CCNP Switch Questions/Answers Implementing Spanning-Tree. A. Will be displayed next hop address for interfaces associated with VLAN 101.

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1 Refer to the show command above, what statement is true? A. Will be displayed next hop address for interfaces associated with VLAN 101. B. Will be displayed IP addresses of interfaces associated with VLAN 100. C. Will be displayed trunk link that carries VLAN 101. D. Will be displayed IP addresses of devices on both end of the trunk. Answer: A Next hop addresses for interfaces and the interfaces associated with VLAN 101 will be displayed with Switch#show ip cef vlan 101 command, that is used to view general information about FIB (Forwarding Information Base) entries, that are associated with the specific VLAN. Thic command does not show information about trunk links associated with VLAN 101. To do that you have to use command Switch#show trunk

2 Which three effects does the interface command switchport host have when entered on a switch? (Choose three) A. Sets the switch port mode to access B. Enables BPDU guard C. Enables spanning tree PortFast D. Enables root guard E. Disables channel grouping F. Enables BPDU filtering Answer: A, C, E Command switchport host is macro command that optimizes the port for Layer 2 host connection. This is done by enabling some and disabling other features. It sets switch port to access mode. Enables spanning tree PortFast and disables EtherChannel grouping. Example of issuing switchport host command on interface gig1/0/27 is shown below

3 One switch in a Layer 2 switched spanning-tree domain is converted to PVRST+ using the spanning-tree mode rapid-pvst global configuration mode command. The remaining switches are running PVST+. What is the effect on the spanning-tree operation? A. Spanning tree is effectively disabled in the network. B. The PVRST+ switch forwards 802.1D BPDUs, but does not participate as a node in any spanning tree. C. All switches default to one 802.1D spanning tree for all VLANs. D. The PVSRT+ reverts to PVST+ to interoperate with the PVST+ switches. Answer: D More on: Link

4 Refer to the exhibit. What conclusion does the output support? A. PortFast is enabled on interface fa0/6. B. IEEE 802.1w is enabled on VLAN1. C. The forward delay timer has been changed from the default value. D. Standard IEEE 802.1D behavior is shown. Answer: D The STP (Spanning Tree Protocol) is a network protocol that ensures a loop-free topology for any bridged Ethernet local area network. The basic function of STP is to prevent bridge loops and the broadcast radiation that results from them. Spanning tree also allows a network design to include spare (redundant) links to provide automatic backup paths if an active link fails, without the danger of bridge loops, or the need for manual enabling/disabling of these backup links. Spanning Tree Protocol (STP) is standardized as IEEE 802.1D. STP switch port states: Blocking - A port that would cause a switching loop, no user data is sent or received but it may go into forwarding mode if the other links in use were to fail and the spanning tree algorithm determines the port may transition to the forwarding state. BPDU data is still received in blocking state. Prevents the use of looped paths. Listening - The switch processes BPDUs and awaits possible new information that would cause it to return to the blocking state. It does not populate the MAC address table and it does not forward frames. Learning - While the port does not yet forward frames it does learn source addresses from frames received and adds them to the filtering database (switching database). It populates the MAC Address table, but does not forward frames

5 Forwarding - A port receiving and sending data, normal operation. STP still monitors incoming BPDUs that would indicate it should return to the blocking state to prevent a loop. Disabled - Not strictly part of STP, a network administrator can manually disable a port

6 Refer to the exhibit. What two conclusions does the output show to be true? (Choose two) A. DLS1 is running IEEE 802.1s on instance 1. B. DLS1 is running IEEE 802.1D on instance 1. C. DLS1 is the Root Bridge for instance 1. D. Interface fa0/12 will move into the err-disable state if a BPDU is received. E. Interfaces fa0/1 through fa0/6 are trunk ports. Answer: A, C From show command above we can see that on DLS1 MST is configured and displayed are MST01 instance information. From ID information, we can see priority, MAC address, that this switch is Root Bridge and values for Hello Time, Max Age and Forward Delay

