2013 Latest Cisco 350-001 Exam Section 1: Transparent (9 Questions)

2013 Latest Cisco 350-001 Exam Section 1: Transparent (9 Questions)

In a bridged LAN, the number if BPDU’s with the TCA bit set is incrementing rapidly. What could be the cause of this? (Choose all that apply).
A. BPDU’s with the TCA bit set is part of the normal operation of a bridged LAN.
B. Improper cabling is being used in the network.
C. There is no spanning tree portfast configured on the ports connecting 2 workstations.
D. The root switch is experiencing problems due to high CPU utilization and is not sending any BPDUs.
E. None of the above.
Answer: B, C
In normal STP operation, a bridge keeps receiving configuration BPDUs from the root bridge on its root port, but it never sends out a BPDU toward the root bridge. So, in order to achieve that, a special BPDU called the topology change notification (TCN) BPDU has been introduced. Thus, when a bridge needs to signal a topology change, it starts sending TCNs on its root port. The designated bridge receives the TCN, acknowledges it, and generates another one for its own root port. And so on until the TCN hits the root bridge.
The TCN is a very simple BPDU that contains absolutely no information that a bridge sends out every hello_time seconds (this is locally configured hello_time, not the hello_time specified in configuration BPDUs). The designated bridge acknowledges the TCN by immediately sending back a normal configuration BPDU with the topology change acknowledgement (TCA) bit set. The bridge notifying the topology change will not stop sending its TCN until the designated bridge has acknowledged it, so the designated bridge answers the TCN even though it does not receive configuration BPDU from its root.
The portfast feature is a Cisco proprietary change in the STP implementation. The command is applied to specific ports and has two effects:
Ports coming up are put directly in the forwarding STP mode, instead of going through the learning and listening process. Note that STP is still running on ports with portfast.
The switch never generates a TCN when a port configured for portfast is going up or
down. Reference:

The TestKing LAN is a bridged network running the 802.1D spanning tree protocol. Which of the following are paramenters that a bridge will receive from the root bridge.
A. Maxage
B. Root Cost
C. Forward delay
D. A,B, and C
E. None of the above
Answer: D
A, B and C are all located in the BPDU which each switch gets from the root bridge.
The BPDUs are in the following format:
Protocol ID – indicates that the packet is a BPDU.
Version – the version of the BPDU being used.
Message Type – the stage of the negotiation.
Flags – two bits are used to indicate a change in topology and to indicate acknowledgement of the TCN BPDU.
Root ID – the root bridge priority (2 bytes) followed by the MAC address (6 bytes).
Root Path Cost – the total cost to from this particular bridge to the designated root bridge.
Bridge ID – the bridge priority (2 bytes) followed by the MAC address (6 bytes), lowest value wins! The default bridge priority is 0x8000 (3276810).
Port ID – the ID of the port from which are transmitted the BPDUs, a root port, this is made up of the configured port priority and the bridge MAC address.
Message Age – timers for aging messages (only has effect on the network if the root bridge is configured with this parameter).
Maximum Age
– the maximum message age before information from a BPDU is dropped because it is too old and no more BPDUs have been received. (only has effect on the network if the root bridge is configured with this parameter). The default value for this is 20 seconds.
Hello Time – the time between BPDU configuration messages sent by the root bridge (only has effect on the network if the root bridge is configured with this parameter). The default value for this is 2 seconds.
Forward Delay – this temporarily stops a bridge from forwarding data to give a chance for information of a topology change to filter through to all parts of the network. This means that ports that need to be turned off in the new topology have a chance to be switched off before the new ports are turned on (only has effect on the network if the root bridge is configured with this parameter).

A small office LAN contains only one switch, which was put in place without any of the default configurations changed. You have noticed that somebody in the office has looped a cable by connecting one end to port 4/37 and the other to port 4/38 as shown below:
All links are 10/100 Configuration is default
Which of the following statements is true?
A. Port 4/38 will be blocked.
B. Both ports will be forwarding.
C. Port 4/37 will be blocking.
D. Both ports will be blocked.
E. Port 4/38 will continuously move between the listening and learning states.
F. Port 4/37 will be stuck in the learning state.
Answer: A
Port priority is based on lowest priority, and lowest port number. Because of this, then 4/37 would become the root port and 4/38 would be blocking. The default mode of a Catalyst switch is to enable the STP process for all VLANs.
Incorrect Answers:
B. Even though this switch will effectively become the root switch, and all ports in a root switch should be in the “forwarding state” a loop will occur in this case, and so one of the ports must be blocking. Since the priority of 4/38 is lower by default, it will be blocking.

