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myCCNP Switch 300-115 > Spanning Tree > Flashcards

Spanning Tree Flashcards

1
Q

How is the root bridge elected in STP?

A

Lowest bridge ID. Bridge ID is made up of lowest priority and MAC address.

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2
Q

What are the 3 port types of STP?

A

Designated Port = FWD Root Port = FWD Non Designated (Alternate) Port = Blocking

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3
Q

What ports are found on a root bridge?

A

All ports on the Root Bridge are Designated Ports

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4
Q

What ports are closest (facing) the Root Bridge?

A

Root Ports

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5
Q

What determines the closest path to a Root Bridge?

A

COST of the links it take to get back to the Root. 1G=4 100M = 19 10M= 100

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6
Q

What is the special frame used to negotiate STP?

A

BPDU (Bridge Protocol Data Unit)

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7
Q

What is the default STP Priority?

A

32768

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8
Q

What are the 4 STP Modes?

A

Blocking Listening Learning Forwarding

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9
Q

How long do each of the modes take?

A

Blocking 20 Sec Listening 15 Sec Learning 15 Sec Forwarding

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10
Q

What is happening during the listening mode?

A

The switch will only be sending and receiving BPDU’s

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11
Q

What is happening during the learning mode?

A

The switch will only be sending and receiving BPDU’s along with learning MAC address. No data transmission until in forward mode.

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12
Q

What are default Hello Times for STP?

A

2 seconds

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13
Q

What is the default Max age timers for STP?

A

20 Seconds

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14
Q

What is CST & PVST+ Reconvergence time?

A

50 Seconds Total

  • 20 Sec Max Age Timer
  • 15 Sec Listening
  • 15 Sec Learning
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15
Q

What is the default STP mode for cisco switches?

A

PVST

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16
Q

What does PortFast do.

A

It skips the Listening and Learning stage of STP.

A port with Portfast will never create a topology change noticication (TCN).

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17
Q

When STP has to make a decesion, what 4 tie breakers will it use?

A

1. Lowest bridge ID: the switch with the lowest bridge ID becomes the root bridge.

2. Lowest path cost to root bridge: when the switch receives multiple BPDUs it will select the interface that has the lowest cost to reach the root bridge as the root port.

3. Lowest sender bridge ID: when a switch is connected to two switches that it can use to reach the root bridge and the cost to reach the root bridge is the same, it will select the interface connecting to the switch with the lowest bridge ID as the root port.

4. Lowest sender port ID: when the switch has two interfaces connecting to the same switch, and the cost to reach the root bridge is the same it will use the interface with the lowest number as the root port.

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18
Q

What is RSTP and PVRSTP average converge times?

A

2-3 seconds.

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19
Q

How do you enable PortFast?

A

under the interface

#spanning-tree portfast

or

Global#spanning-tree portfast Default (enable portfast on all access mode interfaces)

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20
Q

What are the steps of RSTP?

A

1, Discarding

  1. Learning
  2. Forwarding
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21
Q

Who send BPDU after the network has been converged in CST?

A

The root bridge generates BPDU’s and they are relayed by none root bridges.

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22
Q

Who generates BPDU’s in RSTP?

A

All Switches generate BPDU every two seconds (Hello Time)

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23
Q

How are BPDU’s used as keepalive mechanism in RSTP?

A

If a switch misses 3 BPDU from a neighbor it will assume connectivity to ths switch has been lost and it will remove all MAC address.

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24
Q

Name the two types of BPDU’s?

A

Type 1 : configuration BPDU

Type 2: TCN BPDU

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25
Q

How do you set a root bridge for a VLAN?

A

SW2(config)#spanning-tree vlan 20 root Primary

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26
Q

What is uplink Fast?

A

a Manual setting on CST that puts all ports that connect back to RB in a group. The blocking port is in standby. When the Root Port fails the standby port goes straight into forwarding state.

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27
Q

Where would you use uplink fast?

A

Only use uplink fast in the access layer of the network. Uplink fast set switch priority to 49,152 so it does not become root.

