Implementing and Operating Cisco Enterprise Network Core Technologies (ENCOR) v1.2InfrastructureMedium
A network engineer is troubleshooting a Layer 2 loop in a complex enterprise network. The loop is causing severe network instability, including broadcast storms and MAC address table instability. Which Cisco proprietary protocol can be implemented to detect and mitigate these Layer 2 loops while still allowing redundant paths for high availability?
- ARSTP
- BUDLD
- CFlexLink
- DPVST+
Show answer & explanationAnswer & explanation
Correct answer: D. PVST+
PVST+ (Per-VLAN Spanning Tree Plus) is a Cisco proprietary protocol that runs a separate instance of Spanning Tree Protocol for each VLAN, allowing for load balancing across redundant links and preventing Layer 2 loops on a per-VLAN basis. RSTP is an IEEE standard, FlexLink provides fast convergence but doesn't prevent loops, and UDLD detects unidirectional links.
Why the other options are wrong
- A. RSTP (Rapid Spanning Tree Protocol) is an IEEE standard, not Cisco proprietary, and while it prevents loops, it typically doesn't allow per-VLAN load balancing in its standard form.
- B. UDLD (UniDirectional Link Detection) detects unidirectional links, which can cause STP to fail, but it doesn't directly prevent Layer 2 loops or provide redundant path load balancing itself.
- C. FlexLink provides fast failover between redundant links but does not use STP to prevent loops; it relies on one link being active and the other in standby.
PVST+
Per-VLAN Spanning Tree Plus (PVST+) is a Cisco proprietary enhancement to STP that runs a separate Spanning Tree instance for each VLAN, allowing for independent root bridges and load balancing across redundant links.
- Cisco proprietary.
- Runs an STP instance per VLAN.
- Allows load balancing per VLAN.
- Prevents Layer 2 loops.
Memory trick: For each VLAN, a PVST+ path, preventing loops and Wrath.