EIGRP Metrics

EIGRP Metrics
EIGRP utilizes several databases or tables of information to calculate routes. These databases
are as follows:
 The route database (routing table) where the best routes are stored
 The topology database (topology table) where all route information resides
 A neighbor table that is used to house information concerning other EIGRP neighbors
126 Chapter 4  IGRP and EIGRP
Each of these databases exists separately for each routed protocol configured for EIGRP:
 The IP session is called IP-EIGRP.
 The IPX session is called IPX-EIGRP.
 The AppleTalk session is called AT-EIGRP.
Therefore, it is possible for EIGRP to have nine active databases when all three protocols are
configured on the router, even more for multiple concurrently configured autonomous systems.
As stated previously, the metrics used by EIGRP are the same as those used by IGRP. As
with IGRP, metrics decide how routes are selected. The higher the metric associated with a
route, the less desirable the route is. The following equation is used by EIGRP to calculate the
composite metric:
metric = [(K1 × bandwidth) + [(K2 × bandwidth) ÷ (256 – load)] + (K3 × delay)] × [K5 ÷
(reliability + K4)] × 256
The elements in this equation are as follows:
 By default, K1 = K3 = 1, K2 = K4 = K5 = 0. Therefore, by default, the metric formula reduces to:
metric = (1 × bandwidth) + (1 × delay) × 256
metric = (bandwidth + delay) × 256
 Delay is computed as one-tenth the sum of all the measured delays, in microseconds, of the
outbound interfaces of the links along the path, which is to say, the cumulative delay along
the path in tens of microseconds.
 Bandwidth = [10000000 ÷ (BW in Kbps)]. BW is the lowest bandwidth of the links along
the path.
Alternatively, the metric can be described as (bandwidth + delay) as long as
 Delay = [Delay in 10s of microseconds] × 256
 Bandwidth = [10000000 ÷ (BW in Kbps)] × 256
Just as with IGRP, you can set the metrics manually from within the configuration mode.
One important thing to keep in mind when manually setting metrics is that in order for EIGRP
routers to form neighbors, they must have the same K-values in the Hello packet. If the K-values
are different, the routers will not form adjacencies with each other. With that in mind, let’s take
a look at how to tune these settings.