Chap 3 - Advanced STP Tuning

Question 1

When setting the System Priority on a switch what number does the Priority need to be a multiple of?

Answer 1

4096

Question 2

What are 3 steps do you take when manually setting the primary and secondary Root Bridge placement?

Answer 2

• Making the System Priority on the Root lower than all other switches.
• Making the System Priority slightly higher than the Root Bridge on the Secondary
• Set all other switches with higher System Priorities.

Question 3

When using the command ‘spanning-tree vlan (vlan_id) root primary’ what value is used by default?

Answer 3

24,576

Question 4

When using the command ‘spanning-tree vlan (vlan_id) root secondary’ what value is used by default?

Answer 4

28,672

Question 5

When using the command ‘spanning-tree vlan (vlan_id) root primary’ what does the optional parameter ‘diameter’ mean and what does it do?

Answer 5

• ‘diameter’ is the number of Layer 2 hops between a switch and the root
• changes timers on the root to make convergence faster

Question 6

What is the best way to configure switches to be primary and secondary root bridges?

Answer 6

Set the System Priority to 0 for the Root and 4096 for the Secondary

Question 7

When a switch receives a BPDU advertising the root path cost does it already include the cost of the link the BPDU was received on?

Answer 7

No, the receiving switch adds the cost of the interface where the BPDU was received to determine its own root patch cost.

Question 8

What interface command takes precedence - ‘spanning-tree vlan (vlan_id) cost (cost)’ or ‘spanning-tree cost (cost)’

Answer 8

The command that specifies the VLAN will take precedence over the command that doesn’t.

Question 9

What 2 methods can be used to change Root Port and Blocked switchport locations?

Answer 9

• change the interface COST
• change the interface Priority

Question 10

What are 2 symptoms of a Layer 2 forwarding loop?

Answer 10

• high CPU consumption
• low free memory

Question 11

What are 4 causes of Layer 2 forwarding loops?

Answer 11

• STP disabled
• misconfigured Load Balancer
• misconfigured Virtual Switch that bridges 2 physical ports
• End users using a dumb switch or hub

Question 12

When syslog detects a flapping MAC address with an associated VLAN what should you do?

Answer 12

Investigate all switches hosting that VLAN and make sure a forwarding loop does not exist

Question 13

Where should Root Guard be placed?

Answer 13

On DP that face other switches that should never become the Root Bridge.

Question 14

What is the Port Identifier on an interface?

Answer 14

It is the Port Priority followed by the interface number after the slash. Fa0/7 would be 128.7

Question 15

When is the only time you should enable Portfast on a trunk?

Answer 15

When the trunk connects to a server with only 1 NIC that runs a hypervisor with VMs in different VLANs.

Question 16

What is the Error Recovery Service and how can you tell what the reason was for the port being disabled?

Answer 16

• Automatically re-enables a port in err-disabled state after a timeout of 300 seconds
• when enabled ‘show errdisable recovery’ gives reason why err-disabled

Question 17

What does BPDU Filtering do when configured on an interface?

Answer 17

When enabled on an interface it will not send or receive BPDUs.

Question 18

What does BPDU Filtering do when configured globally?

Answer 18

• When enabled all Portfast ports will send a series of 10-12 BPDUs
• If any BPDUs are received the port will lose its Portfast status, BPDU filtering will be removed from that port, and the port will just be a normal port

Question 19

What does Loopguard do?

Answer 19

Prevents a blocked port from transistioning to a forwarding state which would cause a forwarding loop.

Question 20

What does UDLD do and what are the 2 steps it uses to performs this?

Answer 20

• Performs bidirectional monitoring of fiber links.
• Switch A sends UDLD packets to switch B
• Switch B repeats those packets back to A adding its own interface info

Question 21

What is the difference between UDLD Normal mode vs Aggressive mode?

Answer 21

• Normal mode - if a frame is not acknowledged the link is considered Undetermined but port stays active
• Aggressive mode - if a frame is not acknowledged UDLD will try once more, if no response then port is placed in an error state.

Question 22

For UDLD Recovery what is the default interval time?

Answer 22

5 minutes

Question 23

What does Root Guard do, where is it placed and how does it react?

Answer 23

• Prevents a configured port from becoming a Root Port.
• Prevents a down-stream switch from becoming a Root Bridge
• Placed on a Designated Port
• Err-disables a Designated Port if a Superior BPDU is received

Question 24

What is a use-case for Loopguard?

Answer 24

• Fiber interface facing another switch in STP blocked state
• The inbound fiber connection fails
• Switch is no longer receiving BPDUs but continues to send
• Switch transitions the port from blocking to forwarding causing a loop

Question 25

Display a switches Root Path Cost.

Answer 25

show spanning-tree vlan (vlan_id)

Question 26

Check to see if portfast and bpduguard are enabled on an interface.

Answer 26

show spanning-tree interface (interface_id) detail

Question 27

Display ports in err-disable state due to bpduguard when Error Recovery Service is enabled.

Answer 27

show errdisable recovery

Question 28

Display port in an Inconsistent state.

Answer 28

show spanning-tree inconsistantports

Question 29

Verify the status of UDLD neighborship.

Answer 29

show udld neighbors

Question 30

Display more detailed UDLD information.

Answer 30

show udld (interface_id)

Question 31

In RSTP what is the RSTP equivalent of 802.1D port types?

Answer 31

• 802.1D port TYPES = RSTP port ROLES
• 802.1D port types - RP, DP, Blocking
• RSTP port roles - RP, DP, Alternate port, Backup port