Ccna certification Practice Tests

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Appendix 

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  Answers to Practice Test Questions

 120.  B.  The RSTP path cost is the calculation of all of the link costs that lead back to the 

root bridge. The link cost is a numeric value that signifies the speed. The lower the cost, 

the higher the speed of the link. The path cost is not related to the latency of the frame 

traversing the link. There is also no monetary cost associated with a link because it 

pertains to STP link cost. The path cost is not a numeric value associated with the speed 

of a link; this would be the link cost, not the path cost.

 121.  B.  Per-VLAN Spanning Tree+ (PVST+) elects a root bridge for each VLAN and creates 

a topology table for each VLAN. It is a Cisco proprietary protocol due to the bridge ID 

calculation it must perform for each VLAN. The IEEE 802.1w specification details Rapid 

Spanning Tree Protocol (RSTP). The Common Spanning Tree (CST) protocol assumes 

one spanning tree instance for all VLANs. RSTP is the Spanning Tree Protocol that has 

superseded the original Spanning Tree Protocol.

 122.  A.  Rapid Per-VLAN Spanning Tree+ elects a root bridge for each VLAN. It allows for fast 

convergence times and logical placement of the root bridge. However, it requires the most 

CPU and RAM of all implementations. Per VLAN Spanning Tree (PVST) operates similar 

to PVST+; however, it transmits 802.1D BPDUs. The Common Spanning Tree protocol 

assumes one spanning tree instance for all VLANs. There is no protocol called the RSTP+ 

protocol; therefore, it is an invalid answer.

 123.  B.  Common Spanning Tree (CST) elects a single root bridge for the entire network and 

all of the VLANs. This creates a problem when the center of your network may vary upon 

VLAN placement. CST is a variant of STP; therefore, it has slower convergence times. 

CST should not be used in really large networks because the root switch for the various 

VLANs may be in different locations on the network. CST elects only one root bridge for 

all VLANs, which could cause a problem.

 124.  B.  RSTP has three transition modes and converges faster than STP, which is 50 seconds. 

It is, however, backward compatible with STP 802.1D. RSTP by itself does not allow for 

multiple root bridges; however, the extension of Rapid PVST will allow for multiple root 

bridges. RSTP has an extremely fast convergence time, and STP has a convergence time of 

50 seconds. STP has five port states to which an interface could possibly transition; RTSP 

has only three port states.

 125.  B.  Each switch is responsible for sensing changes to the topology; it is not the sole 

responsibility of the root bridge. Whenever the topology changes, a Topology Change 

Notification (TCN) is sent out all root ports and an acknowledgment is sent back. This 

happens until the root bridge sends back a notification. The root bridge does not poll each 

switch participating in STP for changes, and the switches participating in STP do not poll 

the root bridge for changes.

 126.  B.  802.1s, which is called Multiple Spanning Tree (MST), is a standard based upon 

PVST+. It is an open standard created by the IEEE that will allow Per-VLAN Spanning 

in multi-vendor switched networks. 802.1X is the IEEE standard for port security that 

requires end devices to authenticate before traffic will be allowed to pass. The original 

IEEE specification of STP (802.1D) was revamped in 2004 with RSTP 802.1w. This 

revamping of STP was to fix problems with the original specification.

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Chapter 2: Network Access (Domain 2)