ActualTests.Cisco.640-802.Exam.Q.and.A.08.15.08-DDU (1130589), страница 23
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As with some routing metrics, the measure ofcloseness using STP is not necessarily reflected by hop count. Specifically, bridges trackwhat is referred to as Root Path Cost, which is the cumulative cost of all links to the RootBridge.QUESTION 190:Refer to the topology shown in the exhibit below:Actualtests.com - The Power of Knowing640-802Based on the information above, which ports will be STP designated ports if all thelinks are operating at the same bandwidth? (Choose three)A. Switch A - Fa0/1B. Switch B - Fa0/0C. Switch A - Fa0/0D.
Switch B - Fa0/1E. Switch C - Fa0/0F. Switch C - Fa0/1Answer: A, B, DExplanation:To remove the possibility of bridging loops, STP makes a final computation to identifyone Designated Port on each network segment. Suppose that two or more switches haveports connected to a single common network segment. If a frame appears on thatsegment, all the bridges attempt to forward it to its destination. Recall that this behaviorwas the basis of a bridging loop and should be avoided.Instead, only one of the links on a segment should forward traffic to and from thatsegment-the one that is selected as the Designated Port.
Switches choose a DesignatedPort based on the lowest cumulative Root Path Cost to the Root Bridge. For example, aswitch always has an idea of its own Root Path Cost, which it announces in its ownBPDUs. If a neighboring switch on a shared LAN segment sends a BPDU announcing alower Root Path Cost, the neighbor must have the Designated Port. If a switch learnsonly of higher Root Path Costs from other BPDUs received on a port, however, it thencorrectly assumes that its own receiving port is the Designated Port for the segment.Notice that the entire STP determination process has served only to identify bridges andports. All ports are still active, and bridging loops still might lurk in the network. STP hasa set of progressive states that each port must go through, regardless of the type oridentification. These states actively prevent loops from forming and are described in thenext section.Note:In each determination process discussed so far, two or more links might have identicalRoot Path Costs.
This results in a tie condition, unless other factors are considered. Alltie-breaking STP decisions are based on the following sequence of four conditions:Actualtests.com - The Power of Knowing640-8021. Lowest Root Bridge ID2. Lowest Root Path Cost to Root Bridge3. Lowest Sender Bridge ID4. Lowest Sender Port IDThe three switches have chosen their Designated Ports (DP) for the following reasons:1. Catalyst A- Because this switch is the Root Bridge, all its active ports are DesignatedPorts, by definition.
At the Root Bridge, the Root Path Cost of each port is 0.2. Catalyst B- Catalyst A port 1/1 is the DP for the Segment A-B because it has thelowest Root Path Cost (0). Catalyst B port 1/2 is the DP for segment B-C. The Root PathCost for each end of this segment is 19, determined from the incoming BPDU on port1/1. Because the Root Path Cost is equal on both ports of the segment, the DP must bechosen by the next criteria-the lowest Sender Bridge ID. When Catalyst B sends a BPDUto Catalyst C, it has the lowest MAC address in the Bridge ID. Catalyst C also sends aBPDU to Catalyst B, but its Sender Bridge ID is higher. Therefore, Catalyst B port 1/2 isselected as the segment's DP.3.
Catalyst C- Catalyst A port 1/2 is the DP for Segment A-C because it has the lowestRoot Path Cost (0). Catalyst B port 1/2 is the DP for Segment B-C. Therefore, Catalyst Cport 1/2 will be neither a Root Port nor a Designated Port. As discussed in the nextsection, any port that is not elected to either position enters the Blocking state. Whereblocking occurs, bridging loops are broken.Reference: CCNP BCMSN Exam Certification Guide, 3rd Edition, Cisco PressQUESTION 191:Exhibit:Actualtests.com - The Power of Knowing640-802The switches in the exhibit above are configured with the default parameters. Whatcan be determined from the exhibit?A. Fa0/2 on SW-1 and Fa0/2 on SW-2 are nondesignated ports.B.
