Exam 300-115 topic 5 question 25 discussion

Actual exam question from Cisco's 300-115

Question #: 25
Topic #: 5

Examine the following partial output of the show run command. The command was executed from Switch A, which is connected to Switch B through both the
Fa0/1 interface and the Fa0/2 interface. Switch A is the root bridge.

Only one of the links is being used. Your intention was to load share the traffic using both links.
What commands do you need to execute to accomplish this? (Choose two.)

A. switchA(config)# interface fa0/2switchA(config-if)#spanning-tree vlan 1-3 port-priority 16
B. switchA(config)# interface fa0/1switchA(config-if)#spanning-tree vlan 4-6 port-priority 16
C. switchA(config)# interface fa0/1switchA(config-if)#spanning-tree vlan 1-3 port-priority 128
D. switchA(config)# interface fa0/2switchA(config-if)#spanning-tree vlan 4-6 port-priority 128
E. switchA(config)# interface fa0/1switchA(config-if)#spanning-tree port-priority 20
F. switchA(config)# interface fa0/2switchA(config-if)#spanning-tree port-priority 20

Show Suggested Answer

Suggested Answer: AB 🗳️
The correct commands to load share the traffic using both links are: switchA(config)# interface fa0/2 switchA(config-if)# spanning-tree vlan 1-3 port-priority 16 switchA(config)# interface fa0/1 switchA(config-if)# spanning-tree vlan 4-6 port-priority 16
The configuration that was reflected in the exhibit in the show run output indicated that VLANs 1 through 6 were configured under both interfaces. However, the normal operation of STP will block one of the interfaces to prevent a loop. By default, all VLANs are allowed on both trunk links. Load sharing allows you to send some of the VLANs over one of the links and the rest on the other. In this case, the correct option will send VLANs 1-3 over Fa0/1 and VLANs 4-6 over Fa0/2.
By altering the port priority of the VLAN 1-3 on one interface and VLANs 4-6 on the other on the root bridge (Switch A) with the port-priority keyword, the behavior of STP is altered on the other switch. The port priority value must be set in increments of 16. Now Switch A will send VLANs 1-3 over one interface without blocking and 4-6 over the other interface without blocking. The additional benefit to this configuration is that if either link goes down, all VLANs can be sent over the remaining link and until the redundant link comes back up.
The commands below will have no effect because the default port priority is already 128, so the situation will remain the same: switchA(config)# interface fa0/1 switchA(config-if)# spanning-tree vlan 1-3 port-priority 128 switchA(config)# interface fa0/2 switchA(config-if)# spanning-tree vlan 4-6 port-priority 128
The commands below will have no effect because they omit the vlan 1-3 and vlan 4-6 parameters, and therefore change the port priority for all VLANs. Since the port priority is changed equally on both interfaces, there will be no load sharing as a result. More over the priority value is not entered in an increment of 16, which will generate an error message indicating that it must be set in increments of 16. switchA(config)# interface fa0/1 switchA(config-if)# spanning-tree port-priority 20 switchA(config)# interface fa0/2 switchA(config-if)# spanning-tree port-priority 20
Objective:

Layer 2 Technologies -
Sub-Objective:
Configure and verify spanning tree
References:
Cisco > Home > Support > Technology Support > LAN Switching > Spanning Tree Protocol > Design > Design Technotes > VLAN Load Balancing Between
Trunks Using the Spanning-Tree Protocol Port Priority
Cisco > Cisco IOS LAN Switching Command Reference > show vlan through ssl-proxy module allowed-vlan > spanning-tree port-priority

by CiscoNinja at Jan. 4, 2020, 5:22 a.m.

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CiscoNinja

5 years, 5 months ago

something is wrong with the question, how would you load balance/share using 1 root bridge? its impossible

upvoted 1 times

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