Refer to the exhibit. An organization is running H-VPLS on a network comprising four routers in a hub-and-spoke topology with R1 as the hub. An engineer added a new spoke with multiple VCs to the…

Question

Options

• ADisable spanning tree to allow loops to occur within the hub-and-spoke topology.
• BDisable split horizon to allow multiple VCs per spoke
• CDisable Cisco Discovery Protocol to allow MPLS to share labels between the designated
• DDisable Cisco Discovery Protocol to allow for neighbor discovery

• A
  7% (2)
• B
  77% (23)
• C
  3% (1)
• D
  13% (4)

Explanation

In H-VPLS, split horizon prevents a hub router from forwarding traffic received on one Virtual Circuit (VC) back out to another VC on the same spoke - a loop-prevention mechanism that breaks down when a spoke legitimately has multiple VCs connecting to the hub. Disabling split horizon on R1 allows traffic to be properly forwarded across all VCs associated with the new multi-VC spoke, restoring traffic flow without compromising the overall topology.

Why the distractors are wrong:

• A is wrong because disabling spanning tree would enable loops, not fix a split-horizon problem - STP is a safeguard, not the culprit here.
• C is wrong because CDP has no role in MPLS label distribution; labels are exchanged via LDP (Label Distribution Protocol), making CDP irrelevant to the issue.
• D is wrong because VPLS neighbor discovery relies on LDP or BGP signaling, not CDP - CDP is a Cisco proprietary Layer 2 discovery protocol unrelated to VPLS operation.

Memory tip: Think of split horizon as a "no U-turn" rule for hub-and-spoke - normally helpful, but when a spoke has multiple VCs, the hub needs to make U-turns between those VCs. The fix is to remove the restriction (disable split horizon) on the hub, not to touch STP or CDP, which are unrelated protocols.

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