Which of the following statements regarding Transparent Bridge tables are FALSE? (Choose all that apply.)
A. Decreasing the bridge table aging time would reduce flooding.
B. Increasing the bridge table aging time would reduce flooding.
C. Bridge table entries are learned by way of examining the source MAC address of each frame.
D. Bridge table entries are learned by examining destination MAC addresses of each frame.
E. The bridge aging time should always be more than the aggregate time for detection and recalculation of the spanning tree.
Answer: A, D
Basic fundamental behind TB is to learn the network topology by means of storing the source MAC address of a packet, and the corresponding interface from which the packet came in on the network. This information is stored in the bridge table. To keep the bridge table small and manageable entries are deleted after a specified period of time, known as bridge table aging time. Once an entry is removed from the bridge table, and a packet arrives for which the information is no longer there in the bridge table, the packet will be flooded out of all interfaces except the interface on which it was received.
An increase in the bridge table aging time will reduce flooding.
Incorrect Answers:
B. Increasing the aging table time will indeed reduce the flooding, since the source MAC addresses are cached for a longer period of time.
C. This statement is also true. Bridge tables are built by looking at the source MAC address to learn which stations are attached to the bridge ports.
E. The aging time should indeed be longer than the convergence time for the spanning tree algorithm in order to prevent information from timing out and being re-learned, which will just begin the STP process again.

The TestKing network is shown in the following exhibit: You issue the “set spantree root 1” command on Switch TK1. What will happen as a result of this change? (Choose all that apply).
A. No other switch in the network will be able to become root as long as Switch TK1 remains up and running in this topology.
B. Switch TK1 will change its Spanning Tree priority to become the root for Vlan 1, only.
C. The port that used to be blocking on Switch TK3 will be changed to forwarding.
D. The link between Switch TK1 and Switch TK2 will remain forwarding throughout the reconvergence of the Spanning Tree domain.
Answer: B, C
The syntax specified in this question only makes TK1 root for Vlan 1 only. The set spantree root {vlan_id} command sets the priority of the switch to 8192 for the VLAN or VLANs specified in {vlan_id} . Note: The default priority for switches is 32768. This command setting means that the Testking switch will be selected as the root switch because it has the lowest priority.
TestKing-Switch> (enable) set spantree root 1
VLAN 1 bridge priority set to 8192.
VLAN 1 bridge max aging time set to 20.
VLAN 1 bridge hello time set to 2.
VLAN 1 bridge forward delay set to 15.
Switch is now the root switch for active VLAN 1.
TestKing-Switch> (enable)

Because Switch 1 will become the root, the link between TK 3 and TK 1 will be forwarding, which means the link between TK2 and TK3 will change from forwarding to blocking.
Incorrect Answers:
A. It is important to remember that the “set spantree root” command merely changes the spanning tree switch priority from 32768 to 8192 so that it is much more likely to become the root for the particular VLAN. If another switch already has a priority lower than 8192, then this command will make the switch the root by lowering it to one below the existing value. For example, if another switch is already configured with a priority of 8192, then issueing the “set spantree root” command will configure the new switch with a priority of 8191. However, another switch could still become the root if it were configured with a lower priority after this command was issues on another switch.
D: After the “set span root 1” command is set, TK1 will send a PDU to TK2 and a new
STP election will occur. During this time, the port will transition into the blocking,
listening, and learning states and during this time the forwarding of traffic will be
temporarily halted.
Reference: Cisco LAN Switching, Clark and Hamilton, Cisco Press, Page 197.

The TestKing network consists of only 2 routers as below:
You perform the following router configurations:
Router TK1: no ip routing ! interface Ethernet 0 no ip address bridge-group 1 ! interface Ethernet 1 no ip address bridge-group 1 ! bridge 1 protocol ieee
Router TK2: no ip routing ! interface Ethernet 0 no ip address bridge-group 2 ! interface Ethernet 1 no ip address bridge-group 2 ! bridge 2 protocol ieee bridge 2 priority 63500
Based on this configuration, which router will become the root, and which ports will be forwarding?
A. Router TK2 will become the root.
One port on Router TK1 will be forwarding, and the other will be blocking.
One port on Router TK2 will be forwarding, and the other will be blocking.

