There are many things that can be done to alter the behaviour of EIGRP. As we saw before, delay can be changed to make one link look worse than the other. Likewise, bandwidth can be changed, but since EIGRP uses the lowest bandwidth and the sum of delays, it makes more sense to play with bandwidth to make minor alterations.

Using the same network as before:

Since EIGRP is a distance vector protocol, you can use an offset-list to change the composite metric, rather than playing with bandwidth & delay. An offset list changes the advertised metric, so it goes on the router with the route.

Simple example:

r0#show ip route 172.19.0.0
Routing entry for 172.19.0.0/24, 1 known subnets
  Redistributing via eigrp 1

D       172.19.0.0 [90/2195456] via 192.168.1.93, 01:12:42, Ethernet0

I’m going to add 804544 to this to bring it up to an even 3000000.

r3:
! match the network
access-list 51 permit  172.19.0.0 0.0.255.255
router eigrp 1
 ! anything matching that gets added
 offset-list 51 out 804544

And,

r0#show ip route 172.19.0.0
Routing entry for 172.19.0.0/24, 1 known subnets
  Redistributing via eigrp 1

D       172.19.0.0 [90/2707456] via 192.168.1.136, 00:01:23, Ethernet0

Oops! It’s decided to go the long way around! (R2’s side of the link out the frame to R1) However, the route still shows up in the topology table (but is not a successor, since the feasibility condition is not met:

r0#show ip eigrp topology 172.19.0.0/24
IP-EIGRP (AS 1): Topology entry for 172.19.0.0/24
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2707456
  Routing Descriptor Blocks:
  192.168.1.136 (Ethernet0), from 192.168.1.136, Send flag is 0x0
      Composite metric is (2707456/2681856), Route is Internal
      Vector metric:
        Minimum bandwidth is 1544 Kbit
        Total delay is 41000 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 2
  192.168.1.93 (Ethernet0), from 192.168.1.93, Send flag is 0x0
      Composite metric is (3000000/2974400), Route is Internal
      Vector metric:
        Minimum bandwidth is 1544 Kbit
        Total delay is 52427 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 1

If you want to change the way the composite metric is calculated, the metric weights command under the routing process config is used.

The syntax is

metric weights tos k1 k2 k3 k4 k5

Where tos=0 and k1 through k5 are the weightings for the EIGRP formula:

metric = [K1 * bandwidth + (K2 * bandwidth) / (256 - load) + K3 * delay] * [K5 / (reliability + K4)]

(from the article above, the last multiplication of k5/(reliability+k4) is only performed if K5 > 0)

By default, K1 and K3 are 1, and the rest are zero. As you can see, though, you can make reliability and load factor in. Be very careful, though, since these are dynamic values and will cause the metric to change frequently, possibly causing a large amount of churn in your routing tables!

Looking at the formula, K1 controls the importance of bandwidth. K2 controls load, K3 controls delay, and K4 and K5 control reliability. The implications of changing the values can be serious, which is why there are many references to “only do this upon expert guidance” appear in the manuals.

References

1. metric weights