JEX - Chapter 4: Spanning Tree

Question 1

The process a switch goes through to create a spanning tree. (High Level)

Answer 1

1. Switches exchange Bridge Protocol Units (BPDUs)
2. Root bridge is elected
3. Port role and state are determined
4. Tree is fully converged

Question 2

Bridge ID

Answer 2

Unique identifier for each switch

Question 3

Root bridge

Answer 3

Switch with the lowest bridge ID

Question 4

Root port

Answer 4

The port on each bridge closest to the root bridge

Question 5

Root path cost

Answer 5

A bridge’s calculated cost to get from itself to the root bridge

Equal to the received root path cost from configuration BPDUs plus the port cost of the root port on the bridge

Question 6

Port cost

Answer 6

Every interface on a bridge has an assigned port cost value

1. Used in the calculation of the root path cost for the local bridge
2. Configurable value (1-200,000,000)
3. The default value is 20,000 for 1 Gigabit Ethernet

Question 7

Designated bridge

Answer 7

A switch representing the LAN segement

Question 8

Port ID

Answer 8

A unique identifier for each port on each switch

Question 9

Designated port

Answer 9

The designated bridge’s forwarding port on a LAN segment.

The port used by a designated bridge to send traffic from the direction of the root to the LAN or from the LAN toward the root.

Question 10

Bridge protocol data unit

Answer 10

Packets used to exchange information between switches

Types of BPDUs:

1. Configuration BPDU
2. Topology change notification BPDU

Question 11

In STP, what are the four states a bridge’s port can be in?

Answer 11

1. Blocking
2. Listening
3. Learning
4. Forwarding

Question 12

Blocking state (STP)

Answer 12

1. The port drops all data packets and listens to BPDUs

2. The port is not used in active topology

Question 13

Listening state (STP)

Answer 13

1. The port drops all data packets and listens to BPDUs

2. The port is transitioning and will be used in active topology

Question 14

Learning state (STP)

Answer 14

1. The port drops all data packets and listens to BPDUs

2. The port is transitioning and the switch is learning MAC addresses

Question 15

Forwarding state (STP)

Answer 15

1. The port receives and forwards data packets and sends and receives BPDUs.
2. The port has transitioned and the switch continues to learn MAC addresses.

Question 16

Ethernet Frame Format

Answer 16

1. Destination Address: The multicast MAC address that is reserved for STP (01:80:c2:00:00:00)
2. Source Address: MAC of the outgoing port of the originating switch.
3. Length
4. LLC Header: DSAP and SAAP = 0x42
5. BPDU
6. FCS

Question 17

BPDU Format

Answer 17

1. Protocol ID: 2 octets
2. Protocol Version: 1 octet
3. BPDU Type: 1 octet
4. Flags: 1 octet
5. Root ID: 8 octets
6. Root Path Cost: 4 octets
7. Bridge ID: 8 octets
8. Port ID: 2 octets
9. Message Age: 2 octets
10. Max Age: 2 octets
11. Hello Time: 2 octets
12. Forward Delay: 2 octets

Question 18

BPDU Field: Protocol ID

Answer 18

This value is always 0

Question 19

BPDU Field: Protocol Version

Answer 19

This value is always 0

Question 20

BPDU Field: BPDU Type

Answer 20

This field determines which of the two BPDU formats this frame contains:

1. Configuration BPDU (0x00)
2. TCN BPDU (0x80)

Question 21

BPDU Field: Flags

Answer 21

This field is used to handle changes in the active topology

Question 22

BPDU Field: Root ID

Answer 22

This field contains the bridge ID (BID) of the Root Bridge.

After convergence, all configuration BPDUs in the bridge network should contain the same value for this field (for a single VLAN).

Some network sniffers break out the two BID subfields:

1. Bridge Priority
2. Bridge MAC Address