Enhanced Interior Gateway Routing Protocol - EIGRP Composite and Vector Metrics

EIGRP Composite and Vector Metrics

EIGRP associates six (6) different vector metrics with each route and considers only four (4) of the vector metrics in computing the Composite metric:

Router>show ip eigrp topology 10.0.0.1 255.255.255.255 IP-EIGRP topology entry for 10.0.0.1/32 State is Passive, Query origin flag is 1, 1 Successor(s), FD is 40640000 Routing Descriptor Blocks: 10.0.0.1 (Serial0/0/0), from 10.0.0.1, Send flag is 0x0 Composite metric is (40640000/128256), Route is Internal Vector metric: Minimum bandwidth is 64 Kbit Total delay is 25000 microseconds Reliability is 255/255 Load is 197/255 Minimum MTU is 576 Hop count is 1

Bandwidth

• Minimum Bandwidth (in kilobits per second) along the path from router to destination network

Load

• Load (number in range 1 to 255; 255 being saturated)

Delay

• Total Delay (in 10s of microseconds) along the path from router to destination network

Reliability

• Reliability (number in range 1 to 255; 255 being the most reliable)

MTU

• Minimum path Maximum Transmission Unit (MTU) (never used in the metric calculation)

Hop Count

• Number of routers a packet passes through when routing to a remote network, used to limit the EIGRP AS.

The K Values There are five (5) K values used in the Composite metric calculation - K1 through K5. The K values only act as multipliers or modifiers in the composite metric calculation. K1 is not equal to Bandwidth, etc.

By default, only total delay and minimum bandwidth are considered when EIGRP is started on a router, but an administrator can enable or disable all the K values as needed to consider the other Vector metrics.

For the purposes of comparing routes, these are combined together in a weighted formula to produce a single overall metric:

where the various constants ( through ) can be set by the user to produce varying behaviors. An important and unintuitive fact is that if is set to zero, the term is not used (i.e. taken as 1).

The default is for and to be set to 1, and the rest to zero, effectively reducing the above formula to .

Obviously, these constants must be set to the same value on all routers in an EIGRP system, or permanent routing loops will probably result. Cisco routers running EIGRP will not form an EIGRP adjacency and will complain about K-values mismatch until these values are identical on these routers.

EIGRP scales the interface bandwidth and delay configuration values with following calculations:

= 107 / Value of the bandwidth interface command

= Value of the delay interface command

On Cisco routers, the interface bandwidth is a configurable static parameter expressed in kilobits per second (setting this only affects metric calculation and not actual line bandwidth). Dividing a value of 107 kbit/s (i.e. 10 Gbit/s) by the interface bandwidth statement value yields a result that is used in the weighted formula. The interface delay is a configurable static parameter expressed in tens of microseconds. EIGRP takes this value directly without scaling into the weighted formula. However, various show commands display the interface delay in microseconds. Therefore, if given a delay value in microseconds, it must first be divided by 10 before using it in the weighted formula.

IGRP uses the same basic formula for computing the overall metric, the only difference is that in IGRP, the formula does not contain the scaling factor of 256. In fact, this scaling factor was introduced as a simple means to facilitate backward compatility between EIGRP and IGRP: In IGRP, the overall metric is a 24-bit value while EIGRP uses a 32-bit value to express this metric. By multiplying a 24-bit value with the factor of 256 (effectively bit-shifting it 8 bits to the left), the value is extended into 32 bits, and vice versa. This way, redistributing information between EIGRP and IGRP involves simply dividing or multiplying the metric value by a factor of 256, which is done automatically.

EIGRP also maintains a hop count for every route, however, the hop count is not used in metric calculation. It is only verified against a predefined maximum on an EIGRP router (by default it is set to 100 and can be changed to any value between 1 and 255). Routes having a hop count higher than the maximum will be advertised as unreachable by an EIGRP router.