1.3b Best Path Selection

1.3b i RD, FD, FC, successor, feasible successor

RD	Reported Distance, the total metric along a path to a destination network as advertised by an upstream router.
FD	Feasible Distance, the best metric along a path to a destination, includes metric to the neighbour advertising the path. Lowest calculated metric to each destination.
FC	Feasibility Condition, a condition placed on a route when the reported distance is less than the feasible distance which turns that route into a backup route. Serves to ensure that the backup route is loop-free.
Successor	The first next-hop router to a destination.
Feasible Successor	A route that satisfies the feasibility condition and is maintained as a backup route.

Example for RD and FD

Reported Distance (RD) = Cost to get to a destination, as advertised by the neighbor. R1 tells R2 that it sees 20.1.1.0/24 at a cost of 50.

Feasible Distance (FD) = Total path cost to get to a destination. In this example 100 (cost of the link between R1 and R2) + 50 (the RD from R1) = 150

Example for Successor, Feasible successor and FC

Successor = Best route to destination. In this case this would be via R2 (100 + 25 = 125). Installed in routing table.

Feasible Successor = Backup path to same destination. In this case this would be via R3 (100 +50 = 150). Stored in topology table until needed.

FC (Feasibility Condition) = RD < FD (OK route can be installed)

1.3b ii Classic Metrics and Wide Metrics 

EIGRP uses the lowest composite metric base on:

• Bandwidth: The bandwidth value used in the EIGRP metric calculation is determined by dividing 10,000,000 by the bandwidth (in kbps) of the slowest link along the path to the destination network.
  
• Delay: Unlike bandwidth, which represents the “weakest link,” the delay value is cumulative. Specifically, it’s the sum of all delays associated with all of the interfaces that must be exited in order to get to the destination network. The output of the show interfaces command shows an interface’s delay in microseconds. However, the value used in EIGRP’s metric calculation is in tens of microseconds. This means you would add up all of the egress interface delays as seen in the show interfaces output for each egress interface, and then divide by 10 to get the tens of microseconds unit of measure.
  
• Load: Similar to reliability, load is a value used in the numerator of a fraction, with 255 as its denominator. The value of the fraction indicates how busy a link is. For example, a load value of 1 indicates the link is minimally loaded (that is, 1/255 = 0.004 < 1 percent).
  
• Reliablity: The reliability is a value used in the numerator of a fraction, with 255 as its denominator. The value of the fraction indicates the reliability of a link. For example, a reliability value of 255 indicates the link is 100 percent reliable (that is, 255/255 = 1 = 100 percent).

Classic Path Metric formula: 

EIGRP uses K values in order to complete this formula all of which are configurable under EIGRP. 

EIGRP K values are the metrics that EIGRP uses to calculate routes. Mismatched K values can prevent neighbor relationships from being established and can negatively impact network convergence.

Employing these default values, a much shorter way of conveying the formula is as follows: 

Since this method is reliant on the number 256 and measuring delay in microseconds, it has its short comings as bandwidth in networks increases, with 10^7 maxing out it’s potential on a 10Gbps interface.  Any interfaces faster than this are attributed the same value of metric as a 10Gbps creating a problem. 

EIGRP Wide Metrics

The EIGRP Wide Metrics feature supports 64-bit metric calculations and Routing Information Base (RIB) scaling in Enhanced Interior Gateway Routing Protocol (EIGRP) topologies. The 64-bit calculations work only in EIGRP named mode configurations.

The wide metrics are enable by default when using EIGRP name mode.

Wide Metrics measures delay in picoseconds and has a scalability of 65,535 giving the potential for interfaces to be properly scaled to speeds of up to 4.2 Tbps. 

Wide metric formula: 

Wide metric = 65,535 * [(K1 * 10^7 / Min. Bandwidth + K2 * 10^7 / Min. Bandwidth / 256 – load +K3 * latency / 10^6 + K6  * extended) * K5 /K4 + Reliability] 

Notice the additional K6 value that represents an extended attribute to measure jitter, energy or other future attributes. 

Reference: IP Routing EIGRP Configuration Guide, Cisco IOS Release 15SY