Why does an EIGRP router keep a topology table as well as its routing table, and what does each one hold?

## Two tables, two jobs EIGRP is a distance-vector routing protocol, specified publicly as Informational RFC 7868 after starting life as one vendor's protocol. Like every distance-vector protocol, it learns routes from what its neighbours tell it rather than from a map of the whole network. What sets it apart is that it does **not** throw away the advertisements it decides not to use. It keeps two separate structures: - The **topology table** is EIGRP's own working store. RFC 7868 defines it as the structure holding every known destination with its prefix and prefix length, the **feasible distance** (FD), the **reported distance** (RD) of each neighbour advertising it, the **computed distance** (CD) over that neighbour, and the **route state**. - The **routing table** is the router's forwarding decision for every protocol together. EIGRP contributes only the paths it has chosen to forward on. ## The vocabulary of one entry | Term | Meaning | How many | |---|---|---| | Reported distance (RD) | The neighbour's own distance to the destination, as it advertised it | one per neighbour | | Computed distance (CD) | The total distance from this router through that neighbour, built from its RD and the cost of the link to it | one per neighbour | | Feasible distance (FD) | The lowest distance this router has known for the destination since the route last became passive | one per destination | | Successor | A neighbour that provides the least-cost path and passes the loop-freedom test | one or more | | Feasible successor | A neighbour that passes the loop-freedom test, whatever its cost | zero or more | Tools that display the table often show each path as a pair, its total and its reported distance; the total is the CD in the RFC's words. ## What reaches the routing table 1. EIGRP collects every neighbour's advertisement for the destination into the topology table. 2. It computes a CD through each neighbour. 3. The neighbour (or neighbours, on a tie) offering the lowest CD that also passes the **feasibility condition** becomes the **successor**. RFC 7868 says DUAL, the Diffusing Update Algorithm, selects the routes inserted into the routing table based on feasible successors, and calls the least-cost ones successors. 4. Only successors are offered to the routing table. Where several protocols offer the same prefix, the router still chooses between sources by its own ranking (administrative distance), a separate step. The feasibility condition itself, a neighbour's RD strictly below this router's FD, belongs to DUAL; the topology table is where the numbers it compares live. ## Why keep the paths it is not using The payoff is convergence. If the successor's link fails and the topology table already holds a **feasible successor**, the router promotes it to the routing table on the spot. The feasibility condition has already guaranteed that path is loop-free, so no coordination with other routers is needed and the switchover is local. When no feasible successor exists, the route goes active and DUAL starts asking neighbours, which is slower and is DUAL's business rather than the table's. Paths that fail the feasibility condition are also kept. RFC 7868 notes that routes from neighbours that may be upstream are not recorded in the routing table but are saved in the topology table: the router knows they exist, but cannot prove they do not loop back through itself, so it does not forward on them while the route is passive. ## Where candidates go wrong - Treating the topology table as a link-state map of the network. It is not: it holds only what directly connected neighbours reported, per destination. A full map is a link-state protocol's database. - Assuming the feasible successor is pre-installed in the routing table as a hot standby. It is not installed until it is promoted, unless an implementation's unequal-cost load sharing is turned on and admits it. - Believing the topology table holds only feasible paths. It holds every advertisement; the routing table is the filtered view. - Confusing FD with "the current best metric". In a stable network they are equal, but FD is a historical record that can be lower than the current best after a path gets worse. ## Why interviewers ask The question checks whether a candidate understands EIGRP's fast-convergence claim as a consequence of data structure: the backup is computed and proven loop-free before it is needed. A good answer names both tables, states what each entry carries, and says that only successors are installed.