400-007 · Question #200
Which two characteristics are associated with 802.1s? (Choose two.)
The correct answer is A. 802.1s provides for faster convergence over 802.1D and PVST+ D. 802.1s maps multiple VLANs to the same spanning-tree instance. IEEE 802.1s (MSTP) allows multiple VLANs to share a single spanning-tree instance and converges faster than legacy 802.1D and PVST+ by leveraging 802.1w RSTP mechanics internally.
Question
Which two characteristics are associated with 802.1s? (Choose two.)
Options
- A802.1s provides for faster convergence over 802.1D and PVST+
- B802.1s is a Cisco enhancement to 802.1w
- C802.1s supports up to 1024 instances of 802.1w
- D802.1s maps multiple VLANs to the same spanning-tree instance
- ECPU and memory requirements are the highest of all spanning-tree STP implementations
How the community answered
(29 responses)- A93% (27)
- C3% (1)
- E3% (1)
Why each option
IEEE 802.1s (MSTP) allows multiple VLANs to share a single spanning-tree instance and converges faster than legacy 802.1D and PVST+ by leveraging 802.1w RSTP mechanics internally.
802.1s uses the rapid convergence mechanisms defined in 802.1w (RSTP) within each MST instance, achieving sub-second convergence that is significantly faster than the 30-50 second convergence timers of 802.1D and the per-VLAN overhead of PVST+.
802.1s is an IEEE standard, not a Cisco proprietary enhancement - 802.1w (RSTP) is also an IEEE standard, and Cisco's proprietary rapid spanning tree variant is Rapid PVST+.
802.1s supports a maximum of 16 MST instances (instances 0 through 15), not 1024 instances.
The defining feature of MSTP is the ability to map multiple VLANs to a single MST instance, reducing the total number of spanning-tree instances and thereby lowering CPU and memory consumption compared to running one instance per VLAN.
MSTP has lower CPU and memory requirements than PVST+ and Rapid PVST+ precisely because it consolidates many VLANs into far fewer spanning-tree instances.
Concept tested: IEEE 802.1s Multiple Spanning Tree Protocol features
Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/lanswitch/configuration/xe-16/lanswitch-xe-16-book/lsw-mult-span-tree-prot.html
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