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QUESTION 41
Refer to the topology shown in the exhibit. Which ports will be STP designated ports if all the links are operating at the same bandwidth? (Choose three.)
A. Switch A – Fa0/0
B. Switch A – Fa0/1
C. Switch B – Fa0/0
D. Switch B – Fa0/1
E. Switch C – Fa0/0
F. Switch C – Fa0/1
Answer: BCD
Explanation:
This question is to check the spanning tree election problem.
1. First, select the root bridge, which can be accomplished by comparing the bridge ID, the smallest will be selected. Bridge-id= bridge priority + MAC address. The three switches in the figure all have the default priority, so we should compare the MAC address, it is easy to find that SwitchB is the root bridge.
2. Select the root port on the non-root bridge, which can be completed through comparing root path cost. The smallest will be selected as the root port.
3. Next, select the Designated Port. First, compare the path cost, if the costs happen to be the same, then compare the BID, still the smallest will be selected. Each link has a DP. Based on the exhibit above, we can find DP on each link. The DP on the link between SwitchA and SwitchC is SwitchA’Fa0/1, because it has the smallest MAC address.
QUESTION 42
Refer to the exhibit. How should the FastEthernet0/1 ports on the 2950 model switches that are shown in the exhibit be configured to allow connectivity between all devices?
A. The ports only need to be connected by a crossover cable.
B. SwitchX(config)# interface fastethernet 0/1
SwitchX(config-if)# switchport mode trunk
C. SwitchX(config)# interface fastethernet 0/1
SwitchX(config-if)# switchport mode access
SwitchX(config-if)# switchport access vlan 1
D. SwitchX(config)# interface fastethernet 0/1
SwitchX(config-if)# switchport mode trunk
SwitchX(config-if)# switchport trunk vlan 1
SwitchX(config-if)# switchport trunk vlan 10
SwitchX(config-if)# switchport trunk vlan 20
Answer: B
Explanation:
IN order for multiple VLANs to cross switches, the connection between the switches must be a trunk. The “switchport mode trunk” command is all that is needed, the individual VLANs should not be listed over that trunk interface.
QUESTION 43
Which three statements about RSTP are true? (Choose three.)
A. RSTP significantly reduces topology reconverging 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: ABD
Explanation:
One big disadvantage of STP is the low convergence which is very important in switched network. To overcome this problem, in 2001, the IEEE with document 802.1w introduced an evolution of the Spanning Tree Protocol: Rapid Spanning Tree Protocol (RSTP), which significantly reduces the convergence time after a topology change occurs in the network. While STP can take 30 to 50 seconds to transit from a blocking state to a forwarding state, RSTP is typically able to respond less than 10 seconds of a physical link failure.
RSTP works by adding an alternative port and a backup port compared to STP. These ports are allowed to immediately enter the forwarding state rather than passively wait for the network to converge.
RSTP bridge port roles:
* Root port – A forwarding port that is the closest to the root bridge in terms of path cost
* Designated port – A forwarding port for every LAN segment
* Alternate port – A best alternate path to the root bridge. This path is different than using the root port. The alternative port moves to the forwarding state if there is a failure on the designated port for the segment.
* Backup port – A backup/redundant path to a segment where another bridge port already connects. The backup port applies only when a single switch has two links to the same segment (collision domain). To have two links to the same collision domain, the switch must be attached to a hub.
* Disabled port – Not strictly part of STP, a network administrator can manually disable a port
QUESTION 44
Refer to the exhibit. A frame on VLAN 1 on switch S1 is sent to switch S2 where the frame is received on VLAN 2. What causes this behavior?
A. trunk mode mismatches
B. allowing only VLAN 2 on the destination
C. native VLAN mismatches
D. VLANs that do not correspond to a unique IP subnet
Answer: C
Explanation:
Untagged frames are encapsulated with the native VLAN. In this case, the native VLANs are different so although S1 will tag it as VLAN 1 it will be received by S2.
QUESTION 45
At which layer of the OSI model is RSTP used to prevent loops?
A. physical
B. data link
C. network
D. transport
Answer: B
Explanation:
RSTP and STP operate on switches and are based on the exchange of Bridge Protocol Data Units (BPDUs) between switches. One of the most important fields in BPDUs is the Bridge Priority in which the MAC address is used to elect the Root Bridge -> RSTP operates at Layer 2 ?Data Link layer -> .
QUESTION 46
What does a Layer 2 switch use to decide where to forward a received frame?
A. source MAC address
B. source IP address
C. source switch port
D. destination IP address
E. destination port address
F. destination MAC address
Answer: F
Explanation:
When a frame is received, the switch looks at the destination hardware address and finds the interface if it is in its MAC address table. If the address is unknown, the frame is broadcast on all interfaces except the one it was received on.
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