MST

Question 1

In MST, what does IST refer to?

Answer 1

Internal spanning tree–the area within a MST region. It resides on instance 0, and sends the actual MST BPDUs.

Question 2

Briefly describe MST.

Answer 2

Multiple spanning tree allows multiple VLANs to be mapped to instances, allowing the control of PVST+ without the strain on the CPU.

Question 3

What must the switches in a MST region agree upon?

Answer 3

MST region name, VLAN-instance mapping, and MST configuration number.

Question 4

Which protocol defines MST?

Answer 4

802.1s.

Question 5

In MST, switches are divided into ___.

Answer 5

Regions.

Question 6

How many instances can exist in a MST region?

Answer 6

16, 0-15.

Question 7

When CST interacts with MST, what instance is it placed into?

Answer 7

0.

Question 8

In MST, when a VLAN is not assigned to an instance, what will it be placed into automatically?

Answer 8

0.

Question 9

In MST, which instance will BPDUs actually be sent on?

Answer 9

0.

Question 10

How many VLANs can be mapped to MSTI 0?

Answer 10

As many as you want, 0-4096.

Question 11

A network engineer is setting up a new switched network. The network is expected to grow and add many new VLANs in the future. Which Spanning Tree Protocol should be used to reduce switch resources and managerial burdens that are associated with multiple spanning-tree instances?

Answer 11

MST

Question 12

When two MST instances (MST 1 and MST 2) are created on a switch, what is the total number of spanning-tree instances running on the switch?

Answer 12

3

Question 13

Define MST

Answer 13

Defined in IEEE 802.1s, a specification for multiple STP instances when using 802.1Q trunks

Question 14

What does *MST* stand for?

Answer 14

Multiple Spanning Tree

Question 15

What is *MST*?

Answer 15

MST is IEEE’s answer to Cisco’s Per-VLAN STP implementation. With MST, you are able to bundle VLANs together in instances, similar to the per-vlan system except using lesser resources.

Question 16

What configuration command would set up *MST*?

Answer 16

• Sw1(config)# spanning-tree mode mst
• Sw1(config)# spanning-tree mode configuration
• Sw1(config-mst)# revision <#>
• Sw1(config-mst)# name <name></name>
• Sw1(config-mst)# instance <#> vlan <vlan-list></vlan-list>

Question 17

What is the main different between *R/PVST+* and *MST*?

Answer 17

With PVST and RPVST, there is an instance of STP running per VLAN. With MST, there is one instance of STP running per “bundle” of VLANs.

Question 18

What is an *MSTI*?

Answer 18

An MSTI is a MST Instance where one or more VLANs are bundled. Maximum of 16 instances supported, and by default, all VLANs are mapped to Instance-0. Each MSTI represents a different STP topology.

Question 19

What are the compatible parameters required for bridges in *MST*?

Answer 19

The compatible parameters are…

1. MST Region Name
2. MST Revision Number (static)
3. VLAN-to-Instance Mapping Configuration Digest

Question 20

How would you create a separate STP Topology in *MST*?

Answer 20

To create separate STP Topologies, you would need to tweak STP variables per instance.

Question 21

What is the *MST BPDU* composed of?

Answer 21

MST BPDU always contains Instance-0 information, and an mrecord of any other instances that would apply across that link.

Question 22

Why is Instance-0 in MST important?

Answer 22

Instance-0 in MST has all the timer information, and is also used to negotiate with non-MST instances.

Question 23

What is an *MST Boundary Port*?

Answer 23

An MST Boundary Port are ports unable to do MST with the peer.

Question 24

What is an *MST Internal Port*?

Answer 24

An MST internal Port are ports that can communicate using MST BPDUs.

Question 25

What are the two root bridges in MST?

Answer 25

MST has two root bridges, one is the Main Root Bridge (CST) and the other is the IST Master.