The Diffusing Update Algorithm

The Diffusing Update Algorithm (DUAL) is the brain behind the operation of EIGRP. It is an algorithm that tracks all the routes advertised from a neighbor and then selects a loop-free path to the destination. Before discussing the details of DUAL, you must understand several terms and concepts:

• Feasible distance (FD) ‚ Feasible distance is the minimum metric along the path to a destination. Figure 6-1 shows the feasible distance calculation to reach Network 7 for each of Router A's neighbors, from Router A's perspective.

  Figure 6-1. Feasible Distance Calculation

  

• Reported distance (RD) ‚ Reported distance, sometimes also known as advertised distance, is the metric toward the destination, as advertised by the upstream neighbor. In other words, the reported distance is the neighbor's metric going to the destination. Figure 6-2 shows the reported distance calculation to reach Network 7 for each of Router A's neighbors.

  Figure 6-2. Reported Distance Calculation

  

• Feasibility condition (FC) ‚ The feasibility condition (FC) is a condition in which the reported distance (RD) is less than the feasible distance (FD). In other words, the feasibility condition is met when the neighbor's metric to a destination is less than the local router's metric. This condition is important to ensure a loop-free path.

• EIGRP successor ‚ A successor is a neighbor that met the feasibility condition (FC) and has the lowest metric toward the destination. A successor is used as the next hop to forward the packet going to the destination network.

• Feasible successor ‚ A feasible successor is a neighbor that satisfies the feasibility condition (FC) but is not selected as the successor. The feasible successor can be thought of as a potential backup route when the primary route goes away.

  Figure 6-3 illustrates the concepts of successor and feasible successor.

  Figure 6-3. Explanation of Successor and Feasible Successor

  

  Router B is chosen as the successor because Router B has the lowest feasible distance (metric = 121) to Network 7 among all of Router A's neighbors. To select a feasible successor, Router A sees which neighbor has a reported distance (RD) that is less than the feasible distance of its successor. In this case, Router H has a reported distance of 30, which is less than the feasible distance of its successor, which is 121. Therefore, Router H is chosen as the feasible successor. Router D is neither a successor nor a feasible successor because its reported distance is 140, which is larger than 121 and thus does not satisfies the feasibility condition.

• Passive route ‚ A passive route in EIGRP indicates that the router has a valid successor to a destination and EIGRP has no complaints.

• Active route ‚ An active route in EIGRP indicates that the router has lost its successor, it doesn't have any feasible successor available, and the router is currently actively searching for alternate routes to converge.