Spanning Tree

Question 0

What are the two types of STP and which type do Cisco devices run by default?

Answer 0

IEEE Spanning Tree and DECK Spanning Tree

Cisco devices run IEEE

Question 1

What is the purpose of spanning tree protocol?

Answer 1

The basic function of STP is to prevent bridge loops (situation in which frames are forwarded in a loop indefinitely). This makes hardier, more redundant network designs possible (w/o STP, ^redundancy can mean ^potential for loops).

Question 2

How is a device’s Bridge ID determined?

Answer 2

By adding its bridge priority and its MAC address.

Question 3

If not chosen manually, which switch will be designated as the root switch?

Answer 3

The one with the lowest bridge ID.

Question 4

Why is it possible for frames to get stuck in an indefinite loop but not for packets the same?

Answer 4

Because packets have a time-to-live (TTL), after the expiration of which they drop off the network, but frames don’t have a TTL or any equivalent field.

Question 5

In IEEE Spanning Tree, what is the default bridge priority #?

Answer 5

32768

Question 6

In VLAN-specific spanning tree (PVST or per-VLAN spanning tree), how is a VLAN’s bridge priority # determined?

Answer 6

Adding the VLAN# to the default bridge priority #:

VLAN 99
Bridge Priority = 32869 (32768+99)

Question 7

What does RSTP stand for?

What is the command to enable it?

Answer 7

Rapid Spanning Tree Protocol

spanning-tree mode rapid PVST

Question 8

What command can you use to show spanning tree information on a particular VLAN?

Answer 8

show spanning-tree VLAN(#) 
# = the VLAN number

Question 9

What command do you use to designate a switch as the primary root switch?

Answer 9

spanning-tree vlan(#) root primary

Question 10

What is the first thing the spanning tree algorithm does?

Answer 10

Elects a root bridge.

Question 11

What is the second thing the spanning tree algorithm does?

Answer 11

Elects a root port for each non-root switch.

Question 12

What is the third thing the spanning tree algorithm does?

Answer 12

Elects a designated port for each segment.

Question 13

What is the fourth thing the spanning tree algorithm does?

Answer 13

Puts each port into either a forwarding or blocking state.

Question 14

What determines which port will be elected as the root port?

Answer 14

The root path cost of each port; the one with the lowest root path cost will be elected.

Question 15

Which ports transition into a forwarding state and which into a blocking state?

Answer 15

Root and designated ports transition into a forwarding state, other ports into a blocking state.

Question 16

What determines the root path cost of a port?

Answer 16

The speed of the link.

Question 17

What is the purpose of PortFast?

Answer 17

To circumvent the 50-second latency of spanning tree (to shorten the duration of the ST process on ports that you know beforehand cannot create a loop on the network). It prevents edge ports from processing BPDU’s, and from engaging in Spanming Tree

Question 18

What has to occur in order for STP to conclude that a topology change has taken place?

Answer 18

The max_age timer has to have expired without the port having received a BPDU from the connected port. The default max_age timer setting is 20 seconds.