1
Q
What does STP stand for?
A
Spanning Tree Protocol
2
Q
What is the primary purpose of STP?
A
To prevent Layer 2 loops in Ethernet networks
3
Q
Which IEEE standard defines the original STP?
A
IEEE 802.1D
4
Q
What are the three port states in 802.1D STP?
A
Blocking, Listening, Learning
5
Q
Which STP port state allows MAC address learning but not forwarding?
A
Learning
6
Q
Which STP port state does not forward frames or learn MAC addresses?
A
Blocking
7
Q
What is the Bridge ID made up of?
A
Bridge Priority + MAC Address
8
Q
How is the root bridge elected in STP?
A
The switch with the lowest Bridge ID
9
Q
What happens if multiple switches have the same priority during root bridge election?
A
The switch with the lowest MAC address becomes the root
10
Q
What is the default Hello Time in STP?
A
2 seconds
11
Q
What is the default Max Age in STP?
A
20 seconds
12
Q
What is the default Forward Delay in STP?
A
15 seconds
13
Q
What are the port roles in STP?
A
Root Port, Designated Port, Non-Designated Port
14
Q
What is a Root Port?
A
The port on a non-root bridge with the lowest path cost to the root bridge
15
Q
What is a Designated Port?
A
The port on a segment with the lowest path cost to the root bridge
16
Q
What is a Non-Designated Port?
A
A port that is in the blocking state to prevent loops
17
Q
How is STP path cost determined?
A
By the speed of the interface
18
Q
What is the STP path cost of a 100 Mbps link?
A
19
19
Q
What is the STP path cost of a 1 Gbps link?
A
4
20
Q
What is the STP path cost of a 10 Gbps link?
A
2
21
Q
What is a BPDU?
A
Bridge Protocol Data Unit, used by STP to share information between switches
22
Q
Which device sends BPDUs in an STP topology?
A
The root bridge sends BPDUs, others forward them
23
Q
What is the IEEE standard for RSTP?
A
IEEE 802.1w
24
Q
How many port states exist in RSTP?
A
Discarding, Learning, Forwarding
25
What are the port roles in RSTP?
Root, Designated, Alternate, Backup, and Disabled
26
What is MST?
Multiple Spanning Tree
27
What IEEE standard defines MST?
IEEE 802.1s
28
What is the benefit of MST?
Allows multiple VLANs to be mapped to a single STP instance
29
What command shows STP status on a Cisco switch?
`show spanning-tree`
30
How can you change the STP priority on a Cisco switch?
`spanning-tree vlan priority `
31
What command makes a switch the root bridge for a VLAN?
`spanning-tree vlan root primary`
32
How do you disable STP on a port?
spanning-tree bpdufilter enable (only for ports connecting to devices that cannot handle BPDU's)
33
What are the five STP port states in 802.1D?
Blocking, Listening, Learning, Forwarding, Disabled
34
What does a port in the Blocking state do?
It does not forward frames or learn MAC addresses; it only receives BPDUs.
35
What happens during the Listening state in STP?
The switch processes BPDUs to determine the STP topology.
36
What happens during the Learning state in STP?
The switch builds the MAC address table but does not forward frames.
37
What happens during the Forwarding state in STP?
The port forwards frames and learns MAC addresses.
38
What is the Disabled state in STP?
A port that is administratively shut down or not participating in STP.
39
What are the three port states in RSTP (802.1w)?
Discarding, Learning, Forwarding
40
What does the Discarding state do in RSTP?
It prevents forwarding and MAC learning; combines Blocking and Listening from STP.
41
What happens during the Learning state in RSTP?
MAC addresses are learned, but frames are not forwarded.
42
What happens during the Forwarding state in RSTP?
The port forwards frames and learns MAC addresses.
43
What is the purpose of the Hello Time in STP?
It determines how often the root bridge sends BPDUs.
44
What is the purpose of the Max Age timer in STP?
It defines how long a switch stores a BPDU before discarding it.
45
What is the purpose of the Forward Delay timer in STP?
It controls how long the Listening and Learning states last.
46
What is a topology change in STP?
A change in the Layer 2 network that triggers recalculation of the STP tree.
47
What event commonly causes an STP topology change?
A port going up or down, such as a link or switch failure.
48
Which device detects an STP topology change?
The switch that experiences the change (e.g., port up/down).
49
What does a switch do when it detects a topology change in 802.1D STP?
It sends a Topology Change Notification (TCN) BPDU toward the root bridge.
50
How does the root bridge respond to a TCN BPDU in STP?