7 Which three parameters should match all switches within an MST region? (Choose three) A. Port costs on trunk ports. B. Configuration name C. Revision number D. Trunk encapsulation method. E. Bridge priority F. VLAN-to-instance mappings. Answer: B, C, F The default spanning tree mode on Cisco Catalyst switches is PVST+ or, on newer models, Rapid PVST+. These protocols result in a direct one-to-one mapping of spanning tree instances to VLANs. For example, if there are eight VLANs on a switch, the switch must participate in eight independent spanning trees. This is often undesirable, as the number of actually unique spanning tree topologies is typically less than the number of VLANs (at worst, the two will be equal). Running only a single common spanning tree (CST, which is not supported on Cisco Catalyst switches) would be more efficient, but it does not allow for the design flexibility afforded by PVST. Multiple Spanning Tree (MST) was created to allow for multiple spanning tree topologies while preserving scalability. MST enables an administrator to map an arbitrary number of VLANs to a single MST instance, resulting in the minimum number of instances needed to satisfy a design. If, for example, you have six VLANs but only two unique layer two topologies, you need only two MST instances. MST region as a collection of switches, sharing the same view of physical topology partitioning into set of logical topologies. For two switches to become members of the same region, the following attributes must match: Configuration name; Configuration revision number (16 bits); The table of 4096 elements which map the respective VLAN to STP instance number

8 Users complain that they lost connectivity to all resources in the network. A network administrator suspects the presence of a bridging loop as a root cause of the problem. Which two actions will determine the existence of the bridging loop? (Choose two) A. Confirm MAC port security is enabled on all access switches. B. Check the port utilization on devices and look for abnormal values. C. Verify that the management VLAN is properly configured on all root bridges. D. Capture the traffic on the saturated link and verify if duplicate packets are seen. E. Ensure that the root guard and loop guard are properly configured on all distribution links. Answer: B, D Forwarding loops seeming can cause all sorts of wacky behavior in various Cisco switches. Forwarding loops vary greatly both in their origin (cause) and effect. Due to the wide variety of issues that can affect STP, we will provide general guidelines about how to troubleshoot forwarding loops. Before you start to troubleshoot, you must obtain this information: An actual topology diagram that details all of the switches and bridges; Their corresponding (interconnecting) port numbers; STP configuration details, such as which switch is the root and backup root, which links have a non-default cost or priority, and the location of blocking ports. Generally, troubleshooting involves these steps: 1. Identify the loop: When a forwarding loop has developed in the network, these are the usual symptoms: Loss of connectivity to, from, and through affected network regions; High CPU utilization on routers connected to affected segments or VLANs that can lead to various symptoms, such as routing protocol neighbor flapping or Hot Standby Router Protocol (HSRP) active router flapping; High link utilization (often 100 percent); High switch backplane utilization (compared to the baseline utilization); Syslog messages that indicate packet looping in the network (for example HSRP duplicate IP address messages); Syslog messages that indicate constant address relearning or MAC address flapping messages; An increasing number of output drops on many interfaces Note: Any of these reasons alone may indicate different issues (or no issue at all). However, when many of these are observed at the same time, it is highly probable that a forwarding loop has developed in the network. If the current traffic level is well above normal or if the baseline level is not known, check whether the peak level has been achieved recently and whether it is close to the current traffic level. For example, if the peak traffic level is 15 percent and it was reached just two minutes ago and current traffic level is 14 percent, then that would mean that the switch is working under an unusually high load. If the traffic load is at a normal level, then that probably means that there is either no loop or that this device is not involved in the loop. However, it still could be involved in a slow loop

9 2. Discover the topology (scope) of the loop. Once it has been established that the reason for the network outage is a forwarding loop, the highest priority is to stop the loop and restore the network operation. In order to stop the loop, you must know which ports are involved in the loop: look at the ports with the highest link utilization (packets per second). The show interface Cisco IOS software command displays the utilization for each interface. In order to display only the utilization information and the interface name (for a quick analysis), you might use Cisco IOS software regular expression output filtering. Issue the show interface include line \/sec command to display only the packet per second statistics and the interface name. Pay particular attention to the interfaces with the highest link utilization

10 Refer to the exhibit. Switch SW1 is receiving traffic from SW2. However, SW2 is not receiving traffic from SW1. Which STP feature should be implemented to prevent inadvertent loops in the network? A. UDLD B. PortFast C. BPDU guard D. BPDU filtering Answer: A The UDLD protocol allows devices connected through fiber-optic or copper Ethernet cables (for example, Category 5 cabling) to monitor the physical configuration of the cables and detect when a unidirectional link exists. When a unidirectional link is detected, UDLD shuts down the affected port and alerts the user. Unidirectional links can cause a variety of problems, including spanning-tree topology loops. More on: Link

11 Which interfaces should loop guard be enabled on? A. Root ports B. Designated ports C. Root port and alternate ports. D. Ports configured with PortFast. E. Root port and ports configured with PortFast. Answer: C Loop Guard feature sets the switch port to detect potential loop due to inactive UDLD. Loop Guard must be enabled on the non-designated ports (more precisely, on root and alternate ports) for all possible combinations of active topologies