Ch 4: Multiple Spanning Tree Protocol

Question 1

Which of the following issues does MST solve? (Choose two.)

a. Enables traffic load balancing for specific VLANs.
b. Reduces the CPU and memory resources needed for environments with large numbers of VLANs.
c. Overcomes MAC address table scaling limitations for environments with large numbers of devices.
d. Detects issues with cabling that transmits data in one direction.
e. Prevents unauthorized switches from attaching to the Layer 2 domain

Answer 1

A and B. MST enables traffic load balancing for specific VLANs through assignment of VLANs to specific instances that might have different topologies. MST also reduces the amount of CPU and memory processing as multiple VLANs are associated with an MST instance.

Question 2

With MST, VLANs are directly associated with ______.

a. areas
b. regions
c. instances
d. switches

Answer 2

C. VLANs are associated with MST instances, and an instance defines the Layer 2 forwarding topology for the VLANs that are associated to it.

Question 3

What do CST and 802.1D have in common?

a. They support only one topology.
b. They support multiple topologies.
c. They allow for load balancing of traffic across different VLANs.
d. They provide switch authentication so that inter-switch connectivity can occur.

Answer 3

A. The original 802.1D specification accounted for one topology for all the VLANs, and Common Spanning Tree (CST) uses one topology for building a loop-free topology.

Question 4

True or false: The MST root bridge advertises the VLAN-to-instance mappings to all other MST switches.

a. True
b. False

Answer 4

B. False. MST uses an internal spanning tree (IST) to advertise itself and other MST instances for building the topology. The local switch configuration associates VLANs to the MST instances.

Question 5

True or false: The MST configuration version is locally significant.

a. True
b. False

Answer 5

B. False. The MST configuration is relevant to the entire MST region and should be the same for all switches in the region.

Question 6

True or false: The MST topology can be tuned for root bridge placement, just like PVST+ and RSTP.

a. True
b. False

Answer 6

A. True. The MST topology can be tuned by setting priority, port cost, and port priority for each MST instance.

Question 7

MST regions can interact with PVST+/RSTP in which of the following ways? (Choose two.)

a. The MST region is the root bridge for all VLANs.
b. The MST region is the root bridge for some VLANs.
c. The PVST+/RSTP topology is the root bridge for all VLANs.
d. The PVST+/RSTP topology is the root bridge for some VLANs.

Answer 7

A and C. MST can interact with PVST+/RSTP environments by acting as a root bridge for all VLANs or ensuring that the PVST+/RSTP environment is the root bridge for all VLANs. MST cannot be a root bridge for some VLANs and then let the PVST+/RSTP environment be the root bridge for other VLANs.

Question 8

What is 802.1D?

Answer 8

Spanning Tree Protocol.

Question 9

What is 802.1Q?

Answer 9

802.1q (also known as dot1q) is an IEEE industry standard trunking protocol. Cisco switches supports both ISL (Inter Switch Link - Cisco propietory) and 802.1Q. The IEEE 802.1Q standard defines the operation of VLAN Bridges that permit the definition, operation and administration of Virtual LAN topologies within a Bridged LAN infrastructure.

Question 10

Why did Cisco develop PVST?

Answer 10

Cisco developed the Per-VLAN Spanning Tree (PVST) protocol to allow for an STP topology for each VLAN. With PVST, the root bridge can be placed on a different switch or can cost ports differently, on a VLAN-by-VLAN basis. This allows for a link to be blocked for one VLAN and forwarding for another.

Figure 4-2 demonstrates how all three switches maintain an STP topology for each of the 4 VLANs. If 10 more VLANs were added to this environment, the switches would have to maintain 14 STP topologies. With the third STP instance for VLAN 3, the blocking port moves to the SW1 ← → SW3 link due to STP tuning to address the needs of the traffic between SW2 (where servers attach) and SW3 (where clients attach). On the fourth STP instance, devices on VLAN 4 reside only on SW2 and SW3, so moving the blocking port to the SW2 ← → SW1 link allows for optimal traffic flow.

Question 11

Why does PVST not scale well?

Answer 11

In environments with thousands of VLANs, maintaining an STP state for all the VLANs can become a burden to the switch’s processors. The switches must process BPDUs for every VLAN, and when a major trunk link fails, they must compute multiple STP operations to converge the network.

MST provides a blended approach by mapping one or multiple VLANs onto a single STP tree, called an MST instance (MSTI).

Question 12

What is a MST region?

Answer 12

A grouping of MST switches with the same high-level configuration is known as an MST region. MST incorporates mechanisms that make an MST region appear as a single virtual switch to external switches as part of a compatibility mechanism.

Question 13

What is IST?

Answer 13

MST Instances (MSTIs): MST uses a special STP instance called the internal spanning tree (IST), which is always the first instance, instance 0.

The IST runs on all switch port interfaces for switches in the MST region, regardless of the VLANs associated with the ports. Additional information about other MSTIs is included (nested) in the IST BPDU that is transmitted throughout the MST region. This enables the MST to advertise only one set of BPDUs, minimizing STP traffic regardless of the number of instances while providing the necessary information to calculate the STP for other MSTIs.

Question 14

What is the maximum number of MST instances by default?

