Chapter 7


What is a protocol?


Answer: A protocol is the agreed-upon rules governing transmitting and receiving of data between two network devices, such as computers, switches, or routers. Protocols determine the type of error checking to be used, how the sending device indicates it has finished sending a message, and how the receiving device indicates it has received a message; however, not all protocols indicate the completion or reception of a message.


What is a bridge loop?


Answer: A bridge loop occurs when two or more active paths exist between network segments.


What is purpose of the Spanning Tree Protocol (STP)?


Answer: The Spanning Tree Protocol provides path redundancy in a bridged or switched network while preventing undesirable loops created by multiple active paths between hosts.


What is a BPDU?


Answer: BDPU is the acronym for bridge protocol data unit and is a Spanning Tree Protocol message frame describing the attributes of a switch port. These attributes include the port's MAC address, priority, age of message, timers, and cost to reach. BPDUs enable switches participating in an STP to gather information about each other and build a topology map so that each switch has a path to forward network traffic.


What are the STP states? Which state can only be manually configured?


Answer: Blocking, listening, learning, forwarding, and disabled. The only state that can be manually configured is the disabled state.


What is the difference between a blocked port and a disabled port?


Answer: A blocked port does not send or receive any traffic, but listens to the Spanning-Tree BPDU messages, whereas a disabled port is manually shut down by the administrator and can be enabled only in the same fashion.


What is the starting point for the Spanning Tree Protocol called?


Answer: The Spanning Tree Protocol reference point is called the root switch or port.


What two components make up the bridge identifier, how long is the bridge identifier, and how is the bridge identifier used?


Answer: The bridge identifier is made up of the bridge priority (2 bytes) and the MAC address (6 bytes). The bridge identifier is used to elect the root bridge or switch port in a LAN.


From the time it is powered up, how long does it take a switch to enter the forwarding state and begin forwarding LAN traffic?


Answer: 50 seconds. The switch port starts in the blocking state at power up and transitions to the listening state. It takes 20 seconds for the switch port to transition from the blocking to the listening state, 15 seconds to transition from the listening to learning state, and another 15 seconds for the switch port to transition from the learning to the forwarding state.


What is convergence?


Answer: Convergence is the process by which network devices, such as bridges, switches, or routers, learn of a change in network topology and then agree on what the new topology looks like after the change.


What does the Spanning Tree Protocol do when a new bridge is added to the network?


Answer: Adding a bridge to the network changes the topology and thereby causes each bridge and switch to converge on the change by means of the Spanning Tree Protocol. STP puts all bridges and switches that are new or changed into a blocking state, listening for BPDUs, learning what the network looks like, and forwarding traffic through each port as determined by the Spanning Tree Protocol.