How does rapid spanning tree protocol work

These ports are allowed to immediately enter the forwarding state rather than passively wait for the network to converge.

This path is different than using the root port. The alternative port moves to the forwarding state if there is a failure on the designated port for the segment. The backup port applies only when a single switch has two links to the same segment collision domain.

To have two links to the same collision domain, the switch must be attached to a hub. The other port will be Backup port according to the definition of Backup port above. We also use third-party cookies that help us analyze and understand how you use this website.

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These cookies ensure basic functionalities and security features of the website, anonymously. The cookie is used to store the user consent for the cookies in the category "Analytics". The cookies is used to store the user consent for the cookies in the category "Necessary". When a port is selected by the STA to become a designated port, In RSTP, this condition corresponds to a port with a designated role but a blocking state. These diagrams illustrate how fast transition is achieved step-by-step.

Suppose a new link is created between the root and Switch A. Both ports on this link are put in a designated blocking state until they receive a BPDU from their counterpart. When a designated port is in a discarding or learning state and only in this case , it sets the proposal bit on the BPDUs it sends out.

This is what occurs for port p0 of the root bridge, as shown in step 1 of the preceding diagram. Because Switch A receives superior information, it immediately knows that p1 is the new root port. Switch A then starts a sync to verify that all of its ports are in-sync with this new information.

A port is in sync if it meets either of these criteria:. In order to illustrate the effect of the sync mechanism on different kind of ports, suppose there exists an alternate port p2, a designated forwarding port p3, and an edge port p4 on Switch A. Notice that p2 and p4 already meet one of the criteria. In order to be in sync see step 2 of the preceding diagram , Switch A just needs to block port p3, and assign it the discarding state. Now that all of its ports are in sync, Switch A can unblock its newly selected root port p1 and send an agreement message to reply to the root.

This message is a copy of the proposal BPDU, with the agreement bit set instead of the proposal bit. This ensures that port p0 knows exactly to which proposal the agreement it receives corresponds. Once p0 receives that agreement, it can immediately transition to the forwarding state. This is step 4 of the preceding figure. Notice that port p3 is left in a designated discarding state after the sync. In step 4, that port is in the exact same situation as port p0 is in step 1.

It then starts to propose to its neighbor, and attempts to quickly transition to the forwarding state. The proposal agreement mechanism is very fast, as it does not rely on any timers. This wave of handshakes propagates quickly towards the edge of the network, and quickly restores connectivity after a change in the topology. If a designated discarding port does not receive an agreement after it sends a proposal, it slowly transitions to the forwarding state, and falls back to the traditional Cisco introduced an enhancement to the sync mechanism that allows a bridge to put only its former root port in the discarding state when it syncs.

Details of how this mechanism works are beyond the scope of this document. However, one can safely assume that it is invoked in most common reconvergence cases. The scenario described in the Convergence with Another form of immediate transition to the forwarding state included in RSTP is similar to the Cisco UplinkFast proprietary spanning tree extension.

Basically, when a bridge loses its root port, it is able to put its best alternate port directly into the forwarding mode the appearance of a new root port is also handled by RSTP. The selection of an alternate port as the new root port generates a topology change. This removes the need for the dummy multicast generation process of UplinkFast.

UplinkFast does not need to be configured further because the mechanism is included natively and enabled in RSTP automatically. When an Once the root bridge is aware of a change in the topology of the network, it sets the TC flag on the BPDUs it sends out, which are then relayed to all the bridges in the network. When a bridge receives a BPDU with the TC flag bit set, it reduces its bridging-table aging time to forward delay seconds.

This ensures a relatively quick flush of stale information. This topology change mechanism is deeply remodeled in RSTP. Both the detection of a topology change and its propagation through the network evolve. In RSTP, only non-edge ports that move to the forwarding state cause a topology change. This means that a loss of connectivity is not considered as a topology change any more, contrary to When a RSTP bridge detects a topology change, these occur:.

It starts the TC While timer with a value equal to twice the hello-time for all its non-edge designated ports and its root port, if necessary. BPDUs are also sent on the root port while the timer is active. It clears the MAC addresses learned on all its ports, except the one that receives the topology change. This way, the TCN floods very quickly across the whole network. The TC propagation is now a one step process.

In fact, the initiator of the topology change floods this information throughout the network, as opposed to This mechanism is much faster than the This approach results in potentially more temporary flooding, but on the other hand it clears potential stale information that prevents rapid connectivity restitution. However, it is important to note that the inherent fast convergence benefits of Each port maintains a variable that defines the protocol to run on the corresponding segment.

A migration delay timer of three seconds also starts when the port comes up. As soon as the migration delay expires, the port adapts to the mode that corresponds to the next BPDU it receives.

If the port changes its mode of operation as a result of a BPDU received, the migration delay restarts. Because the MAC address table is unstable in a loop arrangement, a switch has no way to understand where data needs to be sent so it broadcasts the information out of every port. This data is received by all switches on the network, and rebroadcast out again because the recipient is still unknown.

This can overwhelm switches and severely degrade network performance. The broadcast storm will continue until one of the switches fails or is disconnected from the system. The following illustration shows three switches which are in a loop on the network.

When one switch sends out a message, the two other switches on the network receive the message and forward the message to other switches on the network out of all other ports. For example, when Switch B sends a message to Switch A and Switch C, those switches receive the message and rebroadcast it to the other switches. When Switch A and Switch C initiate broadcasts across the network as well, the network can become congested with repeated broadcast messages, taking up bandwidth and slowing down the entire network.

RSTP prevents network loops by blocking redundant ports. A blocked port will still receive data, but will not send that data out to other devices on the network.

This ensures that switches will receive only a single copy of a packet. If any of the active paths fail, one of the blocked ports will be used. The port that is selected depends on the topology of the configuration. RSTP is an improvement over STP Spanning Tree Protocol mainly due to its reduction in convergence time — that is, the time it takes all switches on a network to reach a state of convergence, or agreement, on the topology of the network.

In STP, there is substantial convergence time whenever there is a topology change or failure in the network, which typically lasts for seconds. In a modern, high-demand networking environment, there is a constant need for increased speed and reliability and a delay of seconds is generally unacceptable. RSTP reduces the convergence time significantly down to around seconds. Fortunately, many modern switches on the market automatically enable RSTP by default.

Further, for networking environments with a mix of older and newer equipment, it is important to note that RSTP is backward compatible with the older STP standard. RSTP is a set of rules by which switches on the network determine the best way to route data on the network without redundancy.