2013 Latest Cisco 350-001 Exam Section 6: Catalyst IOS Configuration Commands (5 Questions)
QUESTION NO: 1 Troubleshooting STP convergence errors reveals that a switched network has multiple bridging loops, which is periodically causing problems. What Cisco IOS switching feature, if used improperly, would most likely cause these errors?
A. Port Fast
B. Uplink Fast
C. Backbone Fast
D. Dot1q Trunking
E. Fast EtherChannel
Spanning tree PortFast causes a spanning tree port to enter the forwarding state immediately, bypassing the listening and learning states. You can use PortFast on switch ports connected to a single workstation or server to allow those devices to connect to the network immediately, rather than waiting for spanning tree to converge. PortFast should be used only when connecting a single end station to a switch port. Otherwise, you might create a network loop.
B. UplinkFast provides fast convergence after a spanning tree topology change and achieves load balancing between redundant links using uplink groups. An uplink group is a set of ports (per VLAN), only one of which is forwarding at any given time. Specifically, an uplink group consists of the root port (which is forwarding) and a set of blocked ports, except for self-looping ports. The uplink group provides an alternate path in case the currently forwarding link fails.
C. BackboneFast is initiated when a root port or blocked port on a switch receives inferior BPDUs from its designated bridge. An inferior BPDU identifies one switch as both the root bridge and the designated bridge. When a switch receives an inferior BPDU, it indicates that a link to which the switch is not directly connected (an indirect link) has failed (that is, the designated bridge has lost its connection to the root bridge). Under normal spanning tree rules, the switch ignores inferior BPDUs for the configured maximum aging time. The switch tries to determine if it has an alternate path to the root bridge. If the inferior BPDU arrives on a blocked port, the root port and other blocked ports on the switch become alternate paths to the root bridge. (Self-looped ports are not considered alternate paths to the root bridge.) If the inferior BPDU arrives on the root port, all blocked ports become alternate paths to the root bridge. If the inferior BPDU arrives on the root port and there are no blocked ports, the switch assumes that it has lost connectivity to the root bridge, causes the maximum aging time on the root to expire, and becomes the root switch according to normal spanning tree rules.
If the switch has alternate paths to the root bridge, it uses these alternate paths to transmit a new kind of PDU called the Root Link Query PDU. The switch sends the Root Link Query PDU out all alternate paths to the root bridge. If the switch determines that it still has an alternate path to the root, it causes the maximum aging time on the ports on which it received the inferior BPDU to expire. If all the alternate paths to the root bridge indicate that the switch has lost connectivity to the root bridge, the switch causes the maximum aging times on the ports on which it received an inferior BPDU to expire. If one or more alternate paths can still connect to the root bridge, the switch makes all ports on which it received an inferior BPDU its designated ports and moves them out of the blocking state (if they were in blocking state), through the listening and learning states, and into the forwarding state.
D. The 802.1Q trunking method is the industry standard for trunk links, and can be used as an alternative to ISL. The use of either trunking method alone will not cause any bridging loops.
E. Fast Etherchannel simply provides a way to bond multiple Ethernet links into one larger channel. It will not introduce any STP loops into the network.
QUESTION NO: 2 The speed and duplex settings are being configured for each port in a Catalyst switch. When trying to set the duplex mode on Port 1/1, what does the following message mean: “Port 1/1 is in auto-sensing mode”?
A. Port 1/1 has auto-negotiated the duplex correctly.
B. An error has occurred – the duplex setting of auto is not valid.
C. CDP has detected that both sides are set for auto-negotiating.
D. An error has occurred – the duplex is now mismatched.
When a port is in auto-sensing mode, both its speed and duplex are determined by auto-sensing. An error message is generated if you attempt to set the transmission type of auto-sensing ports. On a 10/100 module, if a port speed is set to auto, its transmission type (duplex) will also set to auto automatically, i.e., the duplex of an auto-speed port is not settable. The only two configurable choices for duplex settings are full and half.
QUESTION NO: 3 The TestKing network is experiencing network connectivity problems soon after an end-user disconnected her PC and connects a switch with an unknown configuration into an access layer switch port, which has
spanning-tree portfast configured. What should be configured on the access layer switch to prevent the network connectivity problems? (Select two)
A. TestKing2950(config-if)# spanning-tree portfast bpdufilter enable
B. TestKing2950(config-if)# spanning-tree portfast bpduguard enable
C. TestKing2950(config-if)# no spanning-tree portfast
D. TestKing2950(config-if)# spanning-tree link-type point-to-point
E. TestKing2950(config-if)# spanning-tree link-type shared
F. TestKing2950(config)# no spanning-tree backbonefast
G. TestKing2950(config)# no spanning-tree uplinkfast
Answer: B, C
The following explains the portfast Bridge Protocol Data Unit (BPDU) guard feature. This feature is one of the Spanning-Tree Protocol (STP) enhancements created by Cisco to enhance switch network reliability, manageability, and security. STP configures a meshed topology into a loop-free, tree-like topology. When the link on a bridge port goes up, there is STP calculation done on that port. The result of the calculation will be the transition of the port into forwarding or blocking state, depending on the position of the port in the network, and the STP parameters. This calculation and transition period usually takes about 30-50 seconds. At this time, no user data is passing via the port. Some user applications may timeout during this period. To allow immediate transition of the port into forwarding state, the STP portfast feature is enabled. Portfast transitions the port into STP forwarding mode immediately upon linkup. The port still participates in STP in the event that if the port is to be a part of the loop, it will eventually transition into STP blocking mode. As long as the port is participating in STP, there is a possibility that some device attached to that port and also running STP with lower bridge priority than that of the current root bridge, will assume the root bridge function and affect active STP topology, thus rendering the network suboptimal. Permanent STP recalculation caused by the temporary introduction and subsequent removal of STP devices with low (zero) bridge priority represent a simple form of Denial of Service (DoS) attack on the network.
