STP

Question 1

Acronym BPDU

Answer 1

Bridge Protocol Data Units

Question 2

What is the process for electing a root bridge?

Answer 2

1. Lowest bridge ID wins (bridge priority compared first). Add Bridge priority + extended vlan. From there if there is still a tie, use the MAC address field (which is a part of the Bridge ID field).

Question 3

What is the default bridge priority on switches?

Answer 3

32768

Question 4

Breakdown of the Bridge ID field

Answer 4

Bridge Priority (16 Bits) which encompasses bridge priority field (4 bits), and the extended vlan field (12 bits) + MAC address

Question 5

Which switch (imagine a topology) will send BPDU’s?

Answer 5

Only the root bridge will send BPDUs.

Question 6

How do we determine the root port on the remaining switches?

Answer 6

1. The interface with the lowest root cost will win (Speed of an interface). 2. If the cost is the same, the lowest neighbor bridge ID will take home the pie! 3. If the neighbor bridge ID is the same, we will select the lowest neighbor port ID (interface) to become the root port.

Question 7

How do we determine which ports will be non-designated or blocking?

Answer 7

1. The switch with the lowest root cost will make its port designated. 2. If the root cost is the same, the switch with the lowest bridge ID will make its port designated or forwarding. 3. The other switch will make its port non-designated.

Question 8

STP Port states: Blocking

Answer 8

Will not learn MAC address, will receive BPDU’s, but will not forward them. Do not receive regular network traffic.

Question 9

Listening

Answer 9

Will not learn MAC. Will not receive network traffic. Will receive BPDU’s and will forward BPDU’s. 15 seconds long - root and designated port.

Question 10

Learning

Answer 10

Will learn MAC, will send and receive BPDUs, will not send or receive network traffic. 15 seconds

Question 11

Forwarding

Answer 11

Will send and receive network traffic, will send BPDUs, learns MAC addresses and operates as normal.

Question 12

What is BPDU guard

Answer 12

Security feature found in multiple networking devices. It prevents attacks on network by blocking Bridge Protocol Data Units that are sent from unauthorized devices.

Question 13

What is Portfast?

Answer 13

Cause a switchport to transition almost immediately to a forwarding state when we plug something into that port - like a Laptop.

Question 14

What happens if we remove a laptop, and plug a switch into a port that is enabled with portfast?

Answer 14

Could cause a Layer 2 loop - because you already have port setup to bypass the listening and learning stages. Remember, BPDU’s don’t arrive on a port setup with PortFast.

Question 15

How does BPDU Guard work?

Answer 15

BPDU Guard works hand-in-hand with PortFast. It disables the interface (port) which has been setup for PortFast when it begins receiving BPDU’s on the interface. (Remember, BPDU’s are not received or sent on an interface that is running portfast).