What is the difference between reported distance and feasible distance?
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Say this first
Reported distance, also called advertised distance, is the neighbor's own metric to the destination:
the number it tells me. Feasible distance is my best total metric to the destination, which is a
neighbor's reported distance plus my cost to reach that neighbor, for the best path. In
show ip eigrp topology each line shows (my metric through that neighbor/that neighbor's RD).
Go deeper
Reading the numbers
P 10.50.0.0/24, 1 successors, FD is 3328
via 10.0.12.2 (3328/3072), GigabitEthernet0/0/0
via 10.0.13.3 (26368/3072), GigabitEthernet0/0/1- Through R2: R2 reports 3072, R1 adds its own gigabit link and arrives at 3328.
- Through R3: R3 also reports 3072, but R1's link to R3 is a 100 Mbps circuit, so R1's total is 26368.
- The FD is the best total: 3328.
The precise definition
Strictly, the feasible distance is the lowest metric the router has had to that destination since the route last went from active to passive. In a stable network that is simply the successor's metric. The difference only matters while metrics are changing, and it is what keeps the feasibility check safe during those changes.
Why EIGRP needs both
The reported distance is how DUAL decides whether a neighbor is a safe backup. If the neighbor's own distance is lower than my best distance, the neighbor is closer to the destination than I am, so its path cannot run through me.
They will ask next
Where does the reported distance come from on the wire?
From the neighbor's update. It sends its vector metric (minimum bandwidth, total delay, and the other fields). I compute its RD from that vector, then add my own interface delay and take the lower of the two bandwidths to get my total through that neighbor.
Traps
- Swapping the two numbers inside the brackets. The first is my total, the second is what the neighbor reported.