Vol 1 Ch 9 - Spanning Tree Protocol

Question 1

How does Spanning Tree Protocol (STP) work?

Answer 1

STP works by blocking ports that are not attached to the ROOT switch and have the highest cost to reach the ROOT switch. The ROOT switch is auto-designated after the switches exchange IDs. The switch with the lowest ID is the ROOT switch. The ROOT switch cannot have blocking ports. After that the switches determine the cost for each port to reach the ROOT switch. The port with the highest cost is blocked.

The switches exchange information as soon as a link goes down so that a new port can be chosen for blocking.

Question 2

What is the vocabulary used in Spanning Tree Protocol?

Answer 2

ROOT switch
FORWARDING STATE
LEAST ADMINISTRATIVE COST between switch and the root switch
ROOT COST
ROOT PORT
DESIGNATED SWITCH
DESIGNATED PORT
SPANNING TREE ALGORITHM (STA)
ELECTION OF ROOT SWITCH
BRIDGE ID (BID)
8 BYTES = 2 BYTE PRIORITY AND 6 BYTE MAC ADDRESS
BRIDGE PROTCOL DATA UNITS (BPDU)
CONFIGURATION BPDUs
HELLO BPDU

Question 3

What is inside a BPDU ( a Bridge Protocol Data Unit)?

Answer 3

Root Bridge ID
Sender Bridge ID
Sender’s root cost
Timer values on the root switch

Question 4

Which switch becomes the root switch?

Answer 4

The switch with the lowest value Bridge ID (BID) becomes the root switch.

Question 5

Which port on each switch becomes the root port?

Answer 5

The port with the smallest path cost to the root bridge.

Question 6

What are the steps in the STP process?

Answer 6

1. Elect the root switch.
2. Choose each switch’s root port
3. Choose each switch’s designated port
4. Block the port with the highest path cost to the root bridge.

Question 7

What is a root port?

Answer 7

The root port on an STP device has the smallest path cost to the root bridge and is responsible for forwarding data to the root bridge. Among all STP-capable ports on a device, the port with the smallest root path cost is a root port.

Question 8

How does STP work with EtherChannel?

Answer 8

STP only blocks the link if all the links in EtherChannel fail. This avoids convergence any time a link within the bundle fails, making the network more available.

Question 9

On which layers can EtherChannel be configured?

Answer 9

EtherChannel can be configured with either layer 2 logic or layer 3 logic.

Question 10

How is PortFast used?

Answer 10

PortFast allows a switch to immediately transition from blocking to forwarding. To avoid loops, it only makes sense to apply PortFast to ports that are connected to non-STP speaking devices.

PortFast is most appropriate for connections to end user devices to speed up convergence when a device boots up.

Question 11

How is BPDU Guard used?

Answer 11

The BPDU Guard is used on access ports and blocks any BPDU messages received via these ports.