SW-1 is the backup designated root bridge.C. SW-3 is the root bridge.D. Fa0/3 on SW-1 and Fa0/3 on SW-2 are attached to the lowest-cost paths to the rootbridge.E. None of the aboveAnswer: CExplanation:Remember that one root switch is identified per-VLAN. After the root switchidentification, the switches adhere to these rules:1. STP Rule 1-All ports of the root switch must be in forwarding mode.Next, each switch determines the best path to get to the root. The switches determine thispath by a comparison of the information in all the BPDUs that the switches receive on allports. The switch uses the port with the least amount of information in the BPDU in orderto get to the root switch; the port with the least amount of information in the BPDU is theroot port.
After a switch determines the root port, the switch proceeds to rule 2.1. STP Rule 2-The root port must be set to forwarding mode.In addition, the switches on each LAN segment communicate with each other todetermine which switch is best to use in order to move data from that segment to the rootbridge. This switch is called the designated switch.1.
STP Rule 3-In a single LAN segment, the port of the designated switch that connects to that LANsegment must be placed in forwarding mode.2. STP Rule 4-All the other ports in all the switches (VLAN-specific) must be placed inblocking mode. The rule only applies to ports that connect to other bridges or switches.STP does not affect ports that connect to workstations or PCs. These ports remainforwarded.Our answer can be found in Rule 1 above. Since none of the ports in SW-3 are blocking,we can determine that this switch must be the root.Reference:Actualtests.com - The Power of Knowing640-802http://www.cisco.com/en/US/tech/ CK3 89/ CK621/technologies_configuration_example09186a008009467c.shtmlQUESTION 192:In the Certkiller switched LAN environment, what does the IEEE 802.1Q standarddescribe?A.
VLAN pruningB. A method of VLAN trunkingC. An approach to wireless LAN communicationD. The operation of VTPE. The process for root bridge selectionF. None of the aboveAnswer: BExplanation:A broadcast domain must sometimes exist on more than one switch in the network. Toaccomplish this, one switch must send frames to another switch and indicate whichVLAN a particular frame belongs to.
On Cisco switches, a trunk link is created toaccomplish this VLAN identification. ISL and IEEE 802.1Q are different methods ofputting a VLAN identifier in a Layer 2 frame.The IEEE 802.1Q protocol interconnects VLANs between multiple switches, routers, andservers. With 802.1Q, a network administrator can define a VLAN topology to spanmultiple physical devices.Cisco switches support IEEE 802.1Q for FastEthernet and Gigabit Ethernet interfaces.An 802.1Q trunk link provides VLAN identification by adding a 4-byte tag to anEthernet Frame as it leaves a trunk port.QUESTION 193:You are an administrator of the Certkiller switched network and your goal is toreduce some of the administrative overhead on your network.
You plan onachieving this by configuring a new VLAN for each department in the Certkillernetwork. However, you need to share the VLAN information across numerousswitches. Which of the following would allow you accomplish this?A. STPB. GVRPC. SNMPD. VTPE.
DHCPF. None of the aboveActualtests.com - The Power of Knowing640-802Answer: DExplanation:The VTP (VLAN Trunking Protocol) is a Cisco Layer 2 messaging protocol thatmanages the addition, deletion, and renaming of VLANs on a network-wide basis. Itallows for VLAN information to span multiple switches within the switch domain.Incorrect Answers:A. STP is a switching protocol but it is used for preventing network loops.B.
GVARP is an actual protocol used in switch administration, but it beyond the scope ofwhat a CCNA is expected to know.C. SNMP is a protocol used for managing and monitoring networksE. DHCP is the Dynamic Host Configuration Protocol, which allows for PC's to obtaintheir IP address dynamically upon booting up, along with their DNS and default gatewayinformation.QUESTION 194:Which of the following a true statements regarding the use of VLANs to segment anetwork? (Select three)A. They increase the size of collision domainsB. They allow logical grouping of users by function.C. They can enhance network security.D. They increase the size of the broadcast domain while decreasing the number ofcollision domains.E.
They increase the number of broadcast domains while decreasing the size of thebroadcast domains.F. They simplify switch administration.Answer: B, C, EExplanation:VLANs are used to segment a LAN into multiple, smaller LANs. This can be used toenhance security as local traffic from one VLAN will not be passed to users in otherVLANS.Incorrect Answers:A. VLANs are used to decrease the size of a collision domain, not increase it.D. The opposite is true.F. The default operation of a switch is to allow all traffic and to enable all ports in VLAN1.
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