Chapter 2: STP

Question 1

BPDU

Answer 1

Bridge Protocol Data Unit, This network packet is used for network switches to identify a hierarchy and notify of changes in the topology. A BPDU uses the destination MAC address 01:80:c2:00:00:00, there are two types of BPDUs.

Question 2

Config BPDU

Answer 2

Configuration Bridge Protocol Data Unit, used to identify the root bridge, root ports, designated ports, and blocking ports. Consists of the following: STP type, root path cost, root bridge identifier, local bridge identifier, max age, hello time and forward delay

Question 3

hello time

Answer 3

Time that a BPDU is advertised out a port. The default value is 2 seconds, but can be configured to 1-10 seconds.

Question 4

DP

Answer 4

Designated Port, a network port that recieves and forwards BPDU frames to other switches, provide connectivity to downstream devices and switches. Only one DP per link

Question 5

forward delay

Answer 5

The amount of time a port stays in listening and learning state. The default value is 15 seconds, but can be changed to 15-30 seconds

Question 6

local bridge identifier

Answer 6

Combination of the local switches bridge system MAC address, system ID extension and system priority of the root bridge.

Question 7

Max Age

Answer 7

Maximum length of time that passes before a bridge port saves its BPDU information, default value is 20 seconds. If a switch loses contact with the BPDU source, it assumes the BPDU information is still valid for the duration of the Max Age timer

Question 8

root bridge

Answer 8

Switch that is considered the top of a STP topology for all path calculations, are ports are DPs,

Question 9

root path cost

Answer 9

combined cost for a specific path toward the root switch

Question 10

root port

Answer 10

Network port that connects to the root bridge or an upstream switch in the STP topology, should only be one root port per VLAN on a switch

Question 11

system priority

Answer 11

4 bit value indicates the preference for a switch to be root bridge. The default value is 32, 768

Question 12

system ID extension

Answer 12

12 bit value indicates the VLAN that the BPDU coorelates to. The system priority and System ID extension are combined as part of the switches identification of the root bridge

Question 13

TCN

Answer 13

Topology Change Notification, 2nd type of BPDU, used to communicate changes in the Layer 2 topology to other switches