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28
Q

What is backbone fast?

A

Backbone fast will null the max age timer on a blocking interface. A switch should only receive a Root BPDU on a blocking interface if the connected switch looses connection to the Root Bridge.

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29
Q

What is BPDU Guard?

A

Blockes all BPDU’s from entering a port. If BPDU are received then the port will go into err-disabled.

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30
Q

How to enable BPDU guard?

A

SW1(config-if)#spanning-tree bpduguard enable

Or

SW1(config)#spanning-tree portfast bpduguard default (enable on all ports with portfast enabled)

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31
Q

What port types does RSTP have?

A

Edge ports & P2P ports.

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32
Q

What are the 5 versions of Spanning Tree?

A
  • Common Spanning Tree 802.1D
  • PVLAN+ (Cisco Only)
  • RSTP 802.1W
  • PVRST (Cisco Only)
  • MST 802.1S
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33
Q

Summarize the three-step process that STP uses to initially converge on an active topology.

A

Elect a single Root Bridge for the entire bridged network. Elect one Root Port for every non-Root Bridge. Elect one Designated Port for every segment.

34
Q

When running the Spanning-Tree Protocol, every bridge port saves a copy of the best information it has heard. How do bridges decide what constitutes the best information?

A

Lowest Root BID Lowest Path Cost to Root Bridge Lowest Sender BID Lowest Port ID

35
Q

Why are Topology Change Notification BPDUs important? Describe the TCN process.

A

Topology Change Notification BPDUs play an important role in that they help bridges relearn MAC addresses more quickly after a change in the active STP topology. A bridge that detects a topology change sends a TCN BPDU out its Root Port. The Designated Port for this segment acknowledges the TCN BPDU with the TCA flag in the next Configuration BPDU it sends. This bridge also propagates the TCN BPDU out its Root Port. This process continues until the BPDU reaches the Root Bridge. The Root Bridge then sets the TC flag in all Configuration BPDUs sent for twice the Forward Delay period. As other bridges receive the TC flag, they shorten the bridge table aging period to Forward Delay seconds.

36
Q

How are Root Path Cost values calculated?

A

Root Path Cost is the cumulative cost of the entire path to the Root Bridge. It is calculated by adding a port’s Path Cost value to the BPDUs received on that port.

37
Q

Assume that you install a new bridge and it contains the lowest BID in the network. Further assume that this devices is running experimental Beta code that contains a severe memory leak and, as a result, reboots every 10 minutes. What effect does this have on the network?

A

STP is a preemptive protocol that constantly seeks the Root Bridge with the lowest BID. Therefore, in this network, the new bridge wins the Root War, and the entire active topology converges on this bridge every ten minutes. Where links change state during this convergence process, temporary outages of 30–50 seconds occur. When the bridge fails several minutes later, the network converges on the next most attractive Root Bridge and creates another partial network outage for 30–50 seconds.

38
Q

When do bridges generate Configuration BPDUs?

A

Every Hello Time seconds on all ports of the Root Bridge (unless there is a Physical-Layer loop. When a non-Root Bridge receives a Configuration BPDU on its Root Port, it sends an updated version of this BPDU out every Designated Port. When a Designated Port hears a less attractive BPDU from a neighboring bridge.

39
Q

When do bridges generate Topology Change Notification BPDUs?

A

A bridge port is put into the Forwarding state and the bridge has at least one Designated Port. A port in the Forwarding or Learning states transitions to the Blocking state. A non-Root Bridge receives a TCN (from a downstream bridge) on a Designated Port.

40
Q

When is the Root Bridge placement form of STP load balancing most effective? What command(s) are used to implement this approach?

A

When traffic patterns are well defined and clearly understood.

41
Q

When is the Port Priority form of STP load balancing useful? What command(s) are used to implement this approach? What makes this technique so confusing?

A

This form of load balancing is rarely useful. It can only be used with back-to-back switches. It should only be used in early versions of code or when connecting to non-Cisco devices.

42
Q

What are the components that the default value of Max Age is designed to account for? There is no need to specify the exact formula, just the major components captured in the formula.

