101 · Question #565

101 Question #565: Real Exam Question with Answer & Explanation

The correct answer is D: The physical port MAC address on the bridge for user B's workstation. In some transparent bridge implementations that perform MAC substitution or act as a transparent proxy, the ARP entry User B sees for User A reflects the bridge port MAC on User B's segment, not User A's actual NIC MAC.

Question

User A and User B's workload are on the same VLAN, but connected through a transparent layer 2 bridge in use B's ARP table. Which MAC address is reported for user A's workstation?

Options

AThe physical port MAC address on the bridge for user A's workstation
BThe MAC address of the 12 bridge
CThe MAC address of User A's workstation
DThe physical port MAC address on the bridge for user B's workstation

Explanation

In some transparent bridge implementations that perform MAC substitution or act as a transparent proxy, the ARP entry User B sees for User A reflects the bridge port MAC on User B's segment, not User A's actual NIC MAC.

Common mistakes.

A. The bridge port MAC facing User A's segment is learned internally by the bridge as the ingress MAC, but it is not the address that User B's ARP table records for User A.
B. The bridge's overall management or base MAC is used for traffic directed at the bridge itself, not for forwarded user traffic appearing in downstream ARP caches.
C. In a bridge that performs MAC substitution, User A's actual workstation MAC is not propagated across the bridge, so it does not appear in User B's ARP table.

Concept tested. Layer 2 transparent bridge MAC address substitution behavior

Reference. https://www.cisco.com/c/en/us/tech/lan-switching/bridging/tech-tech-notes-list.html

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