400-007 Question #200: Real Exam Question with Answer & Explanation

The correct answer is A: 802.1s provides for faster convergence over 802.1D and PVST+. 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

Explanation

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.

Common mistakes.

B. 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+.
C. 802.1s supports a maximum of 16 MST instances (instances 0 through 15), not 1024 instances.
E. 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

Reference. 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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