STP & RSTP

Question 1

After a spanning tree protocol is configured on an Ethernet switching network, the protocol calculates the network topology to implement the following functions

Answer 1

Loop prevention: The spanning tree protocol blocks redundant links to prevent potential loops on the network.
Link redundancy: If an active link fails and a redundant link exists, the spanning tree protocol activates the redundant link to ensure network connectivity

Question 2

Root Bridge: A tree topology must have a root. As defined in STP, the device that functions as the root of a tree network is called the root bridge.
There is only one root bridge on the entire STP network

Answer 2

After network convergence, the root bridge generates and sends configuration BPDUs at a specific interval

Question 3

A spanning tree is calculated based on the following metrics

Answer 3

Bridge ID (BID), port ID (PID), and path cost

Question 4

Bridge ID (BID)

Answer 4

According to IEEE 802.1D, a BID is composed of a bridge priority (leftmost 16 bits) and a bridge MAC address (rightmost 48 bits).
On an STP network, the device with the smallest BID is elected as the root bridge

Question 5

Port ID (PID)

Answer 5

A PID is composed of a port priority (leftmost 4 bits) and a port number (rightmost 12 bits).
The PID is used to select the designated port

Question 6

Path Cost

Answer 6

The path cost is a port variable used for link selection. STP calculates path costs to select robust links, block redundant links, and finally trim the network into a loop-free tree topology.
On an STP network, a port’s path cost to the root bridge is the sum of the path costs of all ports between the port and the root bridge. This path cost is the root path cost.