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RIPCCNA 200-301

RIP Route Not Installing Due to 16-Hop Limit

Presenting Symptom

A route to a remote network is missing from the routing table, even though the RIP routing protocol is configured on all routers in the path.

Network Context

The network consists of a chain of five Cisco routers (R1 through R5) running RIP version 2 in a small branch office. Each router is connected via serial links with a metric of 1 hop. The network uses IOS 15.x. The engineer notices that R1 cannot reach the loopback interface on R5, which is 5 hops away.

Diagnostic Steps

1

Check the routing table on the source router

show ip route rip
R     10.0.0.0/8 [120/1] via 192.168.1.2, 00:00:17, Serial0/0/0
R     10.0.1.0/24 [120/2] via 192.168.1.2, 00:00:17, Serial0/0/0
R     10.0.2.0/24 [120/3] via 192.168.1.2, 00:00:17, Serial0/0/0
R     10.0.3.0/24 [120/4] via 192.168.1.2, 00:00:17, Serial0/0/0

The routing table shows RIP routes up to 4 hops away, but the destination network (5 hops away) is missing. This indicates the route is not being installed, likely due to exceeding the hop count limit.

2

Verify the hop count to the destination

traceroute 10.0.4.1
Type escape sequence to abort.
Tracing the route to 10.0.4.1
VRF info: (vrf in name/id, vrf out name/id)
  1 192.168.1.2 4 msec 4 msec 4 msec
  2 192.168.2.2 4 msec 4 msec 4 msec
  3 192.168.3.2 4 msec 4 msec 4 msec
  4 192.168.4.2 4 msec 4 msec 4 msec
  5 10.0.4.1 4 msec 4 msec 4 msec

Traceroute confirms the destination is 5 hops away. RIP has a maximum hop count of 15; routes with a metric of 16 are considered unreachable. Since the path is 5 hops, the metric should be 5, which is within limits. However, if any router has misconfigured the metric offset, the hop count might be inflated.

3

Check the RIP database on the source router

show ip rip database
10.0.0.0/8    auto-summary
10.0.0.0/8
    [1] via 192.168.1.2, 00:00:17, Serial0/0/0
10.0.1.0/24
    [2] via 192.168.1.2, 00:00:17, Serial0/0/0
10.0.2.0/24
    [3] via 192.168.1.2, 00:00:17, Serial0/0/0
10.0.3.0/24
    [4] via 192.168.1.2, 00:00:17, Serial0/0/0
10.0.4.0/24
    [16] via 192.168.1.2, 00:00:17, Serial0/0/0

The RIP database shows the route to 10.0.4.0/24 with a metric of 16 (poisoned route). This indicates that somewhere along the path, a router has set the metric to 16, making it unreachable. This could be due to a route poisoning mechanism or a misconfiguration.

4

Examine the RIP configuration on intermediate routers for metric offset

show running-config | section router rip
router rip
 version 2
 network 10.0.0.0
 network 192.168.1.0
 network 192.168.2.0
 offset-list 0 in 11 Serial0/0/0

The offset-list command adds 11 to the incoming metric on Serial0/0/0. This inflates the hop count, causing routes learned on that interface to have a metric increased by 11. For a route that is 5 hops away, the metric becomes 5+11=16, making it unreachable.

Root Cause

An offset-list is configured on an intermediate router (R4) that adds 11 to the incoming metric on its Serial0/0/0 interface. This causes the route to the destination (5 hops away) to have a metric of 16, which is considered unreachable by RIP. The route is therefore not installed in the routing table.

Resolution

Remove the offset-list configuration on the intermediate router. On R4: ``` configure terminal router rip no offset-list 0 in 11 Serial0/0/0 end ``` This removes the metric offset, allowing the route to be advertised with its correct hop count.

Verification

On R1, verify the route is now installed: ``` show ip route rip ``` Expected output includes the previously missing route: ``` R 10.0.4.0/24 [120/5] via 192.168.1.2, 00:00:17, Serial0/0/0 ``` Also verify the RIP database shows metric 5: ``` show ip rip database ``` Expected output: ``` 10.0.4.0/24 [5] via 192.168.1.2, 00:00:17, Serial0/0/0 ```

Prevention

["Avoid using offset-lists unless absolutely necessary for traffic engineering; if used, ensure the total metric does not exceed 15.","Document all offset-list configurations and review them during network changes to prevent accidental metric inflation.","Use a routing protocol with a larger metric range (e.g., OSPF, EIGRP) if the network has more than 15 hops."]

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify why a RIP route is missing. The exam tests understanding of RIP hop count limit (15) and the effect of offset-lists. Candidates must know that a metric of 16 means unreachable and be able to interpret show ip rip database output.

Exam Tips

1.

Remember that RIP maximum hop count is 15; 16 is unreachable.

2.

Offset-list can artificially increase metric; look for it in running-config.

3.

Use 'show ip rip database' to see the metric of routes; if metric is 16, the route is poisoned.

Commands Used in This Scenario

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.

traceroute [ip]

Traces the route packets take from the source device to a destination IP address, showing each hop along the path, used to diagnose network path issues and latency.

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