2 - STP Concepts Flashcards

Spanning Tree Protocol

1
Q

What is a broadcast storm?

A

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

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2
Q

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

A
  • Broadcast storms
  • MAC table instability
  • Multiple frame transmission
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3
Q

What is the IEEE protocol for STP?

A

802.1D

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4
Q

What is root cost?

A

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

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5
Q

What is the root port?

A

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

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6
Q

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

A
  • Priority field = 2 bytes;
  • System ID based on MAC address = 6 bytes
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7
Q

What does a STP Hello BPDU consist of?

A
  • Root BID
  • Sender’s BID
  • Sender’s Root Cost
  • Timer values set by the Root switch
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8
Q

In a Hello BPDU what are the timers?

A
  • Hello timer
  • MaxAge timer
  • Forward Delay timer
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9
Q

What determines a superior Hello?

A

The Bridge ID.

  • First the lowest Priority field
  • Then lowest MAC address.
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10
Q

What is the process to elect a root bridge?

A
  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.
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11
Q

What is the Designated Port (DP) ?

A
  • 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.
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12
Q

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

A

spanning-tree pathcost method long

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13
Q

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

A

Once every 2 seconds.

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14
Q

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

A
  • Changes Root Cost to its own Root Cost
  • Changes Sender Bridge ID to its own BID
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15
Q

What ports does a non-root switch forward BPDUs?

A

All its Designated Ports.

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16
Q

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

A
  1. Configure the Priority Field in the switch BID
  2. Configure the Port Costs of the switchports.
17
Q

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

A

It continues to receive Hello BPDUs on its Root Port.

18
Q

What are the 3 timers required for the convergence process?

A
  • Hello every 2 sec
  • MaxAge = 20 (10 times the Hello timer)
  • Forward Delay = 15 sec
19
Q

Which switch determines what the timers are set at?

A

Root Switch sends them in Hello BPDUs.

20
Q

What is the MaxAge timer and what is its default

A

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).

21
Q

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

A
  1. Fails to receive Hello BPDUs within the MaxAge time OR
  2. Notices that the RP interface has gone down.
22
Q

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

A

It re-evaluates:

  1. Who Root is
  2. Which port its RP should be
  3. its role as DP on any other interfaces.
23
Q

What do interfaces do when in Blocking state?

A
  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.
24
Q

How long and what happens during the Listening State?

A
  • 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.
25
Q

How long and what happens during the Learning State?

A
  • Default is 15 seconds.
  • Begins to learn the MAC addresses
26
Q

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

A
  • 50 seconds.
    • 20 seconds MaxAge waiting for Hello BPDUs
    • 15 sec Listening
    • 15 sec Learning
27
Q

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?

A

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