EIGRPCCNA 200-301

EIGRP Topology Table Has No Successor or FS

Presenting Symptom

The EIGRP topology table shows routes in active state with no successor or feasible successor, causing routing black holes.

Network Context

A small branch office with two Cisco routers (R1 and R2) running EIGRP AS 100 over a serial link. Both routers are Cisco IOS 15.x. The network has a single LAN subnet 192.168.1.0/24 behind R2. R1 is the hub router connected to the WAN. The problem occurs after a link flap on the serial interface.

Diagnostic Steps

Check EIGRP topology table for missing routes

show ip eigrp topology

P 192.168.1.0/24, 1 successors, FD is 28160
        via 10.1.1.2 (28160/28160), Serial0/0/0

If missing: 
P 192.168.1.0/24, 0 successors, FD is Inaccessible
        via 10.1.1.2 (Inaccessible/Inaccessible), Serial0/0/0

If the route shows 0 successors and FD is Inaccessible, the route is stuck in active state. Normal output shows a successor with a feasible distance.

Verify EIGRP neighbors

show ip eigrp neighbors

H   Address         Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                        (sec)         (ms)       Cnt Num
0   10.1.1.2        Se0/0/0         13   00:12:34   1     100  0  15

If the neighbor is listed, adjacency is up. If not, there is a Layer 2 or EIGRP hello issue. If neighbor is up but route is missing, the problem is likely a stuck-in-active or a query not replied.

Check for stuck-in-active routes

show ip eigrp topology active

EIGRP-IPv4 Topology Table for AS(100)/ID(10.1.1.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

A 192.168.1.0/24, 1 successors, FD is Inaccessible
    1 replies, 0 replies, 0 sia replies
    via 10.1.1.2, Serial0/0/0, r, 00:01:23

If a route is in Active state (A) and has been waiting for replies, it may be stuck-in-active. The 'r' indicates a reply is pending. If the timer exceeds 3 minutes, the route will be SIA (Stuck In Active).

Check EIGRP timers and SIA status

show ip eigrp topology 192.168.1.0/24

EIGRP-IPv4 Topology Entry for AS(100)/ID(10.1.1.1) for 192.168.1.0/24
  State is Active, 0 successors, FD is Inaccessible
  Active timer: 00:02:45
  Query origin flag: 1, 1 outstanding replies
  via 10.1.1.2, Serial0/0/0, r, 00:02:45

If the Active timer is approaching 3 minutes, the route is about to become SIA. This indicates that the neighbor is not replying to queries, possibly due to a unidirectional link or high CPU.

Root Cause

The EIGRP neighbor is not replying to queries because of a unidirectional link failure on the serial interface. The interface is up/up on R1 but the return path is broken, so R2 receives queries but cannot send replies. This causes the route to remain in Active state until the SIA timer expires, after which the route is removed.

Resolution

1. Identify the unidirectional link by checking interface counters for input/output errors. 2. Replace faulty cable or hardware. 3. Clear the EIGRP topology to force re-convergence: R1# clear ip eigrp 100 topology Or reset the neighbor: R1# clear ip eigrp 100 neighbors 10.1.1.2

Verification

Run 'show ip eigrp topology' and verify the route is now Passive with a successor: P 192.168.1.0/24, 1 successors, FD is 28160 via 10.1.1.2 (28160/28160), Serial0/0/0 Also check 'show ip route eigrp' to confirm the route is in the routing table.

Prevention

1. Use bidirectional forwarding detection (BFD) to detect link failures faster. 2. Implement EIGRP stub routing on spoke routers to limit queries. 3. Ensure physical layer redundancy and proper cabling to avoid unidirectional links.

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario appears in troubleshooting questions where a route is missing from the routing table. The exam tests the ability to interpret 'show ip eigrp topology' output to identify a stuck-in-active route. Candidates must know that a route with 0 successors and FD Inaccessible indicates a problem, and that the 'show ip eigrp topology active' command reveals the active state.

Exam Tips

Memorize that a route in Active state with 'r' (reply pending) means the router is waiting for a reply from a neighbor; if it lasts over 3 minutes, it becomes SIA.

Know that 'show ip eigrp topology' shows successors; if 0 successors, the route is not usable.

Remember that clearing the EIGRP process or resetting neighbors can resolve stuck-in-active, but the root cause (e.g., unidirectional link) must be fixed first.

Commands Used in This Scenario

show ip eigrp neighbors

Displays the neighbor table for EIGRP, showing all directly connected EIGRP routers and their status, used to verify EIGRP adjacencies and troubleshoot neighbor relationships.

show ip eigrp topology

Displays the EIGRP topology table, showing all learned routes and their feasible successors, used to verify EIGRP convergence and path selection.

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