In a routed access hierarchical campus design, the access-to-distribution Layer 2 uplink trunks are replaced with Layer 3 point-to-point routed links. Why is it recommended that VLANs are confined…

Question

Options

• Ato allow for better convergence time
• Bto prevent the occurrence of Layer 2 loops
• Cto allow for fault isolation
• Dto prevent routing black holes

• A
  5% (2)
• B
  24% (10)
• C
  10% (4)
• D
  62% (26)

Explanation

In a routed access design, each access switch has a Layer 3 uplink, meaning the distribution layer maintains routing entries with a specific next-hop for each subnet. If a VLAN (and its subnet) spans two access switches, the distribution router can only point that route toward one of them - so traffic destined for hosts sitting on the other access switch gets forwarded to the wrong switch and silently dropped, creating a routing black hole. Confining each VLAN to a single access switch ensures the distribution layer always has an unambiguous, correct next-hop for every subnet.

Why the distractors are wrong:

• A (convergence time): Faster convergence comes from using routing protocols and eliminating STP, not from VLAN placement. This is a benefit of the routed access architecture, not of confining VLANs.
• B (Layer 2 loops): L2 loops are already eliminated by replacing trunk uplinks with routed point-to-point links - STP is no longer needed between access and distribution. VLAN confinement doesn't add to this protection.
• C (fault isolation): While a side benefit, fault isolation is not the reason VLANs must be confined. You'd confine VLANs even if fault isolation weren't a concern.

Memory tip: Picture a delivery address: if the router's "address book" lists one access switch as the delivery point for a subnet, any hosts in that subnet living elsewhere are unreachable - the packet falls into a black hole. One subnet = one exit point.

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