (and thanks for the rating). It makes the forum more useful when people can read a question and can know that they will read a response which resolved the question.

I encourage you to continue your participation in the forum. And good luck with your studies.

HTH

Rick

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