A

The default Max Age value of 20 seconds is designed to take two factors into account: End-to-end BPDU propagation delay and Message Age Overestimate.

43
Q

What are the components that the default value of Forwarding Delay is designed to account for? There is no need to specify the exact formula, just the major components captured in the formula.

A

he default Forward Delay value of 15 seconds is designed to take four factors into account: End-to-End BPDU Propagation Delay, Message Age Overestimate, Maximum Transmission Halt Delay, and Maximum Frame Lifetime.

44
Q

What are the main considerations when lowering the Hello Time from the default of two seconds to one second?

A

Lowering the Hello Time value can allow you to improve convergence time by lowering Max Age or Forward Delay (you have to do this separately) but also doubles the load that STP places on your network. Notice that load here refers to both the load of Configuration BPDU traffic and, more importantly, Spanning Tree CPU load on the switches themselves.

45
Q

Where should PortFast be utilized? What does it change about the STP algorithm?

A

In general, PortFast should only be used on end-station ports. It allows a port to immediately move into the Forwarding state when it initializes. Other than that, the processing is the same. When using redundant NICs that toggle link state, it can also be useful for links to servers.

46
Q

Where should UplinkFast be utilized? In addition to altering the local bridging table to reflect the new Root Port after a failover situation, what other issue must UplinkFast address?

A

UplinkFast should only be utilized in leaf-node, wiring closet switches. After a failover, UplinkFast must generate dummy multicast packets to update bridging tables throughout the network in addition to updating its own bridging table.

47
Q

Where should BackboneFast be utilized?

A

To work correctly, BackboneFast must be enabled on every switch in a given Layer 2 domain.

48
Q

Where is PVST+ useful?

A

PVST+ is useful when you are trying to connect traditional PVST Catalyst devices with 801.Q switches that only support a single instance of the Spanning-Tree Protocol.

49
Q

Can MST regions be connected to PVST regions?

A

MST and PVST regions cannot be connected through trunk links (MST switches only support 802.1Q trunks, and PVST switches only support ISL trunks). However, the two types of switches can be connected through access (non-trunk) links (although this is rarely useful).

50
Q

Can you disable STP on a per-port basis?

A

STP cannot be disabled on a per-port basis on Layer 2 Catalyst equipment such as the 4000s, 5000s, and 6000s. In fact, some Layer 3 Catalyst switches (Sup III with NFFC) require that STP be disabled for the entire device (all VLANs).

51
Q

Why is it important to use a separate management VLAN?

A

It is important to use a separate management VLAN to prevent CPU overload. If the CPU does overload as a result of excessive broadcast or multicast traffic, the Spanning Tree information can become out-of-date. When this occurs, it becomes possible that a bridging loop could open. If this loop forms in the management VLAN, remaining CPU resources are quickly and completely exhausted. This can spread throughout the network and create a network-wide outage.

52
Q

What happens if UplinkFast sends the fake multicast frames to the usual Cisco multicast address of 01-00-0C-CC-CC-CC?

A

If UplinkFast sends the dummy frames to the usual Cisco multicast address of 01-00-0C-CC-CC-CC, older, non-UplinkFast-aware Cisco Layer-2 devices do not flood the frames. Therefore, this does not update bridging tables through the network.

53
Q

What is the goal of *STP*?

A

The goal of STP is to prevent bridging loops.

54
Q

What does *BPDU* stand for?

A

Bridge Protocol Data Unit

55
Q

What is a *BPDU*?

A

BPDU is a frame that is communicated in STP that provides the Root ID, Bridge ID, and cost to the Root.

56
Q

How is the *Root Bridge* determined in STP?

A

The Root Bridge is determined by the lowest bridge ID. The Bridge ID is composed of the priority + mac address.

57
Q

What is considered a topology change in STP?

A

A topology change is triggered by:

  1. Learning –> Forwarding
  2. Learning –> Blocking
  3. Port becomes disabled
58
Q

What does *TCN* stand for?

A

Topology Change Notification

59
Q

What is a *TCN*?

