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| 5.3 |  | EIGRP Components |
| 5.3.3 |  | EIGRP tables (continued) |

The Topology Table
EIGRP uses its topology table to store all the information it needs to calculate a set of distances and vectors to all reachable destinations. EIGRP maintains a separate topology table for each routed protocol.

The topology table is made up of all the EIGRP routing tables in the autonomous system. By tracking this information, EIGRP routers can find alternate routes quickly. The topology table includes the following fields:

- Feasible distance (FD is xxxx) – The feasible distance (FD) is the lowest calculated metric to each destination. In Figure , the feasible distance to 32.0.0.0 is 2195456 as indicated by FD.
- Route source (by way of xxx.xxx.xxx.xxx) – The source of the route is the identification number of the router that originally advertised that route. This field is populated only for routes learned externally from the EIGRP network. Route tagging can be particularly useful with policy-based routing. In Figure , the route source to 32.0.0.0 is 200.10.10.10 through 200.10.10.10.
- Reported distance (FD/RD) – The reported distance (RD) of the path is the distance reported by an adjacent neighbor to a specific destination. In Figure , the reported distance to 32.0.0.0 is 281600 as indicated by 2195456/281600.

In addition to these fields, each entry includes the interface through which the destination is reachable.

EIGRP sorts the topology table so that the successor routes are at the top, followed by feasible successors. At the bottom, EIGRP lists routes that DUAL believes to be loops in the topology table.

How does an EIGRP router determine which routers are successors and which routers are feasible successors? Assume that the routing table for RTA includes a route to Network Z through RTB. From the point of view of RTA, RTB is the current successor for Network Z. RTA will forward packets destined for Network Z to RTB. RTA must have at least one successor for Network Z for DUAL to place it in the routing table.

Can RTA have more than one successor for Network Z? IIf RTC claims to have a route to Network Z with the exact same metric as RTB, then RTA also considers RTC a successor. DUAL will then install a second route to Network Z through RTC.

Any of the other neighbors of RTA, that advertise a loop-free route to Network Z will be identified as feasible successors in the topology table. However, they must have a higher than best-route metric and lower than FD.

A router views its feasible successors as neighbors that are downstream, or closer, to the destination than it is. If something goes wrong with the successor, DUAL can quickly identify a feasible successor from the topology table and install a new route to the destination. If no feasible successors to the destination exist, DUAL places the route in the active state. Entries in the topology table can be in one of two states: active or passive. These states identify the status of the route indicated by the entry rather than the status of the entry itself.

A passive route is one that is stable and available for use. An active route is a route in the process of being recomputed by DUAL. Recomputation happens if a route becomes unavailable and DUAL cannot find any feasible successors. When this occurs, the router must ask neighbors for help in finding a new, loop-free path to the destination. Neighbor routers are compelled to reply to this query. If a neighbor has a route, it will reply with information about the successor(s). If not, the neighbor notifies the sender that it does not have a route to the destination either.

Excess recomputation is a symptom of network instability and results in poor performance. To prevent convergence problems, DUAL always tries to find a feasible successor before resorting to a recomputation. If a feasible successor is available, DUAL can quickly install the new route and avoid recomputation.

"Stuck in Active" Routes
If one or more routers to which a query is sent do not respond with a reply within the active time of 180 seconds or three minutes, the route, or routes, in question are placed in the "stuck in active" state. When this happens, EIGRP clears the neighbors that did not send a reply and logs a "stuck in active" error message for the route(s) that went active.