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| Pregunta | Respuesta |
| Which three statements about RSTP are true? (Choose three) A. RSTP significantly reduces topology reconvening time after a link failure. B. RSTP expands the STP port roles by adding the alternate and backup roles. C. RSTP port states are blocking, discarding, learning, or forwarding. D. RSTP provides a faster transition to the forwarding state on point-to-point links than STP does. E. RSTP also uses the STP proposal-agreement sequence. F. RSTP uses the same timer-based process as STP on point-to-point links. | Answer: A B D |
| Why has this switch not been elected the root bridge for VLAN1? A. It has more than one internee that is connected to the root network segment. B. It is running RSTP while the elected root bridge is running 802.1d spanning tree. C. It has a higher MAC address than the elected root bridge. D. It has a higher bridge ID than the elected root bridge. | Answer: D |
| Which command enables RSTP on a switch? A. spanning-tree mode rapid-pvst B. spanning-tree uplinkfast C. spanning-tree backbonefast D. spanning-tree mode mst | Answer: A |
| Refer to the exhibit. Which statement is true? A. The Fa0/11 role confirms that SwitchA is the root bridge for VLAN 20. B. VLAN 20 is running the Per VLAN Spanning Tree Protocol. C. The MAC address of the root bridge is 0017.596d.1580. D. SwitchA is not the root bridge, because not all of the interface roles are designated. | Answer: D Explanation Only non-root bridge can have root port. Fa0/11 is the root port so we can confirm this switch is not the root bridge -> A is not correct. From the output we learn this switch is running Rapid STP, not PVST -> B is not correct. 0017.596d.1580 is the MAC address of this switch, not of the root bridge. The MAC address of the root bridge is 0017.596d.2a00 -> C is not correct. All of the interface roles of the root bridge are designated. SwitchA has one Root port and 1 Alternative port so it is not the root bridge -> D is correct. |
| Refer to the exhibit. The output that is shown is generated at a switch. Which three of these statements are true? (Choose three) A. All ports will be in a state of discarding, learning or forwarding. B. Thirty VLANs have been configured on this switch. C. The bridge priority is lower than the default value for spanning tree. D. All interfaces that are shown are on shared media. E. All designated ports are in a forwarding state. F. The switch must be the root bridge for all VLANs on this switch. | Answer: A C E Explanation From the output, we see that all ports are in Designated role (forwarding state) -> A and E are correct. The command “show spanning-tree vlan 30″ only shows us information about VLAN 30. We don’t know how many VLAN exists in this switch -> B is not correct. The bridge priority of this switch is 24606 which is lower than the default value bridge priority 32768 -> C is correct. All three interfaces on this switch have the connection type “p2p”, which means Point-to-point environment – not a shared media -> D is not correct. The only thing we can specify is this switch is the root bridge for VLAN 3o but we can not guarantee it is also the root bridge for other VLANs -> F is not correct. |
| Which two states are the port states when RSTP has converged? (choose two) A. blocking B. learning C. disabled D. forwarding E. listening | Answer: A D Explanation RSTP only has 3 port states that are discarding, learning and forwarding. When RSTP has converged there are only 2 port states left: discarding and forwarding but the answers don’t mention about discarding state so blocking state (answer A) may be considered the best alternative answer. |
| Which two of these statements regarding RSTP are correct? (Choose two) A. RSTP cannot operate with PVST+. B. RSTP defines new port roles. C. RSTP defines no new port states. D. RSTP is a proprietary implementation of IEEE 802.1D STP. E. RSTP is compatible with the original IEEE 802.1D STP. | Answer: B E |
| Refer to the exhibit. Each of these four switches has been configured with a hostname, as well as being configured to run RSTP. No other configuration changes have been made. Which three of these show the correct RSTP port roles for the indicated switches and interfaces? (Choose three) A. SwitchA, Fa0/2, designated B. SwitchA, Fa0/1, root C. SwitchB, Gi0/2, root D. SwitchB, Gi0/1, designated E. SwitchC, Fa0/2, root F. SwitchD, Gi0/2, root | Answer: A B F Explanation The question says “no other configuration changes have been made” so we can understand these switches have the same bridge priority. Switch C has lowest MAC address so it will become root bridge and 2 of its ports (Fa0/1 & Fa0/2) will be designated ports -> E is incorrect. Because SwitchC is the root bridge so the 2 ports nearest SwitchC on SwitchA (Fa0/1) and SwitchD (Gi0/2) will be root ports -> B and F are correct. Now we come to the most difficult part of this question: SwitchB must have a root port so which port will it choose? To answer this question we need to know about STP cost and port cost. In general, “cost” is calculated based on bandwidth of the link. The higher the bandwidth on a link, the lower the value of its cost. Below are the cost values you should memorize: Link speed Cost 10Mbps 100 100Mbps 19 1 Gbps 4 |
| Refer to the exhibit. At the end of an RSTP election process, which access layer switch port will assume the discarding role? A. Switch3, port fa0/1 B. Switch3, port fa0/12 C. Switch4, port fa0/11 D. Switch4, port fa0/2 E. Switch3, port Gi0/1 | Answer: C Explanation In this question, we only care about the Access Layer switches (Switch3 & 4). Switch 3 has a lower bridge ID than Switch 4 (because the MAC of Switch3 is smaller than that of Switch4) so both ports of Switch3 will be in forwarding state. The alternative port will surely belong to Switch4. Switch4 will need to block one of its ports to avoid a bridging loop between the two switches. But how does Switch4 select its blocked port? Well, the answer is based on the BPDUs it receives from Switch3. A BPDU is superior than another if it has: 1. A lower Root Bridge ID 2. A lower path cost to the Root 3. A lower Sending Bridge ID 4. A lower Sending Port ID These four parameters are examined in order. In this specific case, all the BPDUs sent by Sswitch3 have the same Root Bridge ID, the same path cost to the Root and the same Sending Bridge ID. The only parameter left to select the best one is the Sending Port ID (Port ID = port priority + port index). In this case the port priorities are equal because they use the default value, so Switch4 wi |
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