Chap 3 - Advanced STP Tuning Flashcards
When setting the System Priority on a switch what number does the Priority need to be a multiple of?
4096
What are 3 steps do you take when manually setting the primary and secondary Root Bridge placement?
- 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.
When using the command ‘spanning-tree vlan (vlan_id) root primary’ what value is used by default?
24,576
When using the command ‘spanning-tree vlan (vlan_id) root secondary’ what value is used by default?
28,672
When using the command ‘spanning-tree vlan (vlan_id) root primary’ what does the optional parameter ‘diameter’ mean and what does it do?
- ‘diameter’ is the number of Layer 2 hops between a switch and the root
- changes timers on the root to make convergence faster
What is the best way to configure switches to be primary and secondary root bridges?
Set the System Priority to 0 for the Root and 4096 for the Secondary
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?
No, the receiving switch adds the cost of the interface where the BPDU was received to determine its own root patch cost.
What interface command takes precedence - ‘spanning-tree vlan (vlan_id) cost (cost)’ or ‘spanning-tree cost (cost)’
The command that specifies the VLAN will take precedence over the command that doesn’t.
What 2 methods can be used to change Root Port and Blocked switchport locations?
- change the interface COST
- change the interface Priority
What are 2 symptoms of a Layer 2 forwarding loop?
- high CPU consumption
- low free memory
What are 4 causes of Layer 2 forwarding loops?
- STP disabled
- misconfigured Load Balancer
- misconfigured Virtual Switch that bridges 2 physical ports
- End users using a dumb switch or hub
When syslog detects a flapping MAC address with an associated VLAN what should you do?
Investigate all switches hosting that VLAN and make sure a forwarding loop does not exist
Where should Root Guard be placed?
On DP that face other switches that should never become the Root Bridge.
What is the Port Identifier on an interface?
It is the Port Priority followed by the interface number after the slash. Fa0/7 would be 128.7
When is the only time you should enable Portfast on a trunk?
When the trunk connects to a server with only 1 NIC that runs a hypervisor with VMs in different VLANs.
What is the Error Recovery Service and how can you tell what the reason was for the port being disabled?
- 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
What does BPDU Filtering do when configured on an interface?
When enabled on an interface it will not send or receive BPDUs.
What does BPDU Filtering do when configured globally?
- 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
What does Loopguard do?
Prevents a blocked port from transistioning to a forwarding state which would cause a forwarding loop.
What does UDLD do and what are the 2 steps it uses to performs this?
- 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
What is the difference between UDLD Normal mode vs Aggressive mode?
- 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.
For UDLD Recovery what is the default interval time?
5 minutes
What does Root Guard do, where is it placed and how does it react?
- 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
What is a use-case for Loopguard?
- 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
Display a switches Root Path Cost.
show spanning-tree vlan (vlan_id)
Check to see if portfast and bpduguard are enabled on an interface.
show spanning-tree interface (interface_id) detail
Display ports in err-disable state due to bpduguard when Error Recovery Service is enabled.
show errdisable recovery
Display port in an Inconsistent state.
show spanning-tree inconsistantports
Verify the status of UDLD neighborship.
show udld neighbors
Display more detailed UDLD information.
show udld (interface_id)
In RSTP what is the RSTP equivalent of 802.1D port types?
- 802.1D port TYPES = RSTP port ROLES
- 802.1D port types - RP, DP, Blocking
- RSTP port roles - RP, DP, Alternate port, Backup port
