Lab Objective
Examine the EIGRP topology table for a specific destination network, identify the successor and any feasible successor, and connect this underlying DUAL data directly to what appears in the IP routing table.
Lab Purpose
The IP routing table shows only the single best path EIGRP has chosen, but the topology table reveals the complete picture DUAL, discussed earlier in this series, actually used to make that choice — including any backup path already verified as loop-free and ready for instant use if the primary path fails.
Lab Topology
R1 has two paths to reach network 192.168.50.0/24:
Path via R2: cost 3072
Path via R3: cost 3840
Both R2 and R3 are directly connected
neighbors of R1, offering redundant paths
to the same destinationTask 1: Verify the Routing Table Shows Only One Path
Confirm the IP routing table lists only the single best (successor) path to 192.168.50.0/24.
Task 2: Examine the Full Topology Table Entry
Display the complete topology table entry for 192.168.50.0/24, showing every known path, not just the installed one.
Task 3: Identify the Successor and Feasible Successor
Determine which path is the successor (installed in the routing table) and which qualifies as a feasible successor (backup, loop-free).
Task 4: Verify the Feasibility Condition
Confirm the feasible successor's reported distance is genuinely lower than the successor's feasible distance, the mathematical condition that makes it safe to use as an instant backup.
Solution and Verification
R1# show ip route 192.168.50.0
Routing entry for 192.168.50.0/24
Known via "eigrp 100", distance 90, metric 3072
Routing Descriptor Blocks:
* 10.1.1.2 (via R2), from 10.1.1.2, ...
-- Only the single best path (via R2) appears
-- here, with no indication a backup even existsR1# show ip eigrp topology 192.168.50.0/24
EIGRP-IPv4 Topology Entry for AS(100)/ID(1.1.1.1)
for 192.168.50.0/24
State is Passive, Query origin flag is 1,
1 Successor(s), FD is 3072
Descriptor Blocks:
10.1.1.2 (Serial0/0/0), from 10.1.1.2,
Composite metric is (3072/2816), route is Internal
10.1.2.2 (Serial0/0/1), from 10.1.2.2,
Composite metric is (3840/2816), route is Internal
-- Both paths are visible here, unlike the
-- routing table view-- Reading the composite metric format (FD/RD):
Via R2: (3072/2816) -- FD is 3072, this router's
own metric to reach the destination
Via R3: (3840/2816) -- higher FD, not the
installed path
-- The successor is the path via R2, since
-- its FD (3072) matches the topology entry's
-- overall FD -- this is the path in the
-- routing table
-- For R3's path to qualify as a feasible
-- successor: R3's REPORTED distance (2816,
-- the second number) must be less than R2's
-- FD (3072) -- checking: 2816 < 3072 ✓
-- This confirms R3's path IS a feasible
-- successor, ready for instant useKey Takeaway
The composite metric shown as (FD/RD) in the topology table directly encodes the feasibility condition discussed earlier in this series: whenever a neighbor's reported distance (the second number) is less than the successor's feasible distance, that neighbor is mathematically guaranteed loop-free and instantly promotable if the successor fails — reading this single line of output is how an engineer confirms whether true near-instant EIGRP failover is actually available for a given destination, or whether a failure would instead trigger the slower query process.