STP Questions Flashcards
Refer to the exhibit. Which statement is true about the output?
CAT1# show spanning-tree interface FastEthernet 0/1 detail
Port 1 (FastEthernet0/1) of VLAN0001 is forwarding
Port path cost 19, Port priority 128, Port Identifier 128.1.
Designated root has priority 32769, address 000a.4107.7400
Designated bridge has priority 32769, address 000a.4107.7400
Designated port id is 128.1, designated path cost 0
Timers: message age 0, forward delay 0, hold 0
Number of transitions to forwarding state: 1
BPDU: sent 237, received 1
CAT2# show spanning-tree interface FastEthernet 0/2 detail
Port 2 (FastEthernet0/2) of VLAN0001 is blocking
Port path cost 19, Port priority 128, Port Identifier 128.2.
Designated root has priority 32769, address 000a.4107.7400
Designated bridge has priority 32769, address 000a.4107.7400
Designated port id is 128.1, designated path cost 0
Timers: message age 1, forward delay 0, hold 0
Number of transitions to forwarding state: 0
BPDU: sent 1, received 242
CAT3# show spanning-tree interface FastEthernet 0/1 detail
Port 1 (FastEthernet0/1) of VLAN0001 is forwarding
Port path cost 19, Port priority 128, Port Identifier 128.1.
Designated root has priority 32769, address 000a.4107.7400
Designated bridge has priority 32769, address 000a.4107.7400
Designated port id is 128.1, designated path cost 0
Timers: message age 0, forward delay 0, hold 0
Number of transitions to forwarding state: 1
BPDU: sent 24, received 0
A. The port on switch CAT1 is forwarding and sending BPDUs correctly.
B. The port on switch CAT1 is blocking and sending BPDUs correctly.
C. The port on switch CAT2 is forwarding and receiving BPDUs correctly.
D. The port on switch CAT2 is blocking and sending BPDUs correctly.
E. The port on switch CAT3 is forwarding and receiving BPDUs correctly.
F. The port on switch CAT3 is forwarding, sending, and receiving BPDUs correctly.
Answer: A
Explanation
From the first lines of the “show” commands and the BPDU sent and received we can conclude:
CAT1 is forwarding and sending BPDUs correctly (BPDU: sent 237, received 1) but it is not receiving BPDUs.
CAT2 is blocking and receiving BPDUs correctly (BPDU: sent 1, received 242) but it is not sending BPDUs.
CAT3 is forwarding and sending BPDUs correctly (BPDU: sent 24, received 0) but it is not receiving BPDUs.
-> only answer A is correct.
Which of the following specifications is a companion to the IEEE 802.1w Rapid Spanning Tree Protocol (RSTP) algorithm, and warrants the use multiple spanning-trees?
A. IEEE 802.1s (MST)
B. IEEE 802.1Q (CST)
C. Cisco PVST+
D. IEEE 802.1d (STP)
Answer: A
Explanation
MST maps multiple VLANs into a spanning tree instance, with each instance having a spanning tree topology independent of other spanning tree instances. This architecture provides multiple forwarding paths for data traffic, enables load balancing, and reduces the number of STP instances required to support a large number of VLANs. MST improves the fault tolerance of the network because a failure in one instance (forwarding path) does not affect other instances (forwarding paths).
Note: RSTP is automatically turned on along with MST (the “spanning-tree mode mst” in global configuration mode will turn on both RSTP & MST)
(Reference: http://www.cisco.com/en/US/docs/switches/datacenter/nexus5000/sw/configuration/guide/cli_rel_4_0_1a/MST.html)
What two things will occur when an edge port receives a BPDU? (Choose two)
A. The port immediately transitions to the Forwarding state.
B. The switch generates a Topology Change Notification (TCN) BPDU.
C. The port immediately transitions to the err-disable state.
D. The port becomes a normal STP switch port.
Answer: B D
Explanation
The concept of edge port basically corresponds to the PortFast feature. An edge port directly transitions to the forwarding state, and skips the listening and learning stages. An edge port that receives a BPDU immediately loses edge port status and becomes a normal spanning tree port.
(Reference: http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml#edge)
Which statement is true about RSTP topology changes?