A

A TCN is a message that is flooded whenever a topology change occurs in the STP instance.

60
Q

When is a *TCN* not triggered?

A

A TCN is not triggered by a port configured with PortFast.

61
Q

What does the *Root Bridge* do when receiving a TCN?

A

The Root Bridge sets the “TC-Flag” when transmitting BPDUs into that VLAN, short for Topology Change BPDUs. The Root Bridge reduces own CAM Aging time to Forwarding Delay for the affected VLAN.

62
Q

What do *Non-Root Bridges* do when receiving TC-BPDU?

A

Non-Root Bridges reduce their CAM Aging time to the forwarding delay for the affected VLAN. Any unheard of MAC Addresses during this time are flushed.

63
Q

What is the *STP Root Port*?

A

The STP Root Port is the port that has the best cost to the root.

64
Q

What is the *STP Designated Port*?

A

The STP Designated Port is the downstream port connecting to a LAN segment with the best root path cost.

65
Q

What is the *STP Alternative Port*?

A

The STP Alternative Port is a port with an alternative path to the root. Effectively blocking as it can only listen to BPDUs.

66
Q

What is the *STP Backup Port*?

A

The STP Backup Port is a port on the same collision domain as the designated port. Effectively blocking as it can only listen to BPDUs.

67
Q

What is the Bridge ID?

A

Bridge ID is a combination of the Switch Priority and MAC Address and in every flavor of STP, lowest bridge ID becomes Root Bridge.

68
Q

What is the default switch priority?

A

32768

69
Q

What is port priority in STP?

A

Port priority is used to determine root port. If there are two or mote equal cost paths to root bridge and same bridge ID, lowest port priority wins.

70
Q

What is the default hello timer in STP?

A

The default hello timer is 2 seconds but this can be tuned.

71
Q

What is the *forwarding delay* in STP?

A

Forwarding delay is the time spent in the listening and learning state.

72
Q

What is the max age in STP?

A

Max age is the maximum length of time before a bridge port saves its BPDU information.

73
Q

What is portfast?

A

Portfast is a feature that bypasses most STP states so the end hosts can converge immediately. This should not be used between switches as it effectively disables STP and can cause loops.

74
Q

What is *BPDUGuard*?

A

BPDUGuard blocks against any BPDU packets that may be received on a port. This causes ports to go into error-disabled state.

75
Q

What is *BPDUFilter*?

A

BPDUFilter prevents interfaces from sending or receiving BPDUs, however if a BPDU is received on the interface, the filter is disabled.

76
Q

What is *LoopGuard*?

A

LoopGuard prevents a blocked port from transitioning into forwarding when it stops receiving BPDUs. This could prevent potential loops in case the neighbor switch is having issues.

77
Q

What is *RootGuard*?

A

RootGuard should be enabled on all ports where root bridge should not appear. RootGuard enabled ports, when receiving a superior BPDU, results in the port going into a root-inconsistent STP state.

78
Q

What useful show commands verifies spanning-tree?

A

Sw1# show spanning-tree details
Sw1# show spanning-tree summary

79
Q

What is the difference between long and short path cost values in *STP*?

A

Short cost is based on a 16-Bit value, and is default in RSTP and STP. Long cost is based on a 32-Bit value, and is default in MST and configurable in RSTP.

80
Q

What configuration command configures portfast?

A

Sw1(config)# spanning-tree portfast default
Sw1(config-if)# spanning-tree portfast

81
Q

What configuration command configures bpduguard and bpdufilter?

A
  • Sw1(config)# spanning-tree portfast bpduguard default
  • Sw1(config)# spanning-tree portfast bpdufilter default
  • Sw1(config-if)# spanning-tree bpduguard enable
  • Sw1(config-if)# spanning-tree bpdufilter enable
82
Q

What configuration command configures rootguard and loopguard?

A
  • Sw1(config)# spanning-tree loopguard default
  • Sw1(config-if)# spanning-tree guard loop
  • Sw1(config-if)# spanning-tree guard root

myCCNP Switch 300-115 (73 decks)

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