CCNP ROUTE: 3. EIGRP Metric tuning: Configuring Bandwidth and Delay

The simplified formula for EIGRP metric is: Metric = 256 * ((10*7 / slowest-bandwidth) + cumulative-delay)

The better choice is to modify the Delay value, since Bandwidth is also used in several other network services like QoS and SNMP.

Let's consider the same route as above on R6:
P 172.16.0.12/30, 1 successors, FD is 2195456
        via 172.16.0.6 (2195456/2169856), FastEthernet0/1
        via 172.16.0.2 (2681856/2169856), Serial0/0

R6#show interfaces fastEthernet 0/1
FastEthernet0/1 is up, line protocol is up
  Hardware is Gt96k FE, address is c003.2cd4.0001 (bia c003.2cd4.0001)
  Internet address is 172.16.0.5/30
  MTU 1500 bytes, BW 10000 Kbit/sec, DLY 1000 usec,
     reliability 255/255, txload 1/255, rxload 1/255

R6#show interfaces serial 0/0
Serial0/0 is up, line protocol is up
  Hardware is GT96K Serial
  Internet address is 172.16.0.1/30
  MTU 1500 bytes, BW 1544 Kbit/sec, DLY 20000 usec,
     reliability 255/255, txload 1/255, rxload 1/255

For the route through 172.16.0.6, the metric calculation is: Metric = 256 * ((10*7 / 1544) + 2100) = 256 * 8576 = 2195456
For the route through 172.16.0.2, the metric calculation is: Metric = 256 * ((10*7 / 1544) + 4000) = 256 * 10476 = 2681856
(decimals will be rounded DOWN to the nearest whole number)

If we want the route through 172.16.0.2 to be installed in the routing table and to be used to reach the 172.16.0.12/30 network, then modifying the Delay parameter is both easyer and recommended:

Before modification:
R6#show interfaces fastEthernet 0/1
...
  MTU 1500 bytes, BW 10000 Kbit/sec, DLY 1000 usec

R6#show ip eigrp topology 
...
P 172.16.0.12/30, 1 successors, FD is 2195456
        via 172.16.0.6 (2195456/2169856), FastEthernet0/1
        via 172.16.0.2 (2681856/2169856), Serial0/0

R6#show ip route
...
D       172.16.0.12/30 [90/2195456] via 172.16.0.6, 05:30:57, FastEthernet0/1

To set bandwidth or delay, simply use the "bandwidth value" or "delay value" commands in interface configuration mode.
R6(config)#interface fastEthernet 0/1
R6(config-if)#delay ?
  <1-16777215>  Throughput delay (tens of microseconds)

R6(config-if)#delay 2001

After modification, the route through 172.16.0.2 is preferred due to a (slightly) lower metric (FD):
R6#show interfaces fastEthernet 0/1
...
  MTU 1500 bytes, BW 10000 Kbit/sec, DLY 20010 usec

R6#show ip eigrp topology
...
P 172.16.0.12/30, 1 successors, FD is 2195456
        via 172.16.0.2 (2681856/2169856), Serial0/0
        via 172.16.0.6 (2682112/2169856), FastEthernet0/1

R6#sho ip route
...
D       172.16.0.12/30 [90/2681856] via 172.16.0.2, 00:00:27, Serial0/0

This time, for the route through 172.16.0.6, the metric calculation is: Metric = 256 * ((10*7 / 1544) + 4001) = 256 * 10477 = 2682112
This time, for the route through 172.16.0.2, the metric calculation is: Metric = 256 * ((10*7 / 1544) + 4000) = 256 * 10476 = 2681856

These calculations use the lowest bandwidth value (1544 Kbps) and the cumulative delay of each link from R6 to either R5 or R4 and the delay of R5's or R4's interface to the 172.16.0.12/30 network.

The AD of both routes in the EIGRP topology table is calculated as: AD = 256 * ((10*7 / 1544) + 2000) = 256 * 8476 = 2169856