Why are bpdus used

The following example, when used with a full bridge configuration with aggregated Ethernet, blocks BPDUs on aggregated interface ae0 for 10 minutes seconds before enabling the interface again:.

The BPDU drop feature can be specified only on interfaces on which no spanning-tree protocol is configured. If you want to enable the BPDU shutdown feature, then it is optional to disable spanning-tree protocols on the interface. This example addresses two scenarios. In the first scenario, an interface is shutdown when it encounters an outside BPDU.

In the second scenario, an interface drops only BPDU packets while retaining the status of the interface as up and allowing all other traffic to pass through the interface. Figure 5 shows the topology for this example. Switch 1 is configured for RSTP while Switch 2 has a spanning-tree protocol configured on it for the first scenario, and does not have a spanning-tree protocol configured on it for the second scenario.

When BPDU protection is enabled with the drop statement, the switch interfaces drop only the BPDUs while allowing remaining traffic to pass through and retaining their status as up if BPDUs generated by the laptops attempt to access Switch 2. Table 5 shows the components that will be configured for BPDU protection. This is the first scenario that explains configuration for the shutdown statement. To quickly configure BPDU protection on Switch 2 for the shutdown statement, copy the following commands and paste them into the switch terminal window:.

To configure BPDU protection for the shutdown statement:. This is the second scenario that explains configuration for the drop statement. To quickly configure BPDU protection on Switch 2 for the drop statement, copy the following commands and paste them into the switch terminal window:. You can also disable RSTP globally using the delete protocols rstp , the set protocols rstp disable , or the set protocols rstp interface all disable command. Use the operational mode command show ethernet-switching interfaces :.

You need to re-enable the blocked interfaces. If you included the statement disable-timeout in the BPDU configuration, the interface returns to service after the timer expires. This command will only re-enable an interface but the BPDU configuration for the interface will continue to exist unless you remove the BPDU configuration explicitly. Help us improve your experience. Let us know what you think. Do you have time for a two-minute survey? Maybe Later.

If the interface shuts down after dropping all BPDUs, you can re-enable the interface as follows: On Juniper Networks EX Series and QFX Series switches running Juniper Networks Junos operating system Junos OS that supports the Enhanced Layer 2 Software ELS configuration style: Include the disable-timeout statement at the [edit protocols layer2-control bpdu-block ] hierarchy level to enable the interfaces to automatically return to service when the specified timer expires.

On EX Series switches running Junos OS that does not support the ELS configuration style: Include the disable-timeout statement at the [edit ethernet-switching-options bpdu-block] hierarchy level to enable the interfaces to automatically return to service when the specified timer expires.

Tip: The maximum age timer should be longer than the configured hello timer. Note: In discussions of spanning-tree protocols, the terms bridge and switch are often used interchangeably. BPDU Protection on All Edge Ports of the Bridge To configure edge port blocking for a particular STP family member, include the bpdu-block-on-edge statement for mstp , rstp , or vstp : bpdu-block-on-edge ; interface interface-name ; In contrast to BPDU protection configured on individual spanning-tree instance interfaces, BPDU protection configured on all edge ports of an entire spanning-tree protocol disables designated edge ports and does not enable them again.

Note: Edge ports can be access or trunk ports. Automatically Unblocking an Interface Using an Expiry timer on Access and Leaf Devices To automatically unblock an interface using an expiry timer on access and leaf devices: Note: The range of seconds is between 10 and Manually Unblocking an Interface on Access and Leaf Devices To manually unblock an interface on access and leaf devices: [edit] user host run clear error bpdu interface all. Configuring BPDU protection For Edge Interfaces In a spanning-tree topology, if a switch is an access switch then interfaces on that switch will be connected to end devices such as PCs, servers, routers, or hubs, that are not connected to other switches.

To configure BPDU protection on an edge interface of a switch: Note: Ensure that the switch is connected to an end device. Configuring BPDU for Interface Protection With Port Shutdown Mode In a spanning-tree topology, if a switch is an access switch then interfaces on that switch will be connected to end devices such as PCs, servers, routers, or hubs, that are not connected to other switches.

To configure BPDU protection for an interface to only drop incompatible BPDU packets and to allow the remaining traffic to pass through, while retaining the interface status as up: Note: Ensure that the switch on which you are configuring BPDU protection is connected to a peer device. Disabled RSTP on device 2. Configuration To configure BPDU protection on the interfaces: CLI Quick Configuration Procedure Results CLI Quick Configuration To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, copy and paste the commands into the CLI at the [edit] hierarchy level, and then enter commit from configuration mode.

Results From configuration mode, confirm your configuration by entering the show configuration protocols rstp command. Verification Verify that the configuration is working properly. Action Use the operational mode command show spanning-tree instance.

Action Use the operational mode command show spanning-tree interface. Configuration To configure BPDU protection on two access interfaces: CLI Quick Configuration Procedure Results CLI Quick Configuration To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, copy and paste the commands into the CLI at the [edit] hierarchy level, and then enter commit from configuration mode.

Note: The BPDU drop feature can be specified only on interfaces on which no spanning-tree protocol is configured. Router 1 Distribution Layer.

One BPDU is transmitted on each one of its designated ports. Only the root bridge generates new BPDUs, but when a bridge is first enabled, it thinks it is the root bridge because it has no other priority vector to compare its own to.

So it places all of its ports into the designated role, starts its Hello timer, and begins to generate BPDUs see the section " Root Bridge Selection ". BPDUs transmitted by nonroot bridges carry the same information as the BPDUs they received, with the exception of the following fields that they update see Figure :.

The transmitter's bridge ID and port ID are replaced by the bridge with its own information. The bridge updates the cost to be the sum of the cost it received and the cost of the port on the local bridge its root port that it received the BPDU on.

Regardless of whether a bridge is the root bridge, it transmits a configuration BPDU in the following cases as well:. When a bridge receives a BPDU with a priority vector that's inferior to the one it would use on the same port, it replies with its own superior information.

Because BPDUs are generated only by the root bridge, and are regenerated by the other bridges only upon the reception of a BPDU on their root port, it should be clear that the time taken by the information generated by the root bridge with its BPDUs to reach the leaf bridges is variable.

On a stable network, the time depends mainly on how loaded the bridges are and how fast they can process BPDUs. BPDU relaying via nonroot bridges. BPDUs carrying stale information should not be used to build the loop-free topology. Sign up. Term of the Day. Best of Techopedia weekly. News and Special Offers occasional.

A bridge protocol data unit BPDU is a data message transmitted across a local area network to detect loops in network topologies. A BPDU contains information regarding ports, switches, port priority and addresses.

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.