CCNA: EIGRP’s Use of Bandwidth and Delay

CCNA students can typically rattle off the fact that EIGRP uses Bandwidth and Delay in its composite metric calculation by default. In fact, they tend to know this as well as their own last name. But I often notice they might have some pretty big misconceptions about how this metric is really calculated, and how they can manipulate it.
Here are some very important “Core Knowledge” facts that we need to keep in mind about the EIGRP metric:

• The metric formula uses the bandwidth and delay values that are set as default on the interface, or those values configured on the interface by an administrator
• The bandwidth value that is used in the calculation is the slowest bandwidth in the path from source to destination; to remember this just think of the “weakest link” in the path
• The delay value used in the calculation is the sum of the delay values in the path
• You can set the bandwidth value of an interface using the BANDWIDTH command and you can set the delay value of an interface using the DELAY command
• Setting bandwidth or delay on an interface does not change any physical properties of the interface at all; you are just changing the values that the interface reports for EIGRP metric purposes

Let’s examine some of this at the command line:

R1#show run interface fa0/0
Building configuration...

Current configuration : 95 bytes
!
interface FastEthernet0/0
 ip address 10.10.10.1 255.255.255.0
 duplex auto
 speed auto

Notice that we have not set BANDWIDTH or DELAY under this interface at all. Let us examine what EIGRP will be using regarding this interface in its overall calculation:

R1#show interface fa0/0
FastEthernet0/0 is up, line protocol is up
 Hardware is Gt96k FE, address is c201.0111.0000 (bia c201.0111.0000)
 Internet address is 10.10.10.1/24
 MTU 1500 bytes, BW 10000 Kbit/sec, DLY 1000 usec,
 reliability 255/255, txload 1/255, rxload 1/255

Notice the values that EIGRP can use by default are in place.
The DELAY command is a powerful command for manipulating EIGRP paths. Since the BANDWIDTH command can end up impacting a lot of other configurations (like QoS), we can use the DELAY command to manipulate EIGRP metrics (and therefore, paths) without having to touch the BANDWIDTH command.

R1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#interface fa0/0
R1(config-if)#delay ?
 <1-16777215>  Throughput delay (tens of microseconds)

Notice from the above output just how easy it is to manipulate this value, and therefore, impact the EIGRP metric.