A. Only nonedge ports moving to the blocking state generate a TC BPDU.
B. Any loss of connectivity generates a TC BPDU.
C. Any change in the state of the port generates a TC BPDU.
D. Only nonedge ports moving to the forwarding state generate a TC BPDU.
E. If either an edge port or a nonedge port moves to a block state, then a TC BPDU is generated.
Answer: D
Explanation
When a Switch (Bridge) discovers topology change, it generates a TCN (Topology Change Notification) BPDU (Bridge Protocol Data Unit) and sends the TCN BPDU on its root port. The upstream Switch (Bridge) responds back the sender with TCA (Topology Change Acknowledgment) BPDU (Bridge Protocol Data Unit) and TCA (Topology Change Acknowledgment) BPDU (Bridge Protocol Data Unit)
The upstream Switch (Bridge) (bridge which received the TCN BPDU) generates another TCN BPDU and sends out via its Root Port. The process continues until the Root Switch (Bridge) receives the TCN BPDU.
When the Root Switch (Bridge) is aware that there is a topology change in the network, it starts to send out its Configuration BPDUs with the topology change (TC) bit set. Configuration BPDUs are received by every Switch (Bridge) in the network and all bridges become aware of the network topology change.
The switch never generates a TCN when a port configured for PortFast goes up or down -> it means no TC will be created for PortFast (or Edge Port) -> D is correct.
(Reference: http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094797.shtml)
Which of the following conditions guarantees that a broadcast storm cannot occur?
A. a native VLAN mismatch on either side of an 802.1Q link
B. BPDU filter configured on a link to another switch
C. Spanning Tree Protocol enabled on both Layer 2 and multilayer switches
D. PortFast enabled on all access and trunk ports
Answer: C
Which two statements are true about port BPDU Guard and BPDU filtering? (Choose two)
A. BPDU guard can be enabled globally, whereas BPDU filtering must be enabled on a per-interface basis.
B. When globally enabled, BPDU port-guard and BPDU filtering apply only to PortFast enabled ports.
C. When globally enabled. BPDU port-guard and BPDU filtering apply only to trunking-enabled ports.
D. When a BPDU is received on a BPDU port-guard enabled port, the interface goes into the err-disabled state.
E. When a BPDU is received on a BPDU filtering enabled port, the interface goes into the err-disabled state.
F. When a BPDU is received on a BPDU filtering enabled port, the interface goes into the STP blocking state.
Answer: B D
Which of the following will generate an RSTP topology change notification?
A. an edge port that transitions to the forwarding state
B. a non-edge port that transitions to the blocking state
C. a non-edge port that transitions to the forwarding state
D. an edge port that transitions to the blocking state
E. any port that transitions to the blocking state
F. any port that transitions to the forwarding state
Answer: C
What is the effect of configuring the following command on a switch?
Switch(config)# spanning-tree portfast bpdufilter default
A. If BPDUs are received by a port configured for PortFast, then PortFast is disabled and the BPDUs are processed normally.
B. If BPDUs are received by a port configured for PortFast, they are ignored and none are sent.
C. If BPDUs are received by a port configured for PortFast, the port will transition to forwarding state.
D. The command will enable BPDU filtering on all ports regardless of whether they are configured for BPDU filtering at the interface level.
Answer: A
Explanation
Please read the explanation of Question 3
Refer to the show spanning-tree mst configuration output shown in the exhibit. What should be changed in the configuration of the switch SW_2 in order for it to participate in the same MST region?
spanning-tree_mst_configuration.jpg
A. Switch SW_2 must be configured with the revision number of 2.
B. Switch SW_2 must be configured with a different VLAN range.
C. Switch SW_2 must be configured with the revision number of 1.
D. Switch SW_2 must be configured with a different MST name.
Answer: C
Switch R1 has been configured with the root guard feature. What statement is true if the spanning tree enhancement Root Guard is enabled?
A. If BPDUs are not received on a non-designated port, the port is moved into the STP loop-inconsistent blocked state
B. If BPDUs are received on a PortFast enabled port, the port is disabled.
D C. If superior BPDUs are received on a designated port, the interface is placed into the root-inconsistent blocked state.
D. If inferior BPDUs are received on a root port, all blocked ports become alternate paths to the root bride.
Answer: C
Based on the show spanning-tree vlan 200 output shown in the exhibit, which two statements about the STP process for VLAN 200 are true? (Choose two)
show_spanning-tree_vlan.jpg
A. BPDUs will be sent out every two seconds.
B. The time spent in the listening state will be 30 seconds.
C. The time spent in the learning state will be 15 seconds.
D. The maximum length of time that the BPDU information will be saved is 30 seconds.
E. This switch is the root bridge for VLAN 200.
F. BPDUs will be sent out every 10 seconds.
Answer: B F
Explanation
From the output you learn that:
+ This is not the root bridge for VLAN 200 (it does not have the line “This bridge is the root” and the root bridge information is shown first. It has a Alternative port).
