Ch.3 - STP Flashcards
What is STP?
A layer 2 loop prevention mechanism.
What is the first step when switches initialize STP?
Elect a root bridge
What determines the switch that becomes the root bridge?
1) Switch with lowest system priority
2) Then the lowest MAC address
What is the second step when switches initialize STP?
Each switch determines their root port (except the root bridge)
How does a switch determine its root port?
1) The interface with the lowest path cost is preferred
2) Then its the lowest port priority of the
3) Then the lowest port number
What is the third step when switches initialize STP?
Set redundant designated ports to block
What are non-root-port (non-RP) ports considered as?
Designated ports (DPs)
How does a switch determine which designated port(s) to block?
1) Interfaces with the lowest path cost to the root bridge (RB)
2) Lowest switch system priority
3) Lowest MAC address
(The lower one will stay up, and the other is blocked)
Where are root bridges usually placed?
On a core switch
What setting do you change to force a switch to become a root bridge?
Set system priority to the lowest one (0 to 61,440 in increments of 4,096)
What does the port cost decide for STP?
Which port is the root port or the alternate port between two switches
What does the port priority decide for STP?
Decides which link on two interconnected switches (where there are multiple redundant links to each switch) is the alternate port
What is Root Guard?
Prevents downstream switches from becoming the root bridge
What is STP Portfast?
This setting disables TCN generation for a port. Usually only set on access ports.
This means that an access port can bypass early .1D states and start forwarding immediately
What happens when a BPDU is received on a port that has Portfast enabled?
Portfast is disabled for that port
