EIGRP adds together weighted values of different network link characteristics in order to calculate a metric for evaluating path selection.
These characteristics include:
- Delay (measured in 10s of microseconds)
- Bandwidth (measured in kilobytes per second)
- Reliability (in numbers ranging from 1 to 255; 255 being the most reliable)
- Load (in numbers ranging from 1 to 255; 255 being saturated)
- MTU Smallest MTU in a path (Not used in metric calculation)
| K Value | Component | Description |
| K1 | Bandwidth | Lowest bandwidth of route |
| K2 | Load | Worst load on route based on packet rate |
| K3 | Delay | Cumulative interface delay of route |
| K4 | Reliability | Worst reliability of route based on keep alive |
| K5 | MTU | Smallest MTU in path |
Various constants (K1 through K5) are able to be set by a user to produce varying routing behaviors. However by default, only delay and bandwidth are used in the weighted formula to produce a single 32bit metric:
By default, the values of K1 and K3 are set to 1, and K2, K4 and K5 are set to 0.
Hence the above equation is deduced to
EIGRP Metric = 256*(Bandwidth + Delay)
Bandwidth (K1) is a static value. It will change only when we make some physical (layer1) changes in route such as changing cable or upgrading link types. EIGPR picks lowest bandwidth from all outing going interfaces of route to the destination network. A smallest bandwidth because all other link has a bandwidth higher or equivalent.
Load (K2) is a dynamic value that changes frequently. It is based on packet rate and bandwidth of interface. It calculates the volume of traffic passing through the interface in comparison of maximum capacity. It is expressed on a scale of 255 where 1 represent that an interface is empty and 255 represent that an interface is fully utilized.
Since data flows from both directions, router maintains two separate metric counters;
- Txload for outgoing traffic
- Rxload for incoming traffic
If K2 is enabled, maximum Txload value will be used in composite metric calculation formula.
Delay (K3) reflects the time taken by a packet in crossing the interface. It is measured in fractions of seconds. Like as bandwidth Cisco has implicit delay values for all interfaces based on the type of interface hardware. For example a FastEthernet has default delay of 100 microseconds. Since it is a static value, we can override it with delay command.
Delay can be set anywhere from 10 to 167,772,140 microseconds.
Default delay value or value set by delay command has nothing to do with the actual delay caused by interface. Just like bandwidth, this value is also an influencer.
It is expressed in terms of tens of microseconds. To define a delay of 1000 microseconds, we need to configure 100(1000/10) on interface. Output of show interface command will automatically multiply it with ten before displaying.
Total delay is used in metric calculation formula.
Total delay = delay received from neighboring router + its own interface delay
EIGRP is an enhanced distance vector routing protocol. It also uses route poisoning, withdrawing route, split horizon and poisoned reverse for loop free optimized network. For all these mentioned techniques EIGRP use the maximum delay as the indication of the unreachable route. To denote the unreachable route EIGRP uses the delay of 16,777,215 tens of the microseconds.
Reliability(K4) is also a dynamic value. It compares all successfully received frames against all received frames. 100% reliability indicates that all the frames which we received were good. We don’t have any issue with physical link. If we have any issue with physical link, this value will be decrease.
Reliability is expressed as the fraction of 255. 255 expresses 100% reliability while 0 represents 0% reliability. If K4 is enabled in metric calculation formula, it will use minimal reliability.
MTU (K5) stands for maximum transmission unit. It is advertised with routing update but it does not actively participate in metric calculation. EIGRP allows us to load balance between equal cost paths (6 maximum, default set to 4). It is used when equal cost paths for same destination exceed the number of allowed paths set from maximum-paths command. For example we set maximum allowed paths for load balancing to 5 and metric calculates 6 equal cost paths for a single destination. In this situation path with lowest MTU will be ignored.
CCNPexperts 