Rapid Spanning Tree Protocol - 802.1d

RSTP bridge port roles:

Root - A forwarding port that is the best port from Nonroot-bridge to Rootbridge

Designated - A forwarding port for every LAN segment
Alternate - An alternate path to the root bridge. This path is different than using the root port.
Backup - A backup/redundant path to a segment where another bridge port already connects.
Disabled - Not strictly part of STP, a network administrator can manually disable a port

RSTP is a refinement of STP and therefore shares most of its basic operation characteristics. However there are some notable differences as summarized below:

Detection of root switch failure is done in 3 hello times, which is 6 seconds if default hello times have not been changed.

Ports may be configured as edge ports if they are attached to a LAN that has no other bridges attached. These edge ports transition directly to the forwarding state. RSTP still continues to monitor the port for BPDUs in case a bridge is connected. RSTP can also be configured to automatically detect edge ports. As soon as the bridge detects a BPDU coming to an edge port, the port becomes a non-edge port.

Unlike in STP, RSTP will respond to BPDUs sent from the direction of the root bridge. An RSTP bridge will "propose" its spanning tree information to its designated ports. If another RSTP bridge receives this information and determines this is the superior root information, it sets all its other ports to discarding. The bridge may send an "agreement" to the first bridge confirming its superior spanning tree information. The first bridge, upon receiving this agreement, knows it can rapidly transition that port to the forwarding state bypassing the traditional listening/learning state transition. This essentially creates a cascading effect away from the root bridge where each designated bridge proposes to its neighbors to determine if it can make a rapid transition. This is one of the major elements that allows RSTP to achieve faster convergence times than STP.

As discussed in the port role details above, RSTP maintains backup details regarding the discarding status of ports. This avoids timeouts if the current forwarding ports were to fail or BPDUs were not received on the root port in a certain interval.

OSI MODEL

OSI MODEL (Open System Interconnection Model)

OSI divides telecommunication into seven layers. The layers are in two groups. The upper four layers are used whenever a message passes from or to a user. The lower three layers are used when any message passes through the host computer. Messages intended for this computer pass to the upper layers. Messages destined for some other host are not passed up to the upper layers but are forwarded to another host. The seven layers are:

Layer 7: The application layer ...This is the layer at which communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. (This layer is not the application itself, although some applications may perform application layer functions.)

Layer 6: The presentation layer ...This is a layer, usually part of an operating system, that converts incoming and outgoing data from one presentation format to another (for example, from a text stream into a popup window with the newly arrived text). Sometimes called the syntax layer.

Layer 5: The session layer ...This layer sets up, coordinates, and terminates conversations, exchanges, and dialogs between the applications at each end. It deals with session and connection coordination.

Layer 4: The transport layer ...This layer manages the end-to-end control (for example, determining whether all packets have arrived) and error-checking. It ensures complete data transfer.

Layer 3: The network layer ...This layer handles the routing of the data (sending it in the right direction to the right destination on outgoing transmissions and receiving incoming transmissions at the packet level). The network layer does routing and forwarding.

Layer 2: The data-link layer ...This layer provides synchronization for the physical level and does bit-stuffing for strings of 1's in excess of 5. It furnishes transmission protocol knowledge and management.

Layer 1: The physical layer ...This layer conveys the bit stream through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier.