STP Questions 4
Here you will find answers to STP Questions – Part 4
Question 1
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 (IST) 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.
Question 2
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.
Question 3
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)
Question 4
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:
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.
Question 5
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.
Question 6
Which two statements correctly describe characteristics of the PortFast feature? (Choose two)
A. STP will be disabled on the port.
B. PortFast can also be configured on trunk ports.
C. PortFast is required to enable port-based BPDU guard.
D. PortFast is used for both STP and RSTP host ports.
E. PortFast is used for STP-only host ports.
Answer: B D
Explanation
You can use PortFast on switch or trunk ports connected to a single workstation, switch, or server to allow those devices to connect to the network immediately, instead of waiting for the port to transition from the listening and learning states to the forwarding state -> B is correct.
Also, PortFast can be used for both STP and RSTP -> D is correct.
(Reference: http://www.cisco.com/en/US/docs/switches/lan/catalyst4000/7.4/configuration/guide/stp_enha.html)
Answer C is not correct because BPDU guard can be enabled without PortFast. But what will happen if the PortFast and BPDU guard features are configured on the same port?
Well, at the reception of BPDUs, the BPDU guard operation disables the port that has PortFast configured. The BPDU guard transitions the port into errdisable state, and a message appears on the console
| 2000 May 12 15:13:32 %SPANTREE-2-RX_PORTFAST:Received BPDU on PortFast enable port. Disabling 2/1 2000 May 12 15:13:32 %PAGP-5-PORTFROMSTP:Port 2/1 left bridge port 2/1 |
(Reference and good resource: http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a008009482f.shtml)
Question 7
Which of the following commands can be issued without interfering with the operation of loop guard?
A. Switch(config-if)#spanning-tree guard root
B. Switch(config-if)#spanning-tree portfast
C. Switch(config-if)#switchport mode trunk
D. Switch(config-if)#switchport mode access
Answer: C
Explanation
In general, Loop Guard is configured on non-designated ports (blocking or root ports) and it prevents them from becoming designated ports when the current designated ports stop sending BPDUs.
Root Guard should be configured on designated ports and prevents them from becoming root ports. Therefore Root Guard is incompatible with Loop Guard.
PortFast should be placed on ports configured as access ports while Loop Guard should be placed on trunk ports -> we can use the “switchport mode trunk” without interfering with the operation of Loop Guard.
Question 8
Which statement correctly describes enabling BPDU guard on an access port that is also enabled for PortFast?
A. Upon startup, the port transmits 10 BPDUs. If the port receives a BPDU, PortFast and BPDU guard are disabled on that port and it assumes normal STP operation.
B. The access port ignores any received BPDU.
C. If the port receives a BPDU, it is placed into the error-disable state.
D. BPDU guard is only configured globally and the BPDU filter is required for port-level configuration.
Answer: C
Explanation
If any BPDU is received on a port where BPDU guard is enabled, that port is put into the err-disable state immediately. The port is shut down in an error condition and must be either manually re-enabled or automatically recovered through the errdisable timeout function.
Note: A port that has PortFast enabled also has BPDU guard automatically enabled. By combining PortFast & BPDU guard we have a port that can quickly enter the Forwarding state from Blocking state and automatically shut down when receiving BPDUs.
Question 9
Why is BPDU guard an effective way to prevent an unauthorized rogue switch from altering the spanning-tree topology of a network?
A. BPDU guard can guarantee proper selection of the root bridge.
B. BPDU guard can be utilized along with PortFast to shut down ports when a switch is connected to the port.
C. BPDU guard can be utilized to prevent the switch from transmitteing BPDUs and incorrectly altering the root bridge election.
D. BPDU guard can be used to prevent invalid BPDUs from propagating throughout the network.
Answer: B
Question 10
Which three statements about STP timers are true? (Choose three)
A. STP timers values (hello, forward delay, max age) are included in each BPDU.
B. A switch is not concerned about its local configuration of the STP timers values. It will only consider the value of the STP timers contained in the BPDU it is receiving.
C. To successfully exchange BPDUs between two switches, their STP timers value (hello, forward delay, max age) must be the same.
D. If any STP timer value (hello, forward delay, max age) needs to be changed, it should at least be changed on the root bridge and backup root bridge.
E. On a switched network with a small network diameter, the STP hello timer can be tuned to a lower value to decrease the load on the switch CPU.
F. The root bridge passes the timer information in BPDUs to all routers in the Layer 3 configuration.
Answer: A B D
Explanation
Each BPDU includes the hello, forward delay, and max age STP timers. An 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. If you lose the root, the new root starts to impose its local timer value on the entire network. So, even if you do not need to configure the same timer value in the entire network, you must at least configure any timer changes on the root bridge and on the backup root bridge.
(Reference: http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094954.shtml)
