11.3 Spanning Tree Protocol (STP) Flashcards
Describe the Spanning Tree Protocol (STP)
also referred to as the spanning tree algorithm (STA), assigns a designated bridge (or switch) for each route. Only the designated bridge can forward packets. Redundant bridges (and switches) are assigned as backups.
Describe Switching forward logic
The source VLAN of the frame is determined, using the access interface’s VLAN or the frame’s trunking header.
The source MAC address, source interface ID, and the VLAN ID are added to the MAC address table.
The MAC address table is searched for the destination MAC address of the frame.
The frame is forwarded to the interface of the matched address entry. If the MAC address is not found in the MAC address table, the frame is flooded out to other access ports in the same VLAN.
Describe the Forwarding state
Criteria for putting an interface into a forwarding state is as follows:
STP elects a root switch and all interfaces on the root switch are in a forwarding state.
On non-root switches, the port with the least administrative cost between the switch and the root switch is elected to the forwarding state.
The switch with the lowest root cost, compared with other switches, is also placed in the forwarding state.
Describe Bridge ID
The bridge ID (BID) is composed of the priority number of the host and its MAC address. A hello packet is the most common Bridge Protocol Data Unit (BPDU). Hello packets contain the root bridge ID, the sender’s bridge ID, the cost to the root, and the timers
Describe the bridge port election process
Every switch sends out a hello packet listing itself as the root bridge ID with a cost of zero.
As each switch receives a hello packet, it compares its root bridge ID to the root bridge ID listed in the hello packets.
If the listed BID is lower, the switch elects that as the root and forwards the hello packet listing itself as the sender.
Once the root bridge has been elected, its ports enter a forwarding state and continue to send hello packets.
The other switches examine the hello packets and determine which port has the lowest cost to the root. The port with the lowest cost to the root becomes the root port.
The port on each LAN segment with the lowest cost to the root becomes the designated port for that segment.
When the age timer expires, STP starts a new election process.
What is a Bridge loop/ switching loop
providing redundant paths between segments causes packets to be passed between the redundant paths endlessly
What are the benefits of STP
Eliminates bridge loops.
Provides redundant paths between devices.
Enables dynamic role configuration.
Recovers automatically from a topology change or device failure.
Identifies the optimal path between any two network devices.
Describe Bridge roles
The bridge role determines how the device functions in relation to other devices and whether the device forwards traffic to other segments.
Describe a Root Bridge
The root bridge is the master or controlling bridge.
Describe the Designated Bridge
A designated bridge is any other device that participates in forwarding packets through the network.
Describe Backup Bridge
All redundant devices are classified as backup bridges.
Describe Port state Disabled
A port in the disabled state is powered on but does not participate in forwarding or listening to network messages. A bridge must be manually placed in the disabled state.
Describe the Blocking Port state
When a device is first powered on, its ports are in the blocking state. In addition, backup bridge ports are always in the blocking state. Ports in a blocking state receive packets and BPDUs sent to all bridges, but will not process any other packets.
Describe the Listening port state
The listening state is a transitory state between blocking and learning. The port remains in the listening state for a specific period of time. This time period allows network traffic to settle down after a change has occurred. For example, if a bridge goes down, all other bridges go to the listening state for a period of time. During this time the bridges redefine their roles.
Describe the Learning port state
A port in the learning state is receiving packets and building the bridge database by associating MAC addresses with ports. A timer is also associated with this state. The port goes to the forwarding state after the timer expires.