+ The root bridge is sending Hello every 10 seconds, Max Age is 20 seconds and Forward Delay is 15 seconds while the local bridge is sending Hello every 2 seconds, Max Age is 20 seconds and Forward Delay is 15 seconds.
Aan IEEE bridge is not concerned about the local configuration of the timers value. The IEEE bridge considers the value of the timers in the BPDU that the bridge receives. Effectively, only a timer that is configured on the root bridge of the STP is important. In this case, the local switch will import STP timers from the root bridge -> The listening state (or learning state) will be 30 seconds, which equals to Forward Delay. Also BPDUs will be sent out every 10 seconds (Hello packets).
(Reference: http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094954.shtml)
While logged into a Company switch you issue the following command:
CompanySwitch(config-mst)#instance 10 vlan 11-12
What does this command accomplish?
A. It enables a PVST+ instance of 10 for vlan 11 and vlan 12
B. It enables vlan 11 and vlan 12 to be part of the MST region 10
C. It maps vlan 11 and vlan 12 to the MST instance of 10.
D. It creates an Internal Spanning Tree (1ST) instance of 10 for vlan 11 and vlan 12
E. It create a Common Spanning Tree (CST) instance of 10 for vlan 11 and vlan 12
F. It starts two instances of MST, one instance for vlan 11 and another instance for vlan 12.
Answer: C
Explanation
MST maps multiple VLANs that have the same traffic flow requirements into the same spanning-tree instance. The main enhancement introduced by MST raises the problem, however, of determining what VLAN is to be associated with what instance. More precisely, based on received BPDUs, devices need to identify these instances and the VLANs that are mapped to the instance.
An example of configuring MST on a switch is shown below:
Configuration Description
Switch(config)# spanning-tree mode mst Turn on MST (and RSTP) on this switch
Switch(config)# spanning-tree mst configuration Enter MST configuration submode
Switch(config-mst)# name certprepare Name MST instance
Switch(config-mst)# revision 5 Set the 16-bit MST revision number. It is not incremented automatically when you commit a new MST configuration.
Switch(config-mst)#instance 1 vlan 5-10 Map instance 1 with respective VLANs (VLAN 5 to 10)
Switch(config-mst)#instance 2 vlan 11-15 Map instance 2 with respective VLANs (VLAN 11 to 15)
Note: To be part of a common MST region, a group of switches must share the same configuration attributes. In particular, the configuration name (or region name – 32 bits), revision number (16 bits), and VLAN mapping (associate VLANs with spanning-tree instances) need to be the same for all the switches within the same region.
By default, all VLANs will belong to which MST instance when using Multiple STP?
A. MST00
B. MST01
C. the last MST instance configured
D. none
Answer: A
Explanation
By default, all VLANs are assigned to MST instance 0. Instance 0 is known as the Internal Spanning-Tree (IST), which is reserved for interacting with other Spanning-Tree Protocols (STPs) and other MST regions.
What will occur when a nonedge switch port that is configured for Rapid Spanning Tree does not receive a BPDU from its neighbor for three consecutive hello time intervals?
A. RSTP information is automatically aged out.
B. The port sends a TCN to the root bridge.
C. The port moves to listening state,
D. The port becomes a normal spanning tree port.
Answer: A
Explanation
In STP 802.1D, a non-root bridge only generates BPDUs when it receives one on the root port. But in RSTP 802.1w, a bridge sends a BPDU with its current information every hello-time seconds (2 by default), even if it does not receive any from the root bridge. Also, on a given port, if hellos are not received three consecutive times, protocol information can be immediately aged out (or if max_age expires). Because of the previously mentioned protocol modification, BPDUs are now used as a keep-alive mechanism between bridges. A bridge considers that it loses connectivity to its direct neighbor root or designated bridge if it misses three BPDUs in a row. This fast aging of the information allows quick failure detection. If a bridge fails to receive BPDUs from a neighbor, it is certain that the connection to that neighbor is lost. This is opposed to 802.1D where the problem might have been anywhere on the path to the root.
(Reference: http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml)
A port in a redundant topology is currently in the blocking state and is not receiving BPDUs. To ensure that this port does not erroneously transition to the forwarding state, which command should be configured to satisfy the requirement?
