STP Advanced Features and Variants 3 — Questions and Answers
Question 1: Which STP protection feature should be enabled on all access ports connected to end devices to prevent them from participating in STP topology changes?
- Root Guard
- BPDU Guard (Correct answer)
- Loop Guard
- UDLD
Correct answer: BPDU Guard
BPDU Guard disables a PortFast-enabled port if any BPDU is received, protecting the STP topology from rogue switches.
Question 2: What does the STP Backbone Fast feature do when an indirect link failure is detected?
- Immediately flushes the MAC table and starts a new election
- Allows a switch to use an alternate port without waiting for MaxAge by sending RLQ messages (Correct answer)
- Enables the port to skip the Listening state
- Sends a TCN BPDU to all ports simultaneously
Correct answer: Allows a switch to use an alternate port without waiting for MaxAge by sending RLQ messages
Backbone Fast uses Root Link Query (RLQ) messages to verify if the root is still reachable via upstream switches, cutting MaxAge wait time.
Question 3: STP UplinkFast is most beneficial in which network scenario?
- Core switches with many redundant equal-cost links
- Access layer switches with one uplink active and one in blocking state (Correct answer)
- Distribution switches running MSTP with multiple instances
- Routed access layer deployments with no STP needed
Correct answer: Access layer switches with one uplink active and one in blocking state
UplinkFast allows an access-layer switch to immediately activate a blocking uplink port upon loss of the active uplink, bypassing listening/learning states.
Question 4: In MSTP, what is IST (Internal Spanning Tree)?
- Instance 0 inside an MST region that carries unassigned VLANs and connects to the CIST (Correct answer)
- A separate spanning tree that runs between MST regions only
- The instance dedicated to management VLAN traffic
- A term for RSTP running inside a single VLAN
Correct answer: Instance 0 inside an MST region that carries unassigned VLANs and connects to the CIST
The IST is MST instance 0 within a region; it carries all VLANs not explicitly mapped to other instances and connects the region to the CIST.
Question 5: Which RSTP port role forwards traffic and is elected based on lowest path cost to the root bridge?
- Alternate Port
- Backup Port
- Root Port (Correct answer)
- Designated Port
Correct answer: Root Port
The Root Port is the port on a non-root switch with the lowest cumulative path cost to the root bridge, and it forwards traffic toward the root.
Question 6: What distinguishes an RSTP Alternate Port from a Backup Port?
- Alternate provides a redundant path to the root via a different switch; Backup provides redundancy on the same switch/segment (Correct answer)
- Alternate is used only in MSTP; Backup is used only in PVST+
- Alternate connects to the root bridge; Backup connects to downstream switches
- There is no difference; they are synonyms in 802.1w
Correct answer: Alternate provides a redundant path to the root via a different switch; Backup provides redundancy on the same switch/segment
An Alternate Port offers an alternate path to the root through a different bridge, while a Backup Port is a redundant port on the same shared segment.
Question 7: When configuring MSTP on Cisco switches, which command sets the MST region name?
- spanning-tree mst name <name>
- spanning-tree region <name>
- spanning-tree mst configuration name <name> (Correct answer)
- mst region-name <name>
Correct answer: spanning-tree mst configuration name <name>
Under the MST configuration submode, the command 'spanning-tree mst configuration' followed by 'name <name>' sets the region name.
STP Advanced Features and Variants 3
Which STP protection feature should be enabled on all access ports connected to end devices to prevent them from participating in STP topology changes?