A network administrator is investigating a report that users on a specific subnet (192.168.10.0/24) are unable to access resources on another subnet (192.168.20.0/24). Users on the 192.168.20.0/24 subnet can successfully access resources on the 192.168.10.0/24 subnet. The router connecting these two subnets is configured correctly, and pings from the router to hosts on both subnets are successful. Which of the following is the MOST likely cause of this asymmetric communication issue?
- AIncorrect DNS server configuration on the 192.168.10.0/24 subnet clients.
- BAn access control list (ACL) on the router blocking traffic from 192.168.10.0/24 to 192.168.20.0/24.
- CA missing static route on the router for the 192.168.20.0/24 subnet.
- DThe default gateway on the 192.168.10.0/24 clients is misconfigured.
Show answer & explanationAnswer & explanation
Correct answer: B. An access control list (ACL) on the router blocking traffic from 192.168.10.0/24 to 192.168.20.0/24.
The issue describes asymmetric communication: 10.0/24 cannot reach 20.0/24, but 20.0/24 CAN reach 10.0/24. This immediately points to a unidirectional block. Since the router is confirmed to be configured correctly and can ping both subnets, routing itself is likely not the issue. A missing static route would affect both directions or prevent any communication. An ACL specifically designed to filter traffic from one subnet to another, while allowing the reverse, perfectly explains this scenario.
Why the other options are wrong
- A. DNS issues prevent name resolution but not direct IP access, and would likely be symmetric or affect external access primarily.
- C. If a static route was missing on the router, either the router wouldn't know how to reach one of the subnets or both directions would be affected, which contradicts the '20.0/24 can reach 10.0/24' part.
- D. If the default gateway on 192.168.10.0/24 clients was misconfigured, they wouldn't be able to reach any other subnets, including 192.168.20.0/24, but the problem states they can access local resources, implying their gateway is correct for local routing.
Asymmetric Communication Block
A network issue where traffic can flow successfully in one direction between two points, but not in the reverse direction.
- Often caused by firewalls or ACLs with unidirectional rules.
- Can also result from asymmetric routing paths with stateful inspection.
- Troubleshooting involves checking device configurations in both directions.
Memory trick: One-way street for data, often a 'guard' at the gate.