A. Switch(config)#spanning-tree loopguard default
B. Switch(config-if)#spanning-tree bpdufilter
C. Switch(config)#udld aggressive
D. Switch(config-if)#spanning-tree bpduguard
Answer: A
Explanation
Loop guard prevents alternate or root ports from becoming the designated port due to a failure that could lead to a unidirectional link. An example is shown below:
STP_loop_guard.jpg
Suppose S1 is the root bridge. S3′s port connected with S2 is currently blocked. Because of unidirectional link failure on the link
between S2 and S3, S3 is not receiving BPDUs from S2.
Without loop guard, the blocking port on S3 will transition to listening (upon max age timer expiration) -> learning -> forwarding state which create a loop.n
With loop guard enabled, the blocking port on S3 will transition into the STP loop-inconsistent state upon expiration of the max age timer. Because a port in the STP loop-inconsistent state will not pass user traffic, no loop is created. The loop-inconsistent state is effectively equal to the blocking state.
To enable loop guard globally use the command spanning-tree loopguard default.
You are the administrator of a switch and currently all host-connected ports are configured with the portfast command. You have received a new directive from your manager that states that, in the future, any host-connected port that receives a BPDU should automatically disable PortFast and begin transmitting BPDUs. Which of the following commands will support this new requirement?
A. Switch(config)# spanning-tree portfast bpduguard default
B. Switch(config-if)# spanning-tree bpduguard enable
C. Switch(config-if)# spanning-tree bpdufilter enable
D. Switch(config)# spanning-tree portfast bpdufilter default
Answer: D
Explanation
The bpdufilter option feature is used to globally enable BPDU filtering on all Port Fast-enabled interfaces and this prevent the switch interfaces connected to end stations from sending or receiving BPDUs.
Note: The spanning-tree portfast bpdufilter default global configuration command can be overridden by the spanning-tree bdpufilter enable command in interface mode.
Refer to the exhibit. The service provider wants to ensure that switch S1 is the root switch for its own network and the network of the customer. On which interfaces should root guard be configured to ensure that this happens?
STP_root_guard.jpg
A. interfaces 1 and 2
B. interfaces 1,2,3, and 4
C. interfaces 1, 3, 5, and 6
D. interfaces 5 and 6
E. interfaces 5, 6, 7, and 8
F. interfaces 11 and 12
Answer: D
Explanation
Let’s see what will happen if we set port 5 & 6 as “root guard” ports:
First, notice that the “root guard” command cannot be used on root switch (because this command is based on blocked port – while a root switch can’t have a blocked port -> two middle switches cannot become root bridges.
Moreover, the neighbor switch which has its port connected with this “root guard” port can’t be the root bridge. For example if we configure port 6 as “root guard” port, the left-bottom switch (the switch with ports 3, 4) can’t be root bridge because that will make port 6 root port. Therefore by configuring port 5 & 6 as “root guard” ports, two switches in the “Customer network” cannot become root bridge.
Examine the diagram. A network administrator has recently installed the above switched network using 3550s and would like to control the selection of the root bridge. Which switch should the administrator configure as the root bridge and which configuration command must the administrator enter to accomplish this?
root_bridge_elect.jpg
A. DSW11(config)# spanning-tree vlan 1 priority 4096
B. DSW12(config)# set spanning-tree priority 4096
C. ASW13(config)# spanning-tree vlan 1 priority 4096
D. DSW11(config)# set spanning-tree priority 4096
E. DSW12(config)# spanning-tree vlan 1 priority 4096
F. ASW13(config)# set spanning-tree priority 4096
Answer: E
Explanation
First, only switches in Distribution section should become root bridge -> only DSW11 or DSW12 should be chosen.
The traffic passing root bridge is always higher than other switches so we should choose switch with highest speed connection to be root bridge -> DSW12 with two 100Mbps connections should be chosen.
Also, the correct command to change priority value for a specific VLAN is spanning-treee vlan VLAN-ID priority Priority-number.
Which three statements about the MST protocol (IEEE 802.1S) are true? (Choose three)
A. To verify the MST configuration, the show pending command can be used in MST configuration mode.
B. When RSTP and MSTP are configured; UplinkFast and BackboneFast must also be enabled.
C. All switches in the same MST region must have the same VLAN-to-instance mapping, but different configuration revision numbers.
D. All switches in an MST region, except distribution layer switches, should have their priority lowered from the default value 32768.