It sets the Topology Change (TC) flag in its BPDUs for the duration of Forward Delay × 2.
51
What happens to the MAC address table after a topology change in STP?
All switches reduce their MAC address aging time to forward delay (15 seconds) temporarily.
52
What is the impact of an STP topology change on switch performance?
Increased flooding due to shortened MAC address aging.
53
How does RSTP handle topology changes differently than STP?
RSTP reacts immediately without TCN BPDUs and updates only affected ports.
54
What happens to MAC addresses in RSTP after a topology change?
Only the MAC addresses on affected ports are aged out.
55
Which port roles in RSTP can signal a topology change?
Edge ports and non-edge ports that transition to forwarding.
56
Which command displays STP topology changes on a Cisco switch?
`show spanning-tree detail`
57
What output in `show spanning-tree detail` indicates a topology change occurred?
`Number of topology changes` and the timestamp of the last change
58
What is the purpose of STP tuning?
To optimize convergence time and control which switch becomes the root bridge.
59
How can you influence which switch becomes the root bridge?
By lowering its STP bridge priority.
60
What is the default bridge priority in STP?
32768
61
What is the increment step for STP priority values?
4096
62
What command sets a switch as the root bridge automatically?
`spanning-tree vlan root primary`
63
How can you influence STP path selection?
By adjusting the port cost or port priority.
64
What command sets STP port cost manually?
`spanning-tree vlan cost `
65
What command sets STP port priority?
`spanning-tree vlan port-priority `
66
What does PortFast do?
Immediately places an access port in the forwarding state, bypassing STP states.
67
When should PortFast be enabled?
Only on access ports connected to end devices.
68
What command enables PortFast on an interface?
`spanning-tree portfast`
69
What is the risk of enabling PortFast on trunk ports?
It may cause loops if connected to another switch.
70
What does BPDU Guard do?
Err-disables a port if it receives a BPDU, used with PortFast.
71
What command enables BPDU Guard globally?
`spanning-tree portfast bpduguard default`
72
What does Root Guard do?
Prevents a port from becoming a root port if superior BPDUs are received.
73
What command enables Root Guard on an interface?
`spanning-tree guard root`
74
What does Loop Guard do?
Prevents alternate or root ports from transitioning to forwarding if BPDUs are not received.
75
What command enables Loop Guard on an interface?
`spanning-tree guard loop`
76
What is UplinkFast?
A Cisco proprietary feature for fast failover on access switches with redundant uplinks.
77
What is BackboneFast?
A Cisco proprietary feature that accelerates convergence when an indirect link failure is detected.
78
What is BPDU Filter?
It suppresses the sending or receiving of BPDUs on a port.
79
What is the effect of enabling BPDU Filter globally?
Ports with PortFast will not send or receive BPDUs unless a BPDU is received, then PortFast and Filter are disabled.
80
What is the effect of enabling BPDU Filter on an interface?
It permanently disables STP on the port; BPDUs are neither sent nor received.
81
What is the difference between BPDU Guard and BPDU Filter?
BPDU Guard disables the port on receiving a BPDU, while BPDU Filter suppresses BPDU transmission/reception.
82
What happens when Root Guard receives a superior BPDU?
The port is placed into a root-inconsistent (blocking) state.
83
What happens when Loop Guard detects a loss of BPDUs?
The port moves to loop-inconsistent state instead of forwarding.
84
Where should you use Root Guard?
On designated ports toward downstream switches where you don't want a new root.
85
Where should you use Loop Guard?
On non-designated ports (alternate/root ports) where BPDUs are expected.
86
What kind of topology is UplinkFast designed for?
Access layer switches with redundant uplinks.
87
How does UplinkFast work?
It immediately transitions the backup (blocked) uplink to forwarding when the root port fails.
88
What is a side effect of UplinkFast?
It floods dummy multicast frames with all MAC addresses in the CAM table to update other switches.
89
What kind of failure does BackboneFast handle?
Indirect link failures not directly connected to the detecting switch.
90
How does BackboneFast improve convergence?
By skipping the Max Age timer wait period when an indirect failure is detected.
91
What is Rapid PVST+?
A Cisco proprietary version of RSTP that runs a separate instance of RSTP per VLAN.
92
What IEEE standard is Rapid PVST+ based on?
IEEE 802.1w
93
What is the main benefit of Rapid PVST+ over PVST?
Faster convergence using RSTP enhancements.
94
How does Rapid PVST+ differ from RSTP?
It supports multiple STP instances, one per VLAN.
95
Which switches support Rapid PVST+?
Cisco switches running IOS.
96
What is the command to enable Rapid PVST+?
