Updating the Routing Table in Active Mode with DUAL

Figure 7-5 shows that the router has no acceptable route to substitute, however, and must therefore go into active mode to query its neighbors.

Figure 7-5 The Use of Feasible and Advertised Distance—Active Mode

Figure 7-5 The Use of Feasible and Advertised Distance—Active Mode

When no alternative route is found in the routing table, the following actions are taken (using the network in Figure 7-5 as an example):

• In Figure 7-5, the topology table of Router A has a path (successor) of A to D to G to X.

• The FD is 20, and the AD from Neighbor D is 15.

• When Router D dies, Router A must find an alternative path to X.

• Neighbors B, C, E, and F have ADs of 27, 27, 20, and 21, respectively.

• Because all the neighbors have an AD that is the same or greater than the successor FD, none of these are acceptable as FSs.

• Router A must go into active mode and send queries to the neighbors.

• Both Routers E and F reply with an FS because both have an AD from G of 5. Remember the equation FD > AD; their FD is 20, and 20 > 5.

• This is acceptable. The topology and routing tables will be updated, DUAL will be calculated, and the network will be returned to passive mode.

• From this information received from Routers E and F in Figure 7-5, the router selects the path through E as the best route because it has the lower cost.

• The result is placed in the routing table as the valid neighboring router. EIGRP refers to this neighboring router as a successor.

• Router F will be stored as an FS in the topology table.

The details on how EIGRP computes successors are complex, but the concept is simple.

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