Answer 14

Cisco supports up to 16 MST instances by default. The IST is always instance 0, so instances 1 to 15 can support other VLANs. There is not a special name for instances 1 to 15; they are simply known as MSTIs

Question 15

Put the MST configuration steps in order:

• Associate VLANs to an MST instance. By default, all VLANs are associated to the MST 0 instance. The MST configuration submode must be entered with the command spanning-tree mst configuration. Then the VLANs are assigned to a different MST instance with the command instance instance-number vlan vlan-id.
• (Optional) Define the MST region name. MST regions are recognized by switches that share a common name. By default, a region name is an empty string. The MST region name is set with the command name mst-region-name.
• (Optional) Define the MST instance priority, using one of two methods:
• spanning-tree mst instance-number priority priority Note: The priority is a value between 0 and 61,440, in increments of 4096.
• spanning-tree mst instance-number root {primary | secondary}[diameter diameter] Note: The primary keyword sets the priority to 24,576, and the secondary keyword sets the priority to 28,672.
• Define MST as the spanning tree protocol with the command spanning-tree mode mst.
• Specify the mst version number. The MST version number must match for all switches in the same MST region. The MST version number is configured with the submode configuration command revision version.

Answer 15

MST is configured using the following process, in this sequence.

Step 1. Define MST as the spanning tree protocol with the command spanning-tree mode mst.

Step 2. (Optional) Define the MST instance priority, using one of two methods:

spanning-tree mst instance-number priority priority Note: The priority is a value between 0 and 61,440, in increments of 4096.

spanning-tree mst instance-number root {primary | secondary}[diameter diameter] Note: The primary keyword sets the priority to 24,576, and the secondary keyword sets the priority to 28,672.

Step 3. Associate VLANs to an MST instance. By default, all VLANs are associated to the MST 0 instance. The MST configuration submode must be entered with the command spanning-tree mst configuration. Then the VLANs are assigned to a different MST instance with the command instance instance-number vlan vlan-id.

Step 4. Specify the mst version number. The MST version number must match for all switches in the same MST region. The MST version number is configured with the submode configuration command revision version.

Step 5. (Optional) Define the MST region name. MST regions are recognized by switches that share a common name. By default, a region name is an empty string. The MST region name is set with the command name mst-region-name.

Question 16

What command will verify the configuration of MST on a switch?

Answer 16

The command show spanning-tree mst configuration provides a quick verification of the MST configuration on a switch.

Example 4-2 shows the output. Notice that MST instance 0 contains all the VLANs except for VLANs 10, 20, and 99, regardless of whether those VLANs are configured on the switch. MST instance 1 contains VLAN 10 and 20, and MST instance 2 contains only VLAN 99.

Question 17

T/F: VLAN information can be shown with the command show spanning-tree if MST is running.

Answer 17

False. Only the MST instance will be shown in the VLAN information area

Question 18

What is the command to show topology information with an MST?

Answer 18

A consolidated view of the MST topology table is displayed with the command show spanning-tree mst [instance-number].

The optional instance-number can be included to restrict the output to a specific instance.

The command is shown in Example 4-4. Notice that the VLANs are displayed next to the MST instance, which simplifies troubleshooting.

Question 19

What is the command to view MST settings on a specific interface?

Answer 19

The specific MST settings are viewed for a specific interface with the command show spanning-tree mst interface interface-id

Notice that the output in this example includes additional information about optional STP features such as BPDU filter and BPDU guard.

Question 20

T/F: MST supports the tuning of port cost and port priority.

Answer 20

MST supports the tuning of both port cost and port priority.

The interface configuration command spanning-tree mst instance-number cost cost sets the interface cost.

The interface configuration command spanning-tree mst instance-number port-priority priority sets the interface priority.

Question 21

Give two common misconfigurations with MST.

Answer 21

There are two common misconfigurations within the MST region that network engineers should be aware of:

1. VLAN assignment to the IST. Solution: Assign all VLANS to other MST instances.
2. Trunk link pruning. Solution: A simple rule to follow is to only prune all the VLANs in the same MSTI for a trunk link.

Question 22

What is an MST region boundary?

Answer 22

The topology for all the MST instances is contained within the IST, which operates internally to the MST region. An MST region boundary is 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 23

What is a PVST simulation mechanism?

Answer 23

MSTIs never interact outside the region. MST switches can detect PVST+ neighbors at MST region boundaries. Propagating the CST (derived from the IST) at the MST region boundary involves a feature called the PVST simulation mechanism.

The PVST simulation mechanism sends out PVST+ (and includes RSTP, too) BPDUs (one for each VLAN), using the information from the IST. To be very explicit, this requires a mapping of one topology (IST) to multiple VLANs (VLANs toward the PVST link). The PVST simulation mechanism is required because PVST+/RSTP topologies do not understand the IST BPDU structure.

When the MST boundary receives PVST+ BPDUs, it does not map the VLANs to the appropriate MSTIs. Instead, the MST boundary maps the PVST+ BPDU from VLAN 1 to the IST instance. The MST boundary engages the PVST simulation mechanism only when it receives a PVST BPDU on a port.

Question 24

T/F: Making the MST region the root bridge ensures that all region boundary ports flood the same IST instance BPDU to all the VLANs in the PVST topology.

Answer 24

True.

Making the MST region the root bridge ensures that all region boundary ports flood the same IST instance BPDU to all the VLANs in the PVST topology. Making the IST instance more preferable than any other switch in the PVST+ topology enables this design. The MST region appears as a single entity, and the PVST+ switches detect the alternate link and place it into a blocking state.

Figure 4-7 shows the IST instance as the root bridge for all VLANs. SW1 and SW2 advertise multiple superior BPDUs for each VLAN toward SW3, which is operating as a PVST+ switch. SW3 is responsible for blocking ports.