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Cisco Certs > Spanning tree > Flashcards

Spanning tree Flashcards

1
Q 
Default STP port costs for the following speeds:
10M
100M
1G
10G
A 
10M = 100
100M = 19
1G = 4
10G = 2
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2
Q

What are the 3 types of BPDUs?

A

Hello BPDU
CBDPU - Configuration BPDU
TCN BPDU - Topology Change Notification BPDU

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3
Q

Command to configure a static etherchannel

A

conf t
interface int
channel-group group number mode on

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4
Q

Command to show the status of an etherchannel

A

show etherchannel group number

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5
Q

What are the 2 protocols that can be used to create dynamic etherchannels?

A

PAgP

LACP

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6
Q

What does PAgP stand for?

A

Port Aggregation Protocol

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7
Q

What does LACP stand for?

A

Link Aggregation Control Protocol

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8
Q

What is the IEEE standard for PAgP and LACP?

A

PAgP is Cisco proprietary

LACP is 802.1ad

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9
Q

Syntax to enable PAgP on a interface

A

conf t
int gi1/0
channel-group group number mode {desirable | auto}

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10
Q

Syntax to enable LACP on a interface

A

conf t
int gi1/0
channel-group group number mode {active | passive}

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11
Q

In PAgP configurations, what does the keyword desirable tell the switch to do vs auto?

A

It tells the switch “use this protocol and begin negotiations”versus “use this protocol and wait for the other switch to begin negotiations.”

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12
Q

In LACP configurations, what does the keyword active tell the switch to do vs passive?

A

It tells the switch “use this protocol and begin negotiations”versus “use this protocol and wait for the other switch to begin negotiations.”

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13
Q

What happens with an etherchannel where both sides are set to either auto or passive?

A

It won’t come up, because both sides are waiting for the other to begin negotiations.

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14
Q

What is the IEEE standard for:
STP
PVST
RSTP

A

802.1D
Cisco Proprietary
802.1w

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15
Q

What does RSTP stand for?

A

Rapid Spanning Tree Protocol

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16
Q

What STP mode do Cisco switches generally default to?

A

802.1D with PVST

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17
Q

Command to set a switch to use PVST

A

spanning-tree mode pvst

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18
Q

What 3 fields make up the BID?

A
  1. Priority
  2. System ID Extension
  3. System ID
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19
Q

What is the range for priority?

A

0-65535

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20
Q

What is unusual about the priority value?

A

It’s in multiples of 4096.

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21
Q

What is the value of an all 1’s priority field?

A

61140

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22
Q

What are the values of each bit in the priority field?

A

32768 16384 8192 4096

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23
Q

What is normally stored in the System ID extension field?

A

VLAN ID

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24
Q

How many bits in the System ID extension field?

A

12

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25
Q

How many bits in the priority field?

A

4

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26
Q

How many bits in the System ID field?

A

6 bytes

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27
Q

Command to set the priority for a given VLAN

A

spanning-tree vlan vlan id priority priority

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28
Q

Command to set a port cost.

A

Conf t
interface gi1/0
spanning-tree [vlan vlan id] cost cost

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29
Q

What is the default BID priority?

A

32768

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30
Q

Is portfast or bpduguard enabled by default?

A

No

31
Q

What are the 3 port types in STP?

A
  1. Root port
  2. Designated port
  3. Blocking port
32
Q

What is a Designated port vs other port types?

A

The Designated Port is the one that advertises lowest-cost hello onto the LAN segment.

33
Q

What 4 fields are shown in a “show mac address-table” output?

A
  1. VLAN
  2. MAC address
  3. Type (dynamic or static)
  4. Output port to get to that MAC
34
Q

What does “show interfaces status” show us (7 fields)?

A
  1. Port
  2. Description
  3. Status (connect / notconnect/ disabled)
  4. VLAN
  5. Duplex
  6. Speed
  7. Type (10/100/1000 TX etc)
35
Q

If a frame came into a switch and was either an unknown destination MAC or a broadcast mac, what ports would the switch forward it out of?

A

All forwarding ports for that VLAN, except the one on which it was received.

36
Q

What makes a switch a designated switch for a given ethernet segment?

A

It’s the switch (and therefor the port) that is forwarding the root’s BPDU.

37
Q

4 important fields in a Hello BPDU

A
  1. Root BID
  2. Sender’s BID
  3. Sender’s root cost
  4. Timers on the root switch
38
Q

What are 3 of the timers on the root switch?

