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CCNP Route Switch > STP basics & CST > Flashcards

STP basics & CST Flashcards

1
Q

Define the BPDU.

A

Bridge protocol data unit. Used to build STP topologies and elect the root switch.

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2
Q

Switching loops are also known as ___.

A

Bridging loops.

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3
Q

What three issues can switching loops cause?

A

Frames can’t reach their destination, unnecessary strain is put on the CPU, more bandwidth than needed is used.

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4
Q

Briefly define a configuration BPDU.

A

BPDUs created by the root bridge. Used by all switches during an election process. Also carry STP timer info.

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5
Q

Define a BID.

A

Bridge ID Priority value. Carries a switch’s Priority value along with its MAC address. Lowest value becomes the root bridge.

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6
Q

How often is a config BPDU sent?

A

Every two seconds.

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7
Q

When does the root bridge election end?

A

Never. If a better BID is introduced into the network, the switch originating it will become the root bridge.

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8
Q

How many root ports can the root bridge have?

A

None. Root ports lead to the root bridge, so the root bridge cannot have root ports.

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9
Q

All ports on the root bridge are in ___.

A

Forwarding/designated mode.

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10
Q

In STP, a switch’s path cost to the root bridge is a ___ value.

A

Local value. It is not advertised to other switches.

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11
Q

Briefly define the root path cost.

A

The cumulative value of all the ports on the way to the root bridge. Faster the port, the lower the value. The root path cost is carried by the configuration BPDU, and is incremented as it passes through the network.

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12
Q

The root path cost increments as the configuration BPDU is ___.

A

Received.

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13
Q

Explain the steps for choosing the root port.

A
  1. Choose the port receiving the superior BPDU.
  2. Choose the port with the lower root path cost.
  3. If multiple ports have the same cost, choose the one with the lower sender BID.
  4. If one port has not been chosen, choose the port with the lower sender port ID.
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14
Q

When running the “show spanning vlan x” command, under interface type, what does P2p Peer(STP) indicate?

A

The connected switch is running CST.

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15
Q

How can a switch tell what version of STP another switch is running?

A

Via the version number in the BPDU.

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16
Q

Briefly define the root path cost.

A

The cumulative value of all the ports on the way to the root bridge. Faster the port, the lower the value. The root path cost is carried by the configuration BPDU, and is incremented as it passes through the network.

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17
Q

Ports on the root bridge are ___.

A

Designated ports. They will always be forwarding.

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18
Q

For each segment, you need ___.

A

One designated port. Both ports in a segment will never be simultaneously blocking.

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19
Q

On a segment, which switch will have its port in designated mode?

A

The switch with the lower root path cost. If it’s a tie, the switch with the lower BID wins.

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20
Q

What are the root path costs for 10 Gbps, 1 Gbps, and 100 Mbps, respectively?

A

2, 4, and 19.

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21
Q

Is the physically shortest path always the logically shortest path?

A

No. Two 10 Gbps links are much logically shorter than one 100 Mbps link.

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22
Q

How could we create link per-VLAN load balancing?

A

By setting manual path costs per-VLAN, through the interface configuration mode.

23
Q

A port that’s been administratively shut down is considered by STP to be in the ___ state.

A

Disabled.

24
Q

Describe the sequence of states a port goes through to become a forwarding state. (CST)

A
  1. Blocking. No frames are forwarded or received, besides BPDUs.
  2. Listening. Frames are still not forwarded or received, but the switch may send its own config BPDUs to participate in spanning tree election.
  3. Learning. Frames are still not forwarded, but they are received and the CAM table begins to be built.
  4. Forwarding. The port is finally receiving and forwarding frames.
25
Q

Describe the blocking state.

A

No frames are forwarded or received, besides BPDUs.

26
Q

Describe the listening state.

A

Frames are still not forwarded or received, but the switch may send its own config BPDUs to participate in spanning tree election.

27
Q

Describe the learning state.

A

Frames are still not forwarded, but they are received and the CAM table begins to be built.

28
Q

Describe the forwarding state.

A

The port is finally receiving and forwarding frames.

29
Q

What is the port ID made of?

A

A combination of the port priority (default 128) and port number. Lower is better.

30
Q

Bridge priority must be set in increments of ___ and port priority must be set in increments of ___.

A

4096 and 16, respectively.

31
Q

Name and describe the three CST timers.

A
  1. Hello time. Sets how often config BPDUs are sent. Default two seconds.
  2. Forward delay. Sets the length of the listening and learning stages. Default 15 seconds.
  3. Maximum age. Sets how long a superior BPDU is kept. Default 20 seconds.
32
Q

In CST, only the ___ timers are used.

A

Root bridge.

33
Q

Explain how the “root primary” command chooses what STP priority to use.

A
  1. 24576 (+ VLAN number) is chosen.

2. If the current root BID is <24576, then the current root BID minus 4096 is used.

34
Q

What is the default STP priority?

A

32768.

35
Q

What are two commands that easily and automatically configure the switch as the root bridge and backup root, respectively?

A

Spanning VLAN _ root primary, spanning VLAN _ root secondary.

36
Q

Define the TCN BPDU.

A

Topology change notification BPDU. Generated by a switch when a port goes into forwarding or blocking mode. Causes the root to generate a config BPDU with the topology change bit set.

37
Q

Define the topology change bit set.

A

Causes all switches to set their MAC table aging time to the Forward Delay time. Helps clear out CAM tables of old MAC entries.

38
Q

When a topology change bit set is received, how long will a switch’s MAC table aging time be affected?

A

The forward delay time + the max age time. 35 seconds by default.

39
Q

What type of ports will not generate a TCN BPDU?

A

Portfast-enabled ports and edge ports.

40
Q

How is an indirect link failure detected through STP?

A

When a switch starts to receive inferior BPDUs on one of their ports. This means that a switch somewhere along the chain has lost connectivity to the root.

41
Q

How many VLANs can PVST support at once?

A

128

42
Q

What multicast address are BPDUs broadcast to?

A

01.80.c2.00.00.00

43
Q

What are the two types of BPDUs?

A

Topology Change Notification BPDUs and Configuration BPDUs.

44
Q

What command will show the root bridge for a particular VLAN?

A

Show spanning vlan ___.

Look under Root ID.

45
Q

The root path cost is carried by the ___.

A

Configuration BPDU.

46
Q

When needed, how does STP determine which port of a segment will be put into blocking mode?

A

Of the two switches on the segment, the one with the higher BID will have its port put into blocking mode.

47
Q

STP will put the ___ number of ports into blocking mode to prevent loops.

A

Least.

48
Q

What’s the command to change the port cost for a specific VLAN? (from interface config)

A

Spanning vlan ___ cost ___

49
Q

Describe the hello timer.

A

Defines how often the root bridge sends configu BPDUs. Default 2 seconds.

50
Q

Describe the forward delay timer.

A

Defines the length of the listening and learning stages, each. Default 15 seconds.

51
Q

Describe the max age timer.

A

Defines how long a superior BPDU is kept. Default 20 seconds.

52
Q

What priority is chosen when the “spanning vlan root secondary” command is issued?

A

28672 (+ VLAN number)

53
Q

Does the “spanning vlan root primary” command dynamically adjust the priority continuously?

A

No, it only changes the priority when the command is run.

54
Q

What protocol is CST defined by?

A

802.1d

CCNP Route Switch (25 decks)

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