E. An MST region is a group of MST switches that appear as a single virtual bridge to adjacent CST and MST regions.
F. Enabling MST with the “spanning-tree mode mst” global configuration command also enables RSTP.
Answer: A E F
Explanation
The show pending command can be used to verify the MST configuration (pending configuration). An example of this command is shown below:
MST_show_pennding.jpg
Note:
The above commands do these tasks:
+ Enter MST configuration mode
+ Map VLANs 10 to 20 to MST instance 1
+ Name the region certprepare
+ Set the configuration revision to 1
+ Display the pending configuration
+ Apply the changes, and return to global configuration mode
The MST region appears as a single bridge to spanning tree configurations outside the region -> a MST region appears as a single virtual bridge to adjacent CST and MST regions -> E is correct.
By enabling MST you also enable RSTP because MST relies on the RSTP configuration to operate -> F is correct.
Refer to the exhibit. All network links are FastEthernet. Although there is complete connectivity throughout the network, Front Line users have been complaining that they experience slower network performance when accessing the Server Farm than the Reception office experiences. Based on the exhibit, which two statements are true? (Choose two)
bridge_priority_network_optimize.jpg
A. Changing the bridge priority of S1 to 4096 would improve network performance.
B. Changing the bridge priority of S1 to 36864 would improve network performance.
C. Changing the bridge priority of S2 to 36864 would improve network performance.
D. Changing the bridge priority of S3 to 4096 would improve network performance.
E. Disabling the Spanning Tree Protocol would improve network performance.
F. Upgrading the link between S2 and S3 to Gigabit Ethernet would improve performance.
Answer: B D
Explanation
All three switches have the same bridge priority (32768 – default value) and S1 has the lowest MAC -> S1 is the root bridge and all traffic must go through it -> Front Line Users (S2) must go through S1 to reach Server Farm (S3). To overcome this problem, S2 or S3 should become the root switch and we can do it by changing the bridge priority of S1 to a higher value (which lower its priority – answer B) or lower the bridge priority value (which higher its priority – answer D)
Refer to the exhibit. Initially, LinkA is connected and forwarding traffic. A new LinkB is then attached between SwitchA and HubA. Which two statements are true about the possible result of attaching the second link? (Choose two)
STP_switch_loop_block.jpg
A. The switch port attached to LinkB will not transistion to up.
B. One of the two switch ports attached to the hub will go into blocking mode when a BPDU is received.
C. Both switch ports attached to the hub will transition to the blocking state.
D. A heavy traffic load could cause BPDU transmissions to be blocked and leave a switching loop.
E. The switch port attached to LinkA will immediately transition to the blocking state.
Answer: B D
Explanation
we know that there will have only one Designated port for each segment (notice that the two ports of SwitchA are on the same segment as they are connected to a hub). The other port will be in Blocking state. But how does SwitchA select its Designated and Blocking port? The decision process involves the following parameters inside the BPDU:
* Lowest path cost to the Root
* Lowest Sender Bridge ID (BID)
* Lowest Port ID
In this case, both interfaces of SwitchA have the same “path cost to the root” and “sender bridge ID” so the third parameter “lowest port ID” will be used. Suppose two interfaces of SwitchA are fa0/1 & fa0/2 then SwitchA will select fa0/1 as its Designated port (because fa0/1 is inferior to fa0/2) -> B is correct.
Suppose the port on LinkA (named portA) is in forwarding state and the port on LinkB (named portB) is in blocking state. In blocking state, port B still listens to the BPDUs. If the traffic passing through LinkA is too heavy and the BPDUs can not reach portB, portB will move to listening state (after 20 seconds for STP) then learning state (after 15 seconds) and forwarding state (after 15 seconds). At this time, both portA & portB are in forwarding state so a switching loop will occur -> D is correct.
Refer to the exhibit. Switch S1 is running mst IEEE 802.1s. Switch S2 contains the default configuration running IEEE 802.1D. Switch S3 has had the command spanning-tree mode rapid-pvst running IEEE 802.1w. What will be the result?
STP_simple.jpg
A. IEEE 802.1D and IEEE 802.1w are incompatible. All three switches must use the same standard or no traffic will pass between any of the switches.
B. Switches S1, S2, and S3 will be able to pass traffic between themselves.
C. Switches S1, S2, and S3 will be able to pass traffic between themselves. However, if there is a topology change, Switch S2 will not receive notification of the change.
D. Switches S1 and S3 will be able to exchange traffic but neither will be able to exchange traffic with Switch S2
Answer: B
Explanation
A switch running both MSTP and RSTP supports a built-in protocol migration mechanism that enables it to interoperate with legacy 802.1D switches. If this switch receives a legacy 802.1D configuration BPDU (a BPDU with the protocol version set to 0), it sends only 802.1D BPDUs on that port. An MST switch can also detect that a port is at the boundary of a region when it receives a legacy BPDU, an MST BPDU (version 3) associated with a different region, or an RST BPDU (version 2).
