4 - LAN Troubleshooting

Question 1

What show cmds help in determining the root switch?

Answer 1

• show spanning-tree vlan 10
• show spanning-tree vlan 10 root

Question 2

What rules out a switch as being a possible root switch?

Answer 2

The presence of an RP. Root switches do not have RPs.

Question 3

What is one way to find the root switch?

Answer 3

Follow the RPs. They will lead you to the root.

Question 4

What show cmd tells you the priority and BID of the root regardless of where the cmd is run?

Answer 4

show spanning-tree 10 root

Question 5

What information does an incoming Hello BPDU that allows a switch to determine the root port?

Answer 5

• root cost of sending switch
  
  • In the event of a tie the BPDU also lists:
    
    • sending switches BID
    • sending interface ID (i.e. fa0/1, fa0/2)

Question 6

How does a switch determine its root port?

Answer 6

adds the root cost of incoming Hello BPDUs to the cost of the interface that BPDU came in on.

Question 7

When determining RP, what is wrong with noting each interface cost on all link on a diagram to figure out everyone’s RP?

Answer 7

• This doesn’t take into account:
  
  • whether the interface is running at its maximum speed or not
  • if an interface cost has been manually configured.

Question 8

What if a there is a tie in costs to the root? How is the tie broken?

Answer 8

• choose the neighbor with the lowest BID
• If there is a tie (two ports going to same switch):
  
  • choose the neighbor with the lowest port priority
  • then choose the neighbor with the lowest internal port number. fa0/1 fa0/2

Question 9

What is a switches internal port number?

Answer 9

Cisco switches assign an internal integer to identify each interface on the switch, like fa0/1, fa0/2,

Question 10

What is a switchports default port priority?

Answer 10

128. The lower the priority the better.

Question 11

What is the command to change a switchports port priority?

Answer 11

spanning-tree vlan 10 port-priority 110

Question 12

A switch is trying to determine its RP. The costs along two paths are identical. The switch then compares BIDs. When is the only time the second two tie-breaking methods would be tried?

Answer 12

When the switch has two or more connections to the same switch.

Question 13

What are valid commands to bring up a PAgP Etherchannel?

Answer 13

channel-group 1 mode auto, channel-group 1 mode desireable

Question 14

What are valid commands to bring up a LACP Etherchannel?

Answer 14

channel-group 1 mode active, channel-group 1 mode passive

Question 15

What show command helps diagnose faulty Etherchannel configuration?

Answer 15

show etherchannel summary

Question 16

What 7 items are checked before a link is allowed to participate in an Etherchannel?

Answer 16

• Speed
• Duplex
• Same Operational State (access or trunking)
• If Access then same VLAN
• If a trunk port
  
  • the allowed VLAN list (per the switchport trunk allowed command)
  • same native VLAN
  • STP interface settings

Question 17

How are the 7 Etherchannel checks performed on the neighboring switch of an Etherchannel?

Answer 17

• Either LACP or PAgP
• If Etherchannel is manually configured then CDP

Question 18

In an Etherchannel, how does the data plane of the switch view the participating interfaces?

Answer 18

Etherchannel tells the data plane not to learn MAC addresses on the physical interfaces but rather on the Etherchannel.

Question 19

What is the default load-balancing method of an Etherchannel?

Answer 19

Based on the low-order (far right) bits of the source MAC address.