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CCNA 200-301 > 9. Spanning Tree Protocol Concepts > Flashcards

9. Spanning Tree Protocol Concepts Flashcards

1
Q

What is the standard for STP

A

802.1D

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2
Q

What are two goals of STP

A
  • All devices in a VLAN can send frames to all other devices by making sure STP does not block too many ports
  • Frames have a short life and do not loop around the network indefinitely
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3
Q

How does STP/RSTP prevent loops

A

Add an additional check on each interface
If port is in STP/RSTP forwarding state in that VLAN, use as normal
If port is in STP/RSTP blocking state, block all user traffic and do not send or receive traffic on that interface in that VLAN

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4
Q

Does STP/RSTP affect the interface’s connected or operational state?

A

No

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5
Q

What is a broadcast storm

A

When any kind of ethernet frames loop around a LAN indefinitely which saturate all the links with copies of that single frame crowding out good frames

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6
Q

What is a broadcast storm

A

The forwarding of a frame repeatedly on the same links, consuming
significant parts of the links’ capacities

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7
Q

What is MAC table instability

A

The continual updating of a switch’s MAC address table with
incorrect entries, in reaction to looping frames, resulting in frames
being sent to the wrong locations

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8
Q

What are three problems by not using STP on redundant links

A

Broadcast storms
MAC table instability
Multiple frame transmission

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9
Q

What is multiple frame transmission

A

A side effect of looping frames in which multiple copies of one frame are delivered to the intended host, confusing the host

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10
Q

How does STP/RSTP prevent loops

A

STP/RSTP prevents loops by placing each switch port in either a forwarding state or a blocking
state.
Interfaces in the forwarding state act as normal, forwarding and receiving frames.
However, interfaces in a blocking state do not process any frames except STP/RSTP messages
(and some other overhead messages).
Interfaces that block do not forward user frames,
do not learn MAC addresses of received frames, and do not process received user frames.

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11
Q

What is STP convergence

A

process by which the switches collectively
realize that something has changed in the LAN topology and determine whether they need
to change which ports block and which ports forward.

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12
Q

Which ports does STP place into forwarding state

A
  • All the root switch’s ports
  • Each nonroot switch’s root port
  • Each LAN’s designated port
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13
Q

What is a root port

A

for a nonroot switch, the port with the least administrative cost back to the root switch

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14
Q

What is a root port

A

for a nonroot switch, the port with the least administrative cost (root cost) back to the root switch

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15
Q

What is a designated port

A

On a single link between two switches, the port with the lowest root cost

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16
Q

What is STP/RSTP disabled state

A

failed or not connected interfaces not considered in the spanning tree algorithm

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17
Q

Which ports does STP place into forwarding state

A
  1. All the root switch’s ports
  2. Each nonroot switch’s root port
  3. Each LAN’s designated port
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18
Q

What is a root port

A

For a nonroot switch, the port with the least administrative cost (root cost) back to the root switch
There is only one root port on each switch

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19
Q

What are the fields inside a Hello BPDU

A

Root Bridge ID
Sender’s bridge ID
Sender’s root cost
Timer values on the root switch

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20
Q

What is the root bridge ID in a Hello BPDU

A

Bridge ID of the of the switch that the sender believes is the root switch

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21
Q

What are the timer values inside a Hello BPDU

A

Hello timer
MaxAge timer
forward delay timer

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22
Q

Which switch becomes the root switch

A

Switch with the lowest BID =

Switch with the lowest priority. If priority is tied then switch with the lowest MAC

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23
Q

How is the root switch chosen

A

All switches claim to be root switch and sends out
Hello BPDU as such
If it encounters a lower BID then it stops advertising itself as the root and forwards Hello BPDU with new root
This continues until the root is chosen

24
Q

How does a switch determine the root cost out of a port

A

It adds its interface cost to the root cost listed in each Hello BPDU

25
Q

If two paths on a switch has the same best root cost, how is a tiebreaker determined

A
  1. Choose based on lowest neighbor BID
  2. Choose based on lowest neighbor port priority
  3. Choose based on the lowest neighbor internal port number
26
Q

