Reported and feasible distance explained
In EIGRP, a local router calculates the metric for each route, but also considers the next-hop router’s metric for that same destination subnet. These metric have their own names:
- Reported (advertised) distance (RD or AD) – the metric advertised by the neighboring router for a specific route. This is the metric of the route used by the neighboring router to reach that specific destionation network.
- Feasible distance (FD) – the local router’s metric of the best route to reach a specific network. The metric is calculated using the metric reported by the neighbor (the advertised distance) plus the metric to the neighbor reporting the route. The route with the lowest FD will be placed in the routing table.
We will try to expain these two metrics with an example:
We have a small network of two routers. Both routers are running EIGRP and the neighbor relationship has been established. R2 is directly connected to the 192.168.0.0/24 subnet and advertises that subnet using EIGRP to R1. The R2’s metric to reach that network is 28160. R1 will use that metric to calculate its own metric to reach the 192.168.0.0/24 subnet. These values are stored in the R1’s topology table:
Notice the numbers in parentheses (30720/28160). The first number (30720) represents the feasible distance (the metric) of R1 to reach the 192.168.0.0/24 subnet. The second number (28160) represents the advertised distance – the metric of R2 to reach that subnet.