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Difficulty: HardTroubleshooting Routing, Default Gateways, and ACLs

A network administrator is troubleshooting an issue where a database server at 192.168.50.10/24192.168.50.10/24 cannot establish a connection to a cloud backup endpoint at 172.24.100.45/24172.24.100.45/24. The database server is configured with 192.168.50.1192.168.50.1 as its default gateway.

A traceroute executed from the database server produces the following output:

C:\> traceroute 172.24.100.45
Tracing route to 172.24.100.45 over a maximum of 30 hops:
1 <1 ms <1 ms <1 ms 192.168.50.1
2 2 ms 1 ms 2 ms 10.200.1.1
3 * * * Request timed out.
4 * * * Request timed out.

The administrator connects to router 10.200.1.110.200.1.1 and reviews its routing table and interface status:

Gateway of last resort is 10.200.2.2 to network 0.0.0.0

S* 0.0.0.0/0 [1/0] via 10.200.2.2
C 10.200.1.0/30 is directly connected, GigabitEthernet0/0
C 192.168.50.0/24 is directly connected, GigabitEthernet0/1
S 172.24.0.0/16 [1/0] via 10.200.3.2

GigabitEthernet0/0 is up, line protocol is up (IP 10.200.1.1/30)
GigabitEthernet0/1 is up, line protocol is up (IP 192.168.50.1/24)
GigabitEthernet0/2 is down, line protocol is down (IP 10.200.3.1/30)

Which of the following is the root cause of the connectivity failure?

  1. The router selects the static route 172.24.0.0/16172.24.0.0/16 based on longest prefix match, but its associated outbound interface is down.Answer
  2. B
    The default gateway on the database server resides on a different IP subnet than the host address.
  3. C
    The subnet mask /16/16 exceeds the usable host range allowed for classful routing protocols.
  4. D
    An implicit deny Access Control List (ACL) rule on interface GigabitEthernet0/0 drops all default gateway traffic.

Answer

The router selects the static route 172.24.0.0/16172.24.0.0/16 based on longest prefix match, but its associated outbound interface is down.
Routers forward packets based on the Longest Prefix Match rule. Because 172.24.0.0/16172.24.0.0/16 is more specific than the default route 0.0.0.0/00.0.0.0/0, traffic destined for 172.24.100.45172.24.100.45 matches the static route via 10.200.3.210.200.3.2. However, interface GigabitEthernet0/2 is down, causing router 10.200.1.110.200.1.1 to drop the traffic rather than forwarding it.

Step-by-Step Solution

1
Analyze traceroute diagnostic output
The packet successfully reaches hop 1 (192.168.50.1192.168.50.1) and hop 2 (10.200.1.110.200.1.1), but times out at hop 3, indicating the breakdown occurs at router 10.200.1.110.200.1.1.
Identify the specific hop where packet forwarding fails.
2
Evaluate the routing decision logic on router 10.200.1.110.200.1.1
The target address is 172.24.100.45172.24.100.45. The router evaluates candidate routes: default route 0.0.0.0/00.0.0.0/0 vs static route 172.24.0.0/16172.24.0.0/16.
Routers apply Longest Prefix Match (LPM) rules, preferring /16/16 over /0/0 regardless of default gateway settings.
3
Correlate the selected route with interface operational state
The route to 172.24.0.0/16172.24.0.0/16 forwards packets via next-hop 10.200.3.210.200.3.2 out interface GigabitEthernet0/2, which has a status of line protocol down.
Traffic matching a static route pointing to an inactive or down interface will be dropped, preventing failover to less specific routes if not dynamically withdrawn.

Key Concept

Longest Prefix Match Routing and Next-Hop/Interface Unreachability
Estimated Time:2m 0s
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Topics

Troubleshooting Routing, Default Gateways, and ACLs

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