EIGRP Metric Calculation Flashcards
Path Metric Calculation
Information
Metric calculation is a critical component for any routing protocol. EIGRP uses multiple factors to calculate the metric for a path.
Metric calculation uses bandwidth and delay by default, but it can include interface load and reliability, too.
EIGRP uses K values to define which factors the formula uses and the associated impact of a factor when calculating the metric.
The EIGRP formula is based on the IGRP metric formula, except the output is multiplied by 256 to change the metric from 24 bits to 32 bits.
EIGRP Classic Metric Formula
By default, K1 and K3 have the value 1, and K2, K4, and K5 are set to 0.
BW represents the slowest link in the path scaled to a 10 Gbps link (107).
Delay is the total measure of delay in the path, measured in tens of microseconds (μs).
Metric Propagation
The EIGRP update packet includes path attributes associated with each prefix. The EIGRP path attributes can include hop count, cumulative delay, minimum bandwidth link speed, and RD.
The attributes are updated each hop along the way, allowing each router to independently identify the shortest path.
Metric Propagation
Example
EIGRP update packet for the 10.1.1.0/24 prefix propagating through the AS.
Hop count increments, minimum bandwidth decreases, total delay increases, and RD changes with each router in the AS.
Metric Calculation
Example
Metric from R1 and R2 for the 10.4.4.0/24 network.
Link speed for both routers is 1 Gbps.
Total delay is 30 μs (10 μs for the 10.4.4.0/24 link, 10 μs for the 10.34.1.0/24 link, and 10 μs for the 10.13.1.0/24 link).
Metric = 256 * (10 ^ 7 / Min.BW + Total Delay / 10)
Metric = 256 * (10 ^ 7 / 1,000,000 + 30 / 10) = 3,328
Custom K Values
K values for the path metric formula are set with the command metric weights TOS K1 K2 K3 K4 K5 under the EIGRP process. The TOS value always uses a value of 0.
To ensure consistent routing logic within an EIGRP autonomous system, the K values must match between EIGRP neighbors to form an adjacency and exchange routes. The K values are included as part of the EIGRP hello packet.
BW / Delay Modification
EIGRP delay is set on an interface-by-interface basis allowing for manipulation of traffic patterns flowing through a specific interface on a router.
delay tens-of-microseconds
Bandwidth modification can have a similar effect on the metric calculation formula but can impact other routing protocols like OSPF at the same time. Modifying interface delay impacts only the EIGRP protocol.
Wide Metrics
The original EIGRP specifications measured delay in 10 μs units and bandwidth in kilobytes per second, which did not scale well with higher-speed interfaces.
Traditional EIGRP metrics are scaled in a way that does not pick up bandwidth differences in links over 10G and delay differences in links over 1G.
Delay is converted to picoseconds and is termed latency.
Two additional metric values are included: jitter and energy. These are controlled via a new constant K6, which is zeroed out.
Wide Metric Formula
An additional K value (K6) is included that adds an extended attribute to measure jitter, energy, or other future attributes.
Just as EIGRP scaled by 256 to accommodate IGRP, EIGRP wide metrics scale by 65,535 to accommodate higher-speed links.
Latency is the total interface delay measured in picoseconds (10-12) instead of measuring in microseconds (10-6).
Metric Backward Compatibility
EIGRP wide metrics were designed with backward compatibility in mind. With EIGRP wide metrics, K1 and K3 are set to a value of 1, and K2, K4, K5, and K6 are set to 0, which allows backward compatibility because the K value metrics match with classic metrics.
As long as K1 through K5 are the same and K6 is not set, the two metrics styles allow adjacency between routers.
