2 - STP Concepts

Question 1

What is a broadcast storm?

Answer 1

When any kind of Ethernet frame loops around the network indefinitely.

Question 2

What are the three types of problems that occur without STP?

Answer 2

• Broadcast storms
• MAC table instability
• Multiple frame transmission

Question 3

What is the IEEE protocol for STP?

Answer 3

802.1D

Question 4

What is root cost?

Answer 4

it is the Administrative Cost from that port to the root switch.

Question 5

What is the root port?

Answer 5

It is the port closest to the root switch. Root ports are always in forwarding state.

Question 6

What are the contents of the 8-byte bridge ID or BID

Answer 6

• Priority field = 2 bytes;
• System ID based on MAC address = 6 bytes

Question 7

What does a STP Hello BPDU consist of?

Answer 7

• Root BID
• Sender’s BID
• Sender’s Root Cost
• Timer values set by the Root switch

Question 8

In a Hello BPDU what are the timers?

Answer 8

• Hello timer
• MaxAge timer
• Forward Delay timer

Question 9

What determines a superior Hello?

Answer 9

The Bridge ID.

• First the lowest Priority field
• Then lowest MAC address.

Question 10

What is the process to elect a root bridge?

Answer 10

1. All switches claim to be root at first
2. Hello BPDUs are circulated and BIDs are examined
3. Once a superior BPDU is encountered then that switch gives up the claim to be Root.
4. The bridge with the LOWEST Bridge ID wins.

Question 11

What is the Designated Port (DP) ?

Answer 11

• Only one per LAN segment.
• It is the port on that LAN segment that advertises the lowest cost to get to the root.
• DPs are always placed in Forwarding State.
• The opposite port on that segment goes into Blocking State.
• Every interface on the Root Switch is a DP for that segment.

Question 12

What command is used to convert 1998 IEEE port costs to 2004 IEEE port costs?

Answer 12

spanning-tree pathcost method long

Question 13

By default, how often does the Root Switch send a new Hello BPDU?

Answer 13

Once every 2 seconds.

Question 14

What fields in a BPDU does a switch change before forwarding it on?

Answer 14

• Changes Root Cost to its own Root Cost
• Changes Sender Bridge ID to its own BID

Question 15

What ports does a non-root switch forward BPDUs?

Answer 15

All its Designated Ports.

Question 16

What tools are available to an engineer to change STP topology?

Answer 16

1. Configure the Priority Field in the switch BID
2. Configure the Port Costs of the switchports.

Question 17

How does a switch know its path to the Root Switch is still working?

Answer 17

It continues to receive Hello BPDUs on its Root Port.

Question 18

What are the 3 timers required for the convergence process?

Answer 18

• Hello every 2 sec
• MaxAge = 20 (10 times the Hello timer)
• Forward Delay = 15 sec

Question 19

Which switch determines what the timers are set at?

Answer 19

Root Switch sends them in Hello BPDUs.

Question 20

What is the MaxAge timer and what is its default

Answer 20

The amount of time a switch will go without hearing BPDUs before it initiates STP Topology change. Default is 20 sec (10 times the Hello timer).

Question 21

What are two ways a switch realizes a link is down?

Answer 21

1. Fails to receive Hello BPDUs within the MaxAge time OR
2. Notices that the RP interface has gone down.

Question 22

What 3 things does a switch do when its RP goes down?

Answer 22

It re-evaluates:

1. Who Root is
2. Which port its RP should be
3. its role as DP on any other interfaces.

Question 23

What do interfaces do when in Blocking state?

Answer 23

1. Only STP messages and some overhead messages are processed.
2. Does not forward frames.
3. Does not learn MAC addresses.
4. Does not process user messages.

Question 24

How long and what happens during the Listening State?

Answer 24

• Default is 15 seconds.
• Does not forward frames.
• Removes stale MAC table entries because these MAC table entries could have been the cause for loops.

Question 25

How long and what happens during the Learning State?

Answer 25

• Default is 15 seconds.
• Begins to learn the MAC addresses

Question 26

In STP how long could it take for a port to transition from Blocking to Forwarding?

Answer 26

• 50 seconds.
  
  • 20 seconds MaxAge waiting for Hello BPDUs
  • 15 sec Listening
  • 15 sec Learning

Question 27

When determining a cost of an interface is the cost based on the maximum possible bandwidth of the port or the actual bandwidth the port is operating at?

Answer 27

It’s based on the bandwidth the port is actually operating at. Not the maximum possible.