A
  1. Hello timer
  2. MaxAge timer
  3. Forward Delay timer
39
Q

What is the most common type of BPDU?

A

Hello BPDU

40
Q

What does STA stand for?

A

Spanning Tree Algorithm

41
Q

What is the root cost in a BPDU received directly from the root bridge?

A
  1. The root bridge has a 0 cost to reach itself.
42
Q

If a switch receives a BPDU with a root cost of 25 on a 1 gig port with default costing, what will it rewrite the root cost to be?

A

29

25 + 4 for the gig port

43
Q

Which switch’s BID would win?
A: 32769:0200.0002.0002
B: 32769:0200.0003.0003

A

A, since it has the numerically lower System ID

44
Q

If a switch is connected to a hub and hear’s it’s own BPDU’s on multiple ports, what are the other 2 tiebreaker values?

A
  1. Lowest interface STP priority

2. If all interfaces are the same priority, lowest interface number.

45
Q

What 2 items will a non-root switch change before forwarding out the hello BPDU received from the root’s direction?

A
  1. Root cost

2. Sender ID

46
Q

What is the default value of the 3 primary STP timers?

A
  1. Hello - 2 seconds
  2. MaxAge - 20 seconds
  3. Forward Delay - 15 seconds
47
Q

What is the calculation to get the MaxAge time?

A

MaxAge = 10*(Hello timer)

48
Q

What does the forward delay timer control?

A

How long a port will transition through Listening and Learning before going to Forwarding. Each phase will take (fwddelay value) seconds.

49
Q

What does the MaxAge timer control?

A

How long a switch will wait for a Hello BPDU before declaring the neighbor down and sending out a TCN BPDU.

50
Q

How many interfaces can be included in an Etherchannel?

A

Up to 8.

51
Q

What does PortFast do?

A

Allows a port to go immediately into Forwarding, without the normal interim stages of Listening and Learning.

52
Q

What kinds of ports must PortFast be restricted to?

A

Access ports facing stubs only, never anything that could be cabled up to form a loop.

53
Q

What does BPDUguard do?

A

When a port has it configured, it will immediately become disabled if a BPDU is received on that port.

54
Q

In RSTP, what is the blocking state called?

A

Discarding

55
Q

What is STP’s convergence time vs RSTP?

A

STP - ~50 seconds

RSTP - max 10 seconds, usually much less

56
Q

What does the command “spanning-tree vlan vlan-id root {primary | secondary}” do?

A

The switch on which this is configured will lower it’s own priority to beat out any other switch to become the root bridge.

57
Q

Command to enable portfast on gi1/0.

A

conf t
int gi1/0
spanning-tree portfast

58
Q

Command to enable BPDUguard on gi1/0

A

conf t
int gi1/0
spanning-tree bpduguard enable

59
Q

Command to make a switch root or secondary root.

A

spanning-tree vlan vlan-id root {primary | secondary}

60
Q

Command to see the root BID.

A

show spanning-tree [vlan vlan id]

61
Q

Command to list the root switches for all vlans.

A

show spanning-tree root

62
Q

Command to show the BID broken out into it’s component parts.

A

show spanning-tree vlan vlan id bridge

63
Q

Command to debug STP

A

debug spanning-tree events

64
Q

When using the root primary command, what is the value that the switch’s priority will be set to if the current root is at 32768?

A

24576

65
Q

When using the root primary command, what is the value that the switch’s priority will be set to if the current root is less than 24576?

A

The highest multiple of 4096 that is still less than the priority of the current root.

66
Q

What are the 3 port states of 802.1w?

A
  1. Learning
  2. Forwarding
  3. Discarding
67
Q

2 commands to see if a switch is in PVST or RPVST mode

A

show spanning-tree

show spanning-tree summary

68
Q

Command to set either PVST or RPVST

A

conf t

spanning-tree mode [pvst | rapid-pvst]

69
Q

If a switch has a root port, is it the root switch?

A

No. The root switch only has designated ports.

70
Q

How many root ports will a switch have?

A

1 per vlan

71
Q

What are the stable states in STP?

A
  1. Forwarding

2. Blocking

72
Q

What are the states in STP that only occur during convergence?

A
  1. Listening

2. Learning

73
Q

What is used to create the BID?

A

2 byte priority field + 6 byte system ID (BIA)

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