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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
If two paths on a switch has the same best root cost, how is a tiebreaker determined
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
How is designated port chosen if the advertise costs tie between 2 switches
Switch with the lower BID wins
27
What can network engineers change to influence STP choices
Configure priority | Change STP/RSTP port costs
28
What is the default port cost of a 10Mbps link
2,000,000
29
What is the default port cost of a 100Mbps link
200,000
30
What is the default port cost of a 1 Gbps link
20,000
31
What is the default port cost of a 10Gbps link
2000
32
What is the default port cost of a 100Gbps link
200
33
What is the default port cost of a 1Tbps link
20
34
What is the steady state of operations for Hellos in a stable STP topology
1, root creates and sends Hellop BPDU with root cost of 0 out of all its working interfaces 2. 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 3. Repeat steps 1 and 2 until something changes
35
What are the three times that manage STP convergence
Hello timer - 2 seconds MaxAge - 10 x Hello timer Forward delay - 15 seconds
36
What is the MaxAge timer in STP
10 x hello timer | How long switch should wait after not hearing Hellos before trying to change STP topology
37
What is the forward delay timer in STP
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
What is the steady state of operations for Hellos in a stable STP topology
1, root creates and sends Hello BPDU with root cost of 0 out of all its working interfaces 2. 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 3. Repeat steps 1 and 2 until something changes
39
What are the three timers that manage STP convergence
Hello timer - 2 seconds MaxAge - 10 x Hello timer (def 20 sec) Forward delay - 15 seconds
40
What is the MaxAge timer in STP
10 x hello timer | How long switch should wait after not hearing Hellos on a port before trying to change STP topology
41
What happens after the MaxAge expires on a switch
It goes over the STP choices again based on the Hellos it receives from other switches
42
Why does STP need to put a port that is going from blocking to forwarding into intermediate states
To prevent temporary loops
43
What are the two intermediate states when a port is going from blocking to forwarding in STP
Listening | Learning
44
What is the listening state in STP
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
What is the learning state in STP
Interface do not forward frames | Switch begins to learn MAC address frames received on the interface
46
With default how long can it take for STP to move an interface from blocking to fowarding
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
What is the standard for RSTP
802.1w
48
In what ways is RSTP different from STP
- 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
What is the MaxAge on RSTP
3 x Hello
50
What are the different port roles in RSTP
``` Root port Alternate port Designated port Backup port Disabled port ```
51
What is the alternate port in RSTP
port that replaces the root port when the root port fails
52
What is the backup port in RSTP
port that replaces a designated port when a designated port fails
53
What is the requirement to be an alternate port
The alternate port must receive Hellos that identify the same root switch
54
What is the RSTP process when a root port fails on a switch
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 3. state is changed from forwarding to discarding 4. role of the alternate port is changed to be the root port and forwarding state immediately without waiting on timers
55
What are the different STP ports states compared to RSTP states
``` Disabled - Discarding Blocking - Discarding Listening - Not used Learning - Learning Forwarding - Forwarding ```

CCNA 200-301 (8 decks)

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