Understanding EIGRP Metric Coefficients and Metrics

In EIGRP (Enhanced Interior Gateway Routing Protocol), the metric coefficients, commonly referred to as K-values, play a vital role in determining routing decisions. RFC 7868: Section 5.5 elaborates on these coefficients and related metrics. Let's explore them in detail:

Metric Coefficients (K-Values):

• Metric coefficients, often referred to as K-values, are crucial components of EIGRP's metric calculation. Each K-value corresponds to a specific metric element:
  • K1 = 1: Represents Throughput
  • K2 = 0: Indicates Load
  • K3 = 1: Denotes Latency
  • K4 = 0: Signifies Reliability
  • K5 = 0: Also Reliability
  • K6 = 0: Reserved for Future Use or Energy/Jitter

Important Note - MTU:

• It's essential to clarify that the Maximum Transmission Unit (MTU) is not considered a K-value in EIGRP. Furthermore, MTU does not factor into the calculation of the composite metric.

Units of Measure:

• Units of measure for primary metrics are defined as follows:
  • Throughput (THROUGHPUT) is measured in Kilobits per second (Kbps).
  • Latency (LATENCY) is expressed in picoseconds (ps).

Compatibility and Scaling:

• EIGRP's metric calculation, with these additional metrics, is 100% backward compatible with Classic Mode. These metrics are automatically enabled when you use Named Mode.
• Bandwidth (BW) and Throughput (TP) are computed as (10^7 / BW) * 256, without differentiation between various speeds like 10Gbps, 40Gbps, or 100Gbps.
• Delay (DELAY) and Latency (LATENCY) are adjusted for metric calculation, where DELAY was originally in microseconds ((DELAY / 10) * 256) and LATENCY is represented in picoseconds ((LATENCY * 65536) / 10^6).

RIB Scaling:

• To ensure metric values fit within the Routing Information Base (RIB), a scaling factor known as RIB_SCALE is applied. This factor can be configured from 1 to 255.
• Wide metrics are scaled to fit into the RIB since the RIB only accommodates 32-bit values.

EIGRP Metric Calculation Examples

EIGRP uses a composite metric formula to calculate routing metrics based on K-values (metric coefficients). Let's explore some practical examples of how EIGRP calculates metrics for different routes.

Example 1: Calculating Metric for a Route with Default K-Values

Suppose we have a route with the following properties:

• Bandwidth (BW) = 1 Mbps
• Delay (DELAY) = 10 microseconds
• Default K-Values: K1=1, K2=0, K3=1, K4=0, K5=0

Using the composite metric formula with default K-values:

Metric = ([K1 * BW] + [K3 * DELAY]) * 256 Metric = ([1 * 1000 Kbps] + [1 * 10 microseconds]) * 256 Metric = (1000 + 10) * 256 Metric = 257,760 The calculated metric for this route is 257,760.

Verification Commands:

To manage and fine-tune EIGRP metrics and metrics scaling, utilize the following verification commands:

• show ip protocols: Provides insights into the configured EIGRP metrics and settings.
• metric rib-scale [1-255]: Adjusts the RIB scaling factor to align metrics with the RIB requirements.
• metric weights [tos] k1 k2 k3 k4 k5 k6: Fine-tunes K-values to influence metric calculations.
• show ip eigrp topology x.x.x.x: Displays detailed EIGRP topology information for specific routes.
• offset-list [acl] [in/out] [offset value] [interface]: Applies offset lists to manipulate metrics for route selection.

In summary, understanding EIGRP Metric Coefficients and associated metrics is essential for optimizing routing decisions. These metrics, along with their scaling and compatibility considerations, are valuable tools for network administrators managing EIGRP networks.