Setting a Preferred Route with Equal Cost Routes in EIGRP

27 Sep 2011 - Lab-Rat

There are basically three ways to prefer one equal cost route over another in EIGRP. All three either change the metric (modify delay or the composite metric), or change the administrative distance.

Method 1, Changing the Metric by Modifying Delay

Since EIGRP only uses the lowest link bandwidth and cumulative delay in the default metric calculation (yes, I know, Load, Reliability, MTU, Bandwidth and Delay- but K values for everything but bandwidth and delay are 0, see the EIGRP configuration guide referenced below), Cisco recommends you change the delay setting to influence the metric.

To see the current route delay value, enter:
show ip eigrp topology x.x.x.x y.y.y.y

where x and y are the prefix and mask.

Look for the line: Total delay is nnnn microseconds

To see the current interface delay value enter:
show interface fa0/0

Look for the the line: MTU 1500 bytes, BW 10000 Kbit, DLY 1000… usec

A brief word about units of delay here.

A millisecond is one-thousandth of a second.
A microsecond is one-millionth of a second.
Delay is entered in tens of microseconds, or one one-hundred thousandth of a second.

We set the delay on the interface-

R7(config-if)# delay 240

would set a delay of 2400 microseconds or .0024 seconds.

By adding delay to one route, you increase the metric, and make the other route preferable.

Method 2, Setting the Administrative Distance

Hopefully you don’t have to do this in production, but this scenario can come up in the CCIE lab when you’re doing redistribution- particularly with EIGRP. Recall that the AD for an EIGRP external route is 170. That means that if you do something like redistribute a connected route into EIGRP on router 1, pass that router on to an EIGRP neighbor, and then redistribute EIGRP into another protocol, you may get a route back to your router 1 connected route with a lower administrative distance. Since the router looks at AD first, it will kick the EIGRP learned route out of the table, and replace it with the other protocol’s route- which creates a loop. To avoid this, you can set the administrative distance on any route using the distance command.

First create an access list for the route:

ip access-list standard Loop123 permit 172.16.5.0 0.0.0.255

then under the EIGRP routing process, use the distance command:
R5(config-router)# distance 100 0.0.0.0 255.255.255.255 Loop123
Which says, set the AD to 100 for routes from any source, for the prefix specified in Loop123.

You can check the AD with: show ip route x.x.x.x

Method 3, Change the Cumulative Metric with and Offset List

Here you’re basically just adding a bit to the metric to sway the router in favor of the other route. To do that, on the upstream router:

Create an access list for the route:

ip access-list standard Loop123 permit 172.16.5.0 0.0.0.255

then under the EIGRP routing process, use the offset-list command:
R1(config-router)# offset-list Loop123 out 40 s0/0/0

which says add 40 to the metric of the route in Loop123, outbound on s0/0/0

Invariably they will eliminate your options until you only have one choice on the CCIE lab, so be sure to know all three methodologies.

Additional Reading:
Setting Preferred EIGRP Route
EIGRP Configuration Guide