STP

Question 1

What IEEE standard defines classic STP?

Answer 1

802.1d

Question 2

How do ports in an STP blocked state behave?

Answer 2

Don’t forward/receive traffic, except for BPDUs

Question 3

In general for STP, does high or low number win?

Answer 3

Low number

Question 4

What is the default STP BPDU message send timer?

Answer 4

2 seconds

Question 5

What are the 3 primary steps in STP initialization

Answer 5

1. Root Bridge Election
2. Root Port Selection
3. Remaining collision domains block one of two ports

Question 6

Does the switch with the highest or lowest bridge ID become the root?

Answer 6

Lowest

Question 7

All ports on the root bridge are in what STP state?

Answer 7

Forwarding / Designated

Question 8

What is the selection criteria order for determining root bridge?

Answer 8

1. Bridge priority
2. Lowest MAC address

Question 9

Explain BPDU Guard

Answer 9

If an interface with BPDU guard enabled receives a BPDU from another switch, the interface will be automatically shut down to prevent a loop from forming.

Question 10

Explain Root Guard

Answer 10

If an interface with Root Guard receives a BPDU with a lower bridge ID, the switch will not accept that new switch as the root, and the interface will be disabled. Useful for if a bad actor plugs into an access switch.

Question 11

Explain loop guard

Answer 11

If an interface is enabled with Loop Guard, even if the interface stops receiving BPDUs, it will not start forwarding, the interface will be disabled. Protects against unidirectional links.

Question 12

What is the default flavor of STP on Cisco switches?

Answer 12

Rapid PVST

Question 13

How does STP port cost scale with bandwidth of the interface?

Answer 13

Goes down.