Spanning Tree

Question 1

What are the three types of BPDUs?

Answer 1

• Configuration
• TCN (topology change notification)
• TCA (topology change acknowledgement)

Question 2

What are Configuration BPDUs used for?

Answer 2

Used to identify the root bridge, root ports, designated ports, and blocking ports.

Question 3

What are TCN BPDUs used for and who sends them?

Answer 3

Used when a bridge discovers a change in topology, usually because of a link failure, bridge failure, or port transitioning to the forwarding state. It is forwarded on the root port toward the root bridge.

Question 4

What are TCA BPDUs used for and who sends them?

Answer 4

Used by the upstream bridge to respond to the receipt of a TCN.

Question 5

What does the BID consist of?

Answer 5

Bridge ID contains the Bridge Priority (Bridge priority + VLAN ID) and the MAC address.

Question 6

If all switches in the network topology have the same priority, who becomes the root bridge?

Answer 6

The one with the lowest MAC address.

Question 7

If a switch has two equal port costs to the bridge, what determines which interface becomes the root port?

Answer 7

The interface with the lowest port ID.

Question 8

What is the port ID?

Answer 8

Port ID is a combination of port priority (128 is the default) and port number.

Question 9

How is STP cost calculated?

Answer 9

STP cost is calculated from the bandwidth of a link.

Question 10

Between two non-root bridges, what determines which part of the segment is blocked and other designated?

Answer 10

If there are two paths to the root bridge with equal cost, STP uses the following criteria for best path determination and consequently for determining the designated and non-designated ports on the segment:

Lowest root path cost to the root bridge

Lowest sender BID

Lowest sender port ID

Question 11

What are the STP Port States?

Answer 11

STP Port States are:

• Blocking
• Listening
• Learning
• Forwarding
• Disabled

Question 12

What are the RSTP Port Roles?

Answer 12

RSTP Port Roles are:

• Root
• Designated
• Alternate
• Disabled
• Backup

Question 13

What are the RSTP Port States?

Answer 13

The RSTP Port states are:

• Discarding
• Learning
• Forwarding

Question 14

What is Root Guard?

Answer 14

Root guard is a feature that prevents another switch with a superior BPDU from becoming the root bridge. Once enabled on the port, it can make the port go into root-inconsistent STP state.

Question 15

What is Loop Guard?

Answer 15

Loop Guard is a loop prevention mechanism that when a non-designated port no longer receives BPDUs, it will move into a loop-inconsistent state rather than remain in forwarding.

Question 16

What is UDLD and what does it do?

Answer 16

Unidirectional Link Detection (UDLD) s a Cisco-proprietary protocol that detects unidirectional links and prevents Layer 2 loops from occurring across fiber-optic cables.

Question 17

What is CST and what does it do?

Answer 17

Common Spanning Tree (CST) is the entire spanning tree topology in an enterprise which can integrate all forms of STP that might be in use.

Question 18

What is IST?

Answer 18

Internal Spanning Tree (IST) is a MST instance within a region that communicates to the outside CST at the boundary to maintain a single STP topology. In other words, a locally significant CST, bounded by the edges of a region.