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. This is done via the use of the “spanning-tree portfast bpduguard enable” command.
QUESTION NO: 4
A TestKing LAN switch has been configured as shown below:
What does the IOS configuration displayed in the exhibit accomplish on a Catalyst 2900 switch?
A. It enables frames with a CoS 0 or CoS 1 marking to be serviced by WRR (Weight Round Robin) queing with a weighting value of 1.
B. It enables frames with a CoS 5 marking to be serviced by the expedite queue.
C. It guarantees 10% of the link bandwidth to Queue 1 and 20% to queue 2 and 70% to queue 3. Queue 4 is not used.
D. It sets up the 3 CoS-to-DSCP mappings and DSCP-to-CoS mappings.
E. It sets up the WRR queueing where frames with a CoS of 3 or 6 or 7 will have the highest priority.
WRR allows bandwidth sharing at the egress port. This command defines the bandwidths for egress WRR through scheduling weights. Four queues participate in the WRR unless you enable the egress expedite queue. The expedite queue is a strict-priority queue that is used until it is empty before using one of the WRR queues. There is no order of dependencies for the wrr-queue bandwidth command. If you enable
otherwise, all four parameters are used. The WRR weights are used to partition the bandwidth between the queues in the event all queues are nonempty. For example, entering weights of 1:3 means that one queue gets 25 percent of the bandwidth and the other queue gets 75 percent as long as both queues have data. Entering weights of 1:3 do not necessarily lead to the same results as entering weights at
10:30. Weights at 10:30 mean that more data is serviced from each queue and the latency of packets being serviced from the other queue goes up. You should set the weights so that at least one packet (maximum size) can be serviced from the lower priority queue at a time. For the higher priority queue, set the weights so that multiple packets are serviced at any one time.
To map CoS values to drop thresholds for a queue, use the wrr-queue cos-map command. Use the no form of this command to return to the default settings. wrr-queue cos-map queue-id threshold-id cos-1 … cos-n no wrr-queue cos-map Syntax Description
threshold-id to 4.
cos-1 … cos-n 7.
The defaults are as follows:
QUESTION NO: 5
The TestKing switched LAN is shown below:
In the shown diagram, Switch TestKingA is the Root of Spanning Tree If there is a Unidirectional link failure between switches TestKingA and TestKingC, and Switch TestKingC stops receiving BPDUs from Switch TestKingA, it will transition its blocked port to the forwarding state and we can have a Spanning Tree loop. What features can we use to prevent this from happening? (Choose Two)
B. Portfast BPDU guard
D. Portfast BPDU filter
Answer: C, E
Loop guard helps prevent bridging loops that could occur because of a uni-directional link failure on a point-to-point link. When enabled globally, the loop guard applies to all point-to-point ports on the system. Loop guard detects root ports and blocked ports and ensures that they keep receiving BPDUs from their designated port on the segment. If a loop guard enabled root or blocked port stop a receiving BPDUs from its designated port, it transitions to the loop-inconsistent blocking state, assuming there is a physical link error on this port. The port recovers from this loop-inconsistent state as soon as it receives a BPDU. You can enable loop guard on a per-port basis. When you enable loop guard, it is automatically applied to all of the active instances or VLANs to which that port belongs. When you disable loop guard, it is disabled for the specified ports. Disabling loop guard moves all loop-inconsistent ports to the listening state. If you enable loop guard on a channel and the first link becomes unidirectional, loop guard blocks the entire channel until the affected port is removed from the channel. Example:
In this example:
Switches A and B are distribution switches.
Switch C is an access switch.
Loop guard is enabled on ports 3/1 and 3/2 on Switches A, B, and C.
Enabling loop guard on a root switch has no effect but provides protection when a root switch becomes a nonroot switch.
You can enable UniDirectional Link Detection (UDLD) to help isolate the link failure. A loop may occur until UDLD detects the failure, but loop guard will not be able to detect it.
Loop guard has no effect on a disabled spanning tree instance or a VLAN.
Topic 3: IP (47 Questions)
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