Loop Guardis designed to protect againstSTP loop conditionsthat may occur whenBPDUs are unexpectedly missingon a point-to-point link. This is especially relevant in cases where UDLD (Unidirectional Link Detection) might not detect the problem quickly enough. When BPDUs are absent, a switch might erroneously assume that the link is safe to forward, potentially leading to atemporary Layer 2 loop.
UsingLoop Guardensures that a port moves into aloop-inconsistentstate instead of forwarding traffic if BPDUs are missing, effectively stopping a loop before it forms. This makes it acomplementary mechanism to UDLD, which focuses on detecting unidirectional links at the physical layer, not STP control plane failures.
This recommendation aligns with CCDE v3.1 best practices forresilient Layer 2 designs, which emphasize using multiple mechanisms in tandem to proactively prevent failure conditions before traffic is impacted.
Why other options are incorrect:
* A. Root Guard: Prevents an unauthorized switch from becoming the root bridge but doesn't protect against missing BPDUs.
* B. BPDU Guard: Used mainly on access ports to prevent BPDU reception; not intended for core loop prevention.
* D. BPDU Filtering: Suppresses BPDU exchange entirely, which increases loop risk and is not suited for loop mitigation.

* B (RPVST+ - Rapid Per VLAN Spanning Tree Plus): Provides fast convergence while blocking redundant paths to prevent loops - fail closed behavior under failure scenarios.
* C (MST - Multiple Spanning Tree): Provides loop prevention and isolates failures to specific instances (VLANs), maintaining fail closed protection.
Why other options are incorrect:
* A: EIGRP is a routing protocol; does not operate at Layer 2 to provide fail-closed loop protection.
* D: L2MP (Layer 2 Multipathing) allows multiple active links but requires additional loop prevention mechanisms.
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The root cause here is likely related to multicast forwarding behavior:
* Multicast forwarding may follow a single path based on unicast routing decisions (shortest-path trees or RPF lookup).
* Equal-cost Layer 3 links do not always achieve perfect load balancing for multicast traffic since the multicast flow may hash consistently to a particular path.
Applying a more granular load-balancing hash algorithm on SW1 allows better distribution of flows across multiple links by considering additional packet header fields (e.g., Layer 4 ports, source/destination IP).
This is directly aligned with CCDE v3.1 design goals:
* Avoiding overutilization by improving hashing entropy.
* Preserving consistent unicast and multicast performance.
* Leveraging hardware-based load balancing optimizations.
Why other options are incorrect:
* A & B: Adding or bundling links doesn't address poor hashing distribution.
* D: The issue is identified at SW1 (where hashing decisions are made).
* E: Filtering IGMP joins may block legitimate receivers, not a valid solution.
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