However, the switch does not automatically revert to the MSTP mode if it no longer receives 802.1D BPDUs because it cannot determine whether the legacy switch has been removed from the link unless the legacy switch is the designated switch
(Reference: http://www.cisco.com/en/US/docs/switches/lan/catalyst2950/software/release/12.1_9_ea1/configuration/guide/swmstp.html)
Refer to the exhibit. Switch S2 contains the default configuration. Switches S1 and S3 both have had the command spanning-tree mode rapid-pvst issued on them. What will be the result?
STP_simple.jpg
A. IEEE 802.1D and IEEE 802.1w are incompatible. All three switches must use the same standard or no traffic will pass between any of the switches.
B. Switches S1, S2. and S3 will be able to pass traffic between themselves.
C. Switches S1, S2. and S3 will be able to pass traffic between themselves. However, if there is a topology change. Switch S2 will not receive notification of the change.
D. Switches S1 and S3 will be able to exchange traffic but neither will be able to exchange traffic with Switch S2.
Answer: B
Which two statements are true when the extended system ID feature is enabled? (Choose two)
A. The BID is made up of the bridge priority value (2 bytes) and bridge MAC address (6 bytes).
B. The BID is made up of the bridge priority (4 bits), the system ID (12 bits), and a bridge MAC address (48 bits).
C. The BID is made up of the system ID (6 bytes) and bridge priority value (2 bytes).
D. The system ID value is the VLAN ID (VID).
E. The system ID value is a unique MAC address allocated from a pool of MAC addresses assigned to the switch or module.
F. The system ID value is a hex number used to measure the preference of a bridge in the spanning-tree algorithm.
Answer: B D
Explanation
In short, with the use of IEEE 802.1t spanning-tree extensions, some of the bits previously used for the switch priority are now used for the extended system ID
extended_system_id_stp.jpg
Only four high-order bits of the 16-bit Bridge Priority field carry actual priority. Therefore, priority can be incremented only in steps of 4096. In most cases, the Extended System ID holds the VLAN ID. For example, if our VLAN ID is 5 and we use the default bridge priority 32768 then the 16-bit Priority will be 32768 + 5 = 32773.
Note: The MAC address is reserved when the extended system ID feature is enabled.
Refer to the exhibit. Switch 15 is configured as the root switch for VLAN 10 but not for VLAN 20. If the STP configuration is correct, what will be true about Switch 15?
STP_forwarding.jpg
A. All ports will be in forwarding mode.
B. All ports in VLAN 10 will be in forwarding mode.
C. All ports in VLAN 10 will be in forwarding mode and all ports in VLAN 20 will be in blocking mode.
D. All ports in VLAN 10 will be in forwarding mode and all ports in VLAN 20 will be in standby mode.
Answer: B
Explanation
All ports on root bridge are designated ports, which are in forwarding state but notice in this case Switch 15 is the root switch for VLAN 10 -> all ports in VLAN 10 will be in forwarding state. We can not say anything about the modes of ports of Switch 15 in other VLANs.
Refer to the exhibit. STP has been implemented in the network. Switch SW_A is the root switch for the default VLAN. To reduce the broadcast domain, the network administrator decides to split users on the network into VLAN 2 and VLAN 10. The administrator issues the command spanning-tree vlan 2 root primary on switch SW_A. What will happen as a result of this change?
STP_root.jpg
A. All ports of the root switch SW_A will remain in forwarding mode throughout the reconvergence of the spanning tree domain.
B. Switch SW_A will change its spanning tree priority to become root for VLAN 2 only.
C. Switch SW_A will remain root for the default VLAN and will become root for VLAN 2.
D. No other switch in the network will be able to become root as long as switch SW_A is up and running.
Answer: C
Explanation
This command sets the switch to become root for a given VLAN. It works by lowering the priority of the switch until it becomes root. Once the switch is root, it will not prevent any other switch from becoming root. In particular, if the current root bridge is greater than 24576 then our switch will drop to 24576. If the current root bridge is less than 24576, our new bridge priority will be (Priority value of the current root bridge – 4096).
This command does not affect other VLAN so SW_A will remain root for the default VLAN -> C is correct.
Note: This command is not shown in a Catalyst switch configuration because the command is actually a macro executing other switch commands.