How is designated port chosen if the advertise costs tie between 2 switches

A

Switch with the lower BID wins

27
Q

What can network engineers change to influence STP choices

A

Configure priority

Change STP/RSTP port costs

28
Q

What is the default port cost of a 10Mbps link

A

2,000,000

29
Q

What is the default port cost of a 100Mbps link

A

200,000

30
Q

What is the default port cost of a 1 Gbps link

A

20,000

31
Q

What is the default port cost of a 10Gbps link

A

2000

32
Q

What is the default port cost of a 100Gbps link

A

200

33
Q

What is the default port cost of a 1Tbps link

A

20

34
Q

What is the steady state of operations for Hellos in a stable STP topology

A

1, root creates and sends Hellop BPDU with root cost of 0 out of all its working interfaces

  1. Non root switches receive the hello on their root port. After they change the hello to list its own BID as the sender BID and adding its root cost, they send out the hello out of all designated ports
  2. Repeat steps 1 and 2 until something changes
35
Q

What are the three times that manage STP convergence

A

Hello timer - 2 seconds
MaxAge - 10 x Hello timer
Forward delay - 15 seconds

36
Q

What is the MaxAge timer in STP

A

10 x hello timer

How long switch should wait after not hearing Hellos before trying to change STP topology

37
Q

What is the forward delay timer in STP

A

Delay that affects the process that occurs when an interface changes from blocking state to forwarding state.
A port stays in an interim listening state, and then an interim learning state, for the number of seconds defined by the forward delay timer.

38
Q

What is the steady state of operations for Hellos in a stable STP topology

A

1, root creates and sends Hello BPDU with root cost of 0 out of all its working interfaces

  1. Non root switches receive the hello on their root port. After they change the hello to list its own BID as the sender BID and adding its root cost, they send out the hello out of all designated ports
  2. Repeat steps 1 and 2 until something changes
39
Q

What are the three timers that manage STP convergence

A

Hello timer - 2 seconds
MaxAge - 10 x Hello timer (def 20 sec)
Forward delay - 15 seconds

40
Q

What is the MaxAge timer in STP

A

10 x hello timer

How long switch should wait after not hearing Hellos on a port before trying to change STP topology

41
Q

What happens after the MaxAge expires on a switch

A

It goes over the STP choices again based on the Hellos it receives from other switches

42
Q

Why does STP need to put a port that is going from blocking to forwarding into intermediate states

A

To prevent temporary loops

43
Q

What are the two intermediate states when a port is going from blocking to forwarding in STP

A

Listening

Learning

44
Q

What is the listening state in STP

A

Interface does not forward frames.
Switch removes old stale MAC table entries for which no frames are received from each MAC address during this period
These MAC table entries could be cause of temporary loops

45
Q

What is the learning state in STP

A

Interface do not forward frames

Switch begins to learn MAC address frames received on the interface

46
Q

With default how long can it take for STP to move an interface from blocking to fowarding

A

20 sec MaxAge to determine there is an issue
15 sec in listening
15 sec in learning
—————————————–
50 seconds total for an interface to converge

47
Q

What is the standard for RSTP

A

802.1w

48
Q

In what ways is RSTP different from STP

A
  • RSTP adds mechanism by which a switch can replace its root port without any waiting to reach a forwarding state
  • RSTP adds mechanism to replace a designated port without waiting to reach a forwarding state
  • RSTP lowers waiting times for cases in which RSTP must wait for a timer
  • Each switch generates their own hellos instead of forwarding from the root switch
  • Neighbors can query each other for information instead of waiting on timers to expire
49
Q

What is the MaxAge on RSTP

A

3 x Hello

50
Q

What are the different port roles in RSTP

A 
Root port
Alternate port
Designated port
Backup port
Disabled port
51
Q

What is the alternate port in RSTP

A

port that replaces the root port when the root port fails

52
Q

What is the backup port in RSTP

A

port that replaces a designated port when a designated port fails

53
Q

What is the requirement to be an alternate port

A

The alternate port must receive Hellos that identify the same root switch

54
Q

What is the RSTP process when a root port fails on a switch

A
  1. the switches with the link that contains the alternate port exchange RSTP messages that the alt will now be the root and the switch on the other end flushes required MAC entries to avoid a loop
    2 role of the root port is changed from root to disabled
  2. state is changed from forwarding to discarding
  3. role of the alternate port is changed to be the root port and forwarding state immediately without waiting on timers
55
Q

What are the different STP ports states compared to RSTP states

A 
Disabled - Discarding
Blocking - Discarding
Listening - Not used
Learning -  Learning
Forwarding - Forwarding

CCNA 200-301 (8 decks)

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