EIGRPv6 uses the Diffusing Update Algorithm (DUAL) to:

  • Calculate the Distances of routes from the source network to destination network.
  • Inject loop-free routes into the EIGRPv6 topology table.
  • Select the optimal route to a destination network from the topology table and injects it into the IPv6 routing table. (Sub-optimal routes remain in the topology table.)
  • If the primary route fails and the topology has no backup routes, DUAL will search the EIGRPv6 domain for a backup route.

This lesson will focus on how DUAL calculates Distances for EIGRPv6.

Being able to calculate Distance is important because when we design a network, we want to ensure that it behaves as expected (load balancing, selection of primary and backup routes, etc.).

There are two Distances calculated for each route. These Distances are used to ensure loop-free routes.

  1. Feasible Distance (FD) – The Distance from the local network to the destination network.
  2. Reported Distance (RD) – The Distance from the local network's next-hop router.

Feasible and Reported Distances
Feasible and Reported Distances

In this topology, we're interested in the Distances from R-1 to R-3. The Feasible Distance is the cost of all links from the local router (R-1) to the destination router (R-3). The Reported Distance is the cost from the local router’s next-hop router (R-2) to the destination router.

Next we'll discuss DUAL's formula so that we can calculate Distances manually.

DUAL Default Metrics

When EIGRP neighbors share routes, they also share information about those routes. This includes metrics called "K Values".

  • K1 = Bandwidth – The value of the slowest link on the path to the destination network in Kilo-bits per second.
  • K2 = Load – Value is 0-255 (255 is 100 percent loading).
  • K3 = Delay – The sum of all delays on the path in tens of microseconds.
  • K4 = Reliability - Value is 0-255 (255 is 100 percent reliable).
  • K5 = MTU – The Maximum Transmission Unit size along the route.

K values are used to calculate the Feasible and Reported Distances.

For a neighborship to form, neighbors on both sides of the link must have matching K values.

K values can be verified by using the show ipv6 protocols command. If a K value is disabled it will have the value of 0, enabled 1.

Sample output:

R-1#show ipv6 protocols  
[Output Abbreviated]
EIGRP-IPv6 Protocol for AS(6) 
 Metric weight K1=1, K2=0, K3=1, K4=0, K5=0

Here we can see that only K1 (Bandwidth) and K3 (Delay) are enabled as they have the value of 1. These are the only K values that are enabled by default.

Pro Tip: Using only bandwidth (K1) and delay (K3) is considered a best practice. If, for example, Load or Reliability K values are enabled the network could become unstable. These values could change often causing DUAL to rapidly change the primary and backup routes.

Because it's so strongly recommended to only use the default metrics of Bandwidth of Delay, we'll focus here first. Then, we'll further discuss the using more K value metrics.

Bandwidth Metric

The formula for the bandwidth metric is:
Bm = 10,000,000 / Lowest Link Bandwidth (Kbps). If needed, round down to a whole number.

Bandwidth and Delay Topology

In the above topology, there are two paths from R-1 to LAN-A.

From R-1 to LAN-A via R-2, the link from R-4 to LAN-A has the lowest bandwidth value at 100,000 Kbps.

From R-1 to LAN-A via R-3, the links from R-1>R-3 and from R-4 to LAN-A both have the lowest bandwidth value at 100,000 Kbps. It doesn’t matter how many links have the lowest bandwidth value. It is the value that we’re interested in.

For both paths from R-1 to LAN A, the Bandwidth value to calculate the Bandwidth metric is 100,000 Kbps.

Bandwidth Metric = 10,000,000 / Lowest Link Bandwidth
Bm = 10,000,000 / 100,000 = 100

Delay Metric

When we talk about using DUAL's Delay metric, we’re referring to the sum of the delays for all links on the path from the local network to the destination network divided by 10.

Delay Metric = Sum of all link delays / 10.

From R-1 to LAN-A via R-2, the sum of all delays is:
R-1>R-2 = 10
R-2>R-4 = 10
R-4>LAN-A = 100
Sum of all delays = 120
Dm = 120/10 = 12

From R-1 to LAN-A via R-3, the sum of all delays is:
R-1>R-3 = 100
R-3>R-4 = 10
R-4>LAN-A = 100
Sum of all delays = 210
Dm = 210/10 = 21


To calculate the Distance (Composite Metric) using only the (Individual Metrics) of Bandwidth and Delay the DUAL formula is simplified to:

EIGRP Default Metric = (Bandwidth Metric + Delay Metric) * 256.
Path via R-2 = (100+12) * 256 = 28672
Path via R-3 = (100+21) * 256 = 30976

Next, we'll walk through a lab that will expand on the above.


In this lab, we'll manually calculate the Feasible and Reported Distances for the two paths from R-1 to LAN-A using the default DUAL metrics(bandwidth metric and delay metric). Then, we’ll compare our manual calculations with what EIGRPv6 automatically calculated.

DUAL Lab FD and RD Topology
FD and RD Topology

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