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Difficulty: MediumTroubleshooting Routing, Default Gateways, and ACLs

A network administrator is troubleshooting connectivity issues on a Linux backup server. The server is assigned the static IP address 172.16.88.200/25172.16.88.200/25. Local diagnostic testing reveals that the server can communicate with hosts on its immediate local network segment, but traffic destined for remote subnets fails completely. The administrator verifies the host routing table output:

text
default via 172.16.88.1 dev eth0
172.16.88.128/25 dev eth0 proto kernel scope link src 172.16.88.200

Which of the following identifies the root cause of the external connectivity failure?

  1. The assigned default gateway IP address resides on a different IP subnet than the host server.Answer
  2. B
    The access control list on the upstream router contains an implicit deny statement blocking outbound traffic.
  3. C
    The server is missing a DHCP relay agent configuration on interface eth0 to negotiate remote routes.
  4. D
    The local interface is experiencing a duplex mismatch leading to high packet drop rates on remote frames.

Answer

The assigned default gateway IP address resides on a different IP subnet than the host server.
The server's IP address 172.16.88.200/25172.16.88.200/25 places it in the 172.16.88.128/25172.16.88.128/25 subnet (range 172.16.88.128172.16.88.128 - 172.16.88.255172.16.88.255). The configured default gateway 172.16.88.1172.16.88.1 resides in the adjacent 172.16.88.0/25172.16.88.0/25 subnet. A default gateway must be on the same local subnet as the host interface for Layer 2 ARP resolution and off-subnet forwarding to function.

Step-by-Step Solution

1
Analyze host IP address and subnet mask.
Host IP 172.16.88.200172.16.88.200 with CIDR /25/25 (255.255.255.128255.255.255.128) resides in subnet 172.16.88.128/25172.16.88.128/25. Usable host range is 172.16.88.129172.16.88.129 to 172.16.88.254172.16.88.254.
Subnet math determines the boundaries of the local broadcast domain.
2
Evaluate the configured default gateway address.
The configured default gateway is 172.16.88.1172.16.88.1, which falls into subnet 172.16.88.0/25172.16.88.0/25 (usable range 172.16.88.1172.16.88.1 to 172.16.88.126172.16.88.126).
Default gateways must reside on the same IP subnet as the local interface.
3
Determine the impact on host routing.
Because 172.16.88.1172.16.88.1 is not on the 172.16.88.128/25172.16.88.128/25 local network segment, ARP resolution for the default gateway fails, preventing remote traffic delivery.
Layer 3 forwarding requires the next-hop router IP to be directly reachable at Layer 2.

Key Concept

Default Gateway Subnet Alignment
Estimated Time:1m 30s
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Topics

Troubleshooting Routing, Default Gateways, and ACLs

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