B. Both Router TK1 and Router TK2 will become the root in an independent spanning
All ports on Router TK1 and Router TK2 will be forwarding.

C. Router TK1 will become the root.
Both ports on Router TK1 will be forwarding.
Both ports on Router TK2 will be forwarding.

D. Router TK2 will become the root.
Both ports on Router TK1 will be forwarding.
One port on Router TK2 will be forwarding, and the other will be blocking.

E. Router TK1 will become the root.
Both ports on Router TK1 will be forwarding.
One port on Router TK2 will be forwarding, and the other will be blocking.

Answer: E
Bridge 1’s priority is at default 32768, Bridge 2 is at 63500, Bridge 1 (with a lower Bridge ID) will be Root Bridge. All ports on the root bridge are always in forwarding state, hence both the ports on Bridge 1 will be in forwarding state. As per STP any other Bridge can only have one connection to the Root Bridge in the forwarding state, hence only one port on Bridge 2 will be forwarding.
Incorrect Answers:
A, D. TK2 has a bridge priority configured as 63500, while TK1 is left with the default. Since the default value is 32768 and lower is preferred, TK1 will become the root.
C. Only the single root port will be forwarding.

QUESTION NO: 7 What spanning-tree protocol timer determines how often the root bridge send configuration BDPUs?
A. STP Timer
B. Hold Timer
C. Hello Timer
D. Max Age Timer
E. Forward Delay Timer
Answer: C
The STP Hello Time is the time between each Bridge Protocol Data Unit (BPDU) that is sent on a port. This is equal to two seconds by default, but can be tuned to be between one and ten seconds.
Incorrect Answers:
A. The Max Age, Forward Delay, and Hello Timers are all considered to be STP timers.
B. Hold timers are used in routing protocols to avoid inconsistent information and loops, but they are not an STP timer.
D. The Max Age Timer controls the maximum length of time a bridge port saves its configuration BPDU information. This is 20 seconds by default and can be tuned to be between six and 40 seconds.
E. The Forward Delay Timer is the time spent in the listening and learning state. This is by default equal to 15 seconds, but can be tuned to be between four and 30 seconds.

The Testking network topology is displayed below:

WGSW3 has been set up to provide access to mobile users in a conference room. Portfast has been enabled on all access ports. The following command is entered on WGSW3:
Switch(config)#spanning-tree portfast bpduguard
What happens if a switch or bridge is connected to one of the access ports?
A. Any access port that receives a BDPU packet will be disabled.
B. The access port will reject any BDPU packets that they receive.
C. Portfast will be disabled on any access port that receives a BDPU packet.
D. The bridge can join the BDPU topology, but it is blocked form becoming the root bridge.
E. Only BDPU packets that are NOT superior to the current root bridge will be accepted on the access port.
Answer: A
The STP portfast BPDU guard enhancement is designed to allow network designers to enforce the STP domain borders and keep the active topology predictable. The devices behind the ports with STP portfast enabled are not allowed to influence the STP topology. This is achieved by disabling the port with portfast configured upon reception of BPDU. The port is transitioned into errdisable state, and a message is printed on the console. The following is an example of the message printed out as a result of BPDU guard operation:
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

The following was executed on switch TestKingC:

If all switches in the TestKing network run the same type of spanning tree, what is the total number of spanning tree topology changes that occurred in this network?
A. 7
B. 35
C. Can not be determined. Only the root bridge tracks the complete amount of topology changes
D. 0
E. 2
Answer: A
The role of the TC mechanism is to correct L2 forwarding tables after the forwarding topology has changed. This is necessary to avoid a connectivity outage because, after a TC, some MAC addresses previously accessible through particular ports might become accessible through different ports. TC shortens the forwarding table aging time on all
age-out and flooding will occur to ensure packets reach the destination MAC address. TC is triggered by the change of a port’s STP state to or from the STP forwarding state. Note:With Rapid STP or Multiple STP (IEEE 802.1w and IEEE 802.1s), TC is triggered by a change of the port’s state to forwarding, as well as the role change from designated to root. With Rapid STP, the L2 forwarding table is immediately flushed, as opposed to 802.1d, which shortens the aging time. The immediate flushing of the forwarding table restores connectivity faster, but will cause more flooding. In the output shown above, the total number of topology changes for VLAN 2 is shown to be 7, with the last change occurring 3 weeks and 5 days ago.
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