Static Route Recursive Lookup Failing — Route Flaps
Presenting Symptom
A static route configured on a router intermittently disappears from the routing table, causing traffic to be dropped for a few seconds before reappearing.
Network Context
The network is a small branch office with a single router (Cisco 4321, IOS 15.9) connected to an ISP router via a serial link (PPP encapsulation). The branch router has a static default route pointing to the ISP router's IP address (10.0.0.2). The ISP router is reachable via a directly connected serial interface (Serial0/0/0, IP 10.0.0.1/30). The static route uses recursive lookup to reach the next-hop.
Diagnostic Steps
Check the routing table for the static route
show ip route staticS* 0.0.0.0/0 [1/0] via 10.0.0.2
or
(no output if route is missing)If the static route is present, note its next-hop. If it is missing, the route has flapped. The route may appear and disappear intermittently.
Check the routing table for the next-hop reachability
show ip route 10.0.0.2Routing entry for 10.0.0.0/30
Known via "connected", distance 0, metric 0 (connected)
* directly connected via Serial0/0/0
or
Routing entry for 10.0.0.2/32
Known via "connected", distance 0, metric 0 (connected)
* directly connected via Serial0/0/0The static route requires recursive lookup to find the outgoing interface. If the route to 10.0.0.2 is not present or is flapping, the static route will be removed. Look for the route to be present and stable.
Check the interface status of the serial link
show interfaces serial0/0/0Serial0/0/0 is up, line protocol is up
Hardware is HD64570
Internet address is 10.0.0.1/30
MTU 1500 bytes, BW 1544 Kbit/sec, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255If the interface or line protocol is down, the directly connected route will be removed, causing the static route to flap. Look for 'up/up' status. If flapping, the counters may show resets or errors.
Check for interface flaps or errors
show interfaces serial0/0/0 | include (resets|CRC|errors) Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: weighted fair
Output queue: 0/1000/64/0 (size/max total/threshold/drops)
Conversations 0/1/256 (active/max active/max total)
Reserved Conversations 0/0 (allocated/max allocated)
Available Bandwidth 1158 kilobits/sec
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
0 packets input, 0 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
0 packets output, 0 bytes, 0 underruns
0 output errors, 0 collisions, 1 interface resets
0 unknown protocol drops
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier, 0 pause output
0 output buffer failures, 0 output buffers swapped outLook for 'interface resets' or 'input errors'. A high number of resets indicates the interface is flapping, which causes the directly connected route to be removed and re-added, leading to static route removal.
Check the routing table for the directly connected route
show ip route connectedC 10.0.0.0/30 is directly connected, Serial0/0/0
If the connected route is missing, the static route will be removed. The connected route should be present and stable. If it disappears intermittently, the interface is flapping.
Root Cause
The serial interface (Serial0/0/0) is experiencing intermittent flaps due to a faulty cable or line card. When the interface goes down, the directly connected route for 10.0.0.0/30 is removed from the routing table. Since the static default route uses recursive lookup to reach next-hop 10.0.0.2, and that next-hop is no longer reachable, the static route is also removed. When the interface comes back up, the connected route reappears, and the static route is re-installed. This causes the static route to flap.
Resolution
Verification
1. Verify the static route is now stable: show ip route static Expected output: S* 0.0.0.0/0 [1/0] via Serial0/0/0 (or via 10.0.0.2 if using next-hop) The route should remain present over time. 2. Verify the interface is stable: show interfaces serial0/0/0 | include (resets|up) Expected: Serial0/0/0 is up, line protocol is up No recent resets. 3. Monitor the routing table for flaps: debug ip routing Expected: No route removal/addition messages for the static route.
Prevention
1. Use static routes with exit interfaces instead of next-hop IPs when the next-hop is directly connected, to avoid recursive lookup dependency. 2. Implement interface monitoring and alerting to detect physical layer issues early. 3. Use redundant links with floating static routes or dynamic routing protocols to provide failover.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario tests understanding of static route recursive lookup behavior and the conditions under which a static route is removed from the routing table. The exam may present a troubleshooting scenario where a static route is flapping, and the candidate must identify that the next-hop is unreachable due to a down interface. The key fact is that a static route with a next-hop IP requires that the next-hop be reachable via a route in the routing table; if that route disappears, the static route is removed.
Exam Tips
Remember that a static route with a next-hop IP is only installed if the next-hop is reachable via another route in the routing table (recursive lookup).
If a static route is missing, always check the reachability of the next-hop IP first.
The command 'show ip route next-hop-ip' is the quickest way to verify if the next-hop is reachable.
Commands Used in This Scenario
debug ip routing
Use debug ip routing to monitor real-time IP routing table updates and routing protocol events, helping troubleshoot route installation or removal issues.
show interfaces
Displays detailed status and statistics for all interfaces or a specific interface, used to verify interface operational state, errors, and performance.
show ip route
Displays the current IP routing table on a Cisco router, used to verify routes, check next-hop addresses, and troubleshoot connectivity issues.
Test Your CCNA Knowledge
Practice with scenario-based questions to prepare for the CCNA 200-301 exam.
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