Chap 4 - Multiple STP

Question 1

What does CST stand for, what is the RFC, and what is unique about it?

Answer 1

• Common Spanning Tree
• Original 802.1D standard
• Only one spanning tree instance for the whole network

Question 2

What is an MST Region?

Answer 2

A grouping of MST switches with the same high-level configuration. An MST Region appears like a single virtual switch to external switches.

Question 3

What does IST stand for, what is it, where does it run, and what is in the IST BPDU and what does it allow for?

Answer 3

• Internal Spanning Tree
• A special STP instance used by MST, the first instance, instance 0.
• Runs on all switchport interfaces
• Additional info about other MST instances is included (nested) in the IST BPDU.
• This allows MST to advertise only 1 set of BPDUs.

Question 4

How many instances of MST does Cisco support?

Answer 4

16 with IST always being the first followed by MSTI’s 1 through 15.

Question 5

In MST when using the command ‘show spanning-tree’ what defines the sys-id-ext?

Answer 5

The MST instance number.

Question 6

Which ‘show’ command allows you to see which VLANs are mapped to which MSTIs?

Answer 6

• show spanning-tree mst configuration
• VLANs are shown under the ‘VLANs Mapped’ line.

Question 7

What are 2 common misconfigurations within the MST region?

Answer 7

• VLAN assignment to the IST
• Trunk link pruning

Question 8

What is the fix for the problem that occurs due to a VLAN being assigned to the IST?

Answer 8

Move the VLAN to a different MSTI.

Question 9

What is the fix for the problem that occurs due to Trunk Link Pruning?

Answer 9

Only prune all the VLANs in the same MSTI for a trunk link.

Question 10

What is an MST Region Boundary?

Answer 10

Any port that connects to a switch that is in a different MST Region or that connects to 802.1D or 802.1W BPDUs.

Question 11

What feature is used to propogate CST into PVST+?

Answer 11

PVST Simulation Mechanism

Question 12

What does the PVST Simulation Mechanism do for incoming packets and for outgoing packets?

Answer 12

• Outgoing - derives VLAN info from the IST and advertises PVST+ / RSTP BPDUs out for each VLAN
• Incoming - maps PVST+ / RSTP VLANs to the MST IST

Question 13

What are 2 design considerations when integrating MST Region with a PVST+ environment?

Answer 13

• The MST is the Root Bridge
• The MST is not the Root Bridge for any VLANs

Question 14

What is PVST Simulation Check?

Answer 14

When the MST region is the root bridge and an MST boundary switch detects a better BPDU for a specific VLAN coming from PVST domain the switch will use BPDU Guard to block the port placing it into an inconsistent state.

Question 15

Verify the MST configuration.

Answer 15

show spanning-tree mst configuration

Question 16

Display the MST topology for a particular instance.

Answer 16

show spanning-tree mst (instance_number)

Question 17

Display the MST settings for a specific interface.

Answer 17

show spanning-tree mst interface gig0/1

Question 18

Verify the MST cost and priority of an interface.

Answer 18

show spanning-tree mst (instance_number)