EIGRP - Source of values for metric calculation
EIGRP uses the EIGRP formula to determine the metric of a particular path to a specific destination. The formula includes the values of the following parameters:
These values are all maintained on a per-interface basis on all the interfaces on all the routing devices that are traversed along the path to the intended destination. Take a look at the example output of the
show interfaces command of a routed interface on a router below:
Here you can see the following values:
- Bandwidth, labeled BW is at 100000 Kbit
- Delay, labeled DLY is 100 microseconds (usec)
- Reliability is at 255/255
- Load is shown as txload and rxload each at 1/255
Now, the bandwidth and delay are actually values that you can manually change. They’re labels that are assigned particular values by default based on the type of interface. For example:
- a 10Mbps link will have a BW of 10000 Kbit and a DLY of 1000 usec
- a 100Mbps link will have a BW of 100000 Kbit and a DLY of 100 usec
- a 1Gbps link will have a BW of 1000000 Kbit and a DLY of 10 usec
But you can go in and change these values manually. It doesn’t change the actual bandwidth of the interface, but it does affect the metric that is calculated by EIGRP and other routing protocols such as OSPF, for traffic that traverses this interface.
Now reliability and load are calculated based on the actual number of errors detected on the interface and on the actual traffic on the link, respectively. These values are updated every 5 minutes.