`spanning-tree mode rapid-pvst`
97
What does MST stand for?
Multiple Spanning Tree
98
What is the purpose of MST?
To map multiple VLANs to a single STP instance, reducing CPU load.
99
What is an MST instance?
A logical STP process inside an MST region that manages a group of VLANs.
100
What is MST0?
The Internal Spanning Tree (IST) instance for region-to-region communication.
101
What is an MST region?
A group of switches with identical MST configuration (name, revision, VLAN mapping).
102
How do you configure an MST region?
`spanning-tree mst configuration`
103
What happens if MST configuration does not match between switches?
They treat each other as different regions and fall back to CST.
104
What command maps VLANs to MST instances?
`instance vlan `
105
What does CST stand for in MST?
Common Spanning Tree, used between MST regions and legacy STP domains.
106
How does EtherChannel interact with STP?
STP treats all links in an EtherChannel as a single logical link.
107
What happens if one link in an EtherChannel goes down?
STP does not reconverge; the EtherChannel remains up if other links are active.
108
What is the STP port state of an EtherChannel?
The entire EtherChannel is either forwarding or blocking.
109
How does STP select the root port when using EtherChannel?
It uses the aggregate cost of the entire bundle.
110
What should you ensure when configuring STP with EtherChannel?
Consistent STP settings across all member ports.
111
What is the risk of BPDU Filter on a port connected to another switch?
It can cause Layer 2 loops by disabling STP protection.
112
What is the global BPDU Filter behavior?
Applies to all PortFast ports unless a BPDU is received.
113
What is the interface-level BPDU Filter behavior?
Disables STP completely on that interface.
114
What is PVST?
Per VLAN Spanning Tree - one instance of STP per VLAN.
115
What is the benefit of PVST?
Allows load balancing by using different root bridges per VLAN.
116
What is a drawback of PVST?
High resource use with many VLANs.
117
How does STP pruning affect VLANs?
Prevents unnecessary VLAN traffic on trunk links.
118
How is STP pruning configured?
`vtp pruning` or manually using `switchport trunk allowed vlan`
119
How do you verify the STP root bridge?
`show spanning-tree` and check 'this bridge is root'
120
What command shows topology changes?
`show spanning-tree detail`
121
What does 'loop-inconsistent' mean?
A port is blocked by Loop Guard due to missing BPDUs.
122
What does 'root-inconsistent' mean?
A port is blocked by Root Guard after receiving a superior BPDU.
123
What is the RSTP sync process?
A handshake mechanism using Proposal and Agreement messages to transition ports to forwarding safely.
124
What triggers the RSTP sync process?
When a designated port is about to transition to forwarding.
125
What is an RSTP Proposal?
A BPDU sent by a bridge proposing itself as the designated bridge on a port.
126
What must a switch do before accepting a Proposal in RSTP?
It must block all non-edge designated ports to ensure loop-free topology.
127
What is an RSTP Agreement?
A response sent back to the proposer, confirming all downstream ports are in sync and loops are prevented.
128
What happens after an Agreement is received in RSTP?
The proposing port immediately transitions to the forwarding state.
129
Why is the RSTP sync process important?
It ensures rapid convergence while maintaining loop prevention.
130
Why would an engineer tune RPVST per-VLAN root priorities?
To load balance VLAN traffic across different uplinks, spreading utilization.
131
How does RPVST per-VLAN load balancing work?
Each VLAN can have a different root bridge/uplink, steering VLANs to different links.
132
What is a downside of RPVST per-VLAN tuning?
It is manual and can lead to uneven traffic distribution if one VLAN grows heavily.
133
How does EtherChannel handle multiple physical links?
It bundles them into a single logical link, seen as one interface by STP.
134
How does EtherChannel perform load balancing?
By hashing fields (MAC, IP, or TCP/UDP ports), not per VLAN.
135
What happens in STP when using an EtherChannel?
STP sees the EtherChannel as one logical interface, simplifying the topology.
136
What is the advantage of EtherChannel over per-VLAN tuning?
Simpler design, automatic load distribution, redundancy, and faster convergence.
137
When would per-VLAN RPVST tuning still be useful?
When links cannot form an EtherChannel (different speeds/vendors) or for deterministic VLAN steering.
138
What is Cisco’s modern best practice for redundant uplinks?
Use EtherChannel (preferably LACP) with QoS for latency-sensitive traffic.
139
Exam key point: RPVST vs EtherChannel?
RPVST tuning = legacy load balancing, EtherChannel = modern best practice for scalability and redundancy.
CCNP ENCOR
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