Rapid Spanning Tree Protocol and Multiple Spanning Tree Protocol

The Rapid Spanning Tree Protocol (RSTP or IEEE 802.1w) reduces the recovery time after a network breakdown. It also maintains backward compatibility with IEEE 802.1d (the spanning tree implementation prior to RSTP). In certain configurations, the recovery time of RSTP can be reduced to less than 1 second. RSTP also reduces the amount of flooding in the network by enhancing the way Topology Change Notification (TCN) packets are generated.

With Multiple Spanning Tree Protocol (MSTP or IEEE 802.1s), you can configure multiple instances or Spanning Tree groups on the same device. Each instance or Spanning Tree group can include one or more VLANs.

By using RSTP and MSTP, the switch achieves the following:

The following sections relate to RSTP and MSTP:

RSTP interoperability with STP

RSTP provides a parameter called ForceVersion to provide backward compatibility with standard STP. A user can configure a port in either STP-compatible mode or RSTP mode:

Note

Note

You must configure protocol migration to true on all spanning-tree enabled interfaces when you change the spanning tree version from STP-compatible to MSTP for those interfaces to work in the proper mode.

Before implement MSTP or RSTP you must be aware of the following:

Differences in port roles for STP and RSTP

RSTP is an enhanced version of STP. These two protocols have almost the same parameters.

The following table lists the differences in port roles for STP and RSTP. STP supports two port roles, while RSTP supports four port roles.

Table 1. Differences in port roles for STP and RSTP

Port Role

STP

RSTP

Description

Root

Yes

Yes

This port receives a better BPDU than its own and has the best path to reach the Root. The root port is in Forwarding state. The root port and designated ports can be in the Discarding state before they go to root forwarding.

Designated

Yes

Yes

This port has the best BPDU on the segment. The designated port is in the Forwarding state.

Alternate

No

Yes

This port receives a better BPDU than its own BPDU, and a root port exists within the same device. The alternate port is in the Discarding state.

Backup

No

Yes

This port receives a better BPDU than its own BPDU, and this BPDU is from another port within the same device. The backup port is in the Discarding state.

Port roles: root forwarding role

MSTP and RSTP root forwarding roles are as follows:

Port roles: designated forwarding role

MSTP and RSTP designated forwarding roles are as follows:

Port roles: alternate blocking role

MSTP and RSTP alternate blocking roles are as follows:

Port roles: backup blocking role

MSTP and RSTP backup blocking roles are as follows:

Edge port

RSTP uses a parameter called the edge port. After a port connects to a nonswitch device, such as a PC or a workstation, it must be configured as an edge port. An active edge port enters the forwarding state without delay. An edge port becomes a nonedge port if it receives a BPDU.

Path cost values

RSTP and MSTP path cost values support a wide range of link speeds. The following table lists the path cost values.

Table 2. Path cost values

Link speed

Value

Less than or equal to 100 Kbps

1 Mbps

10 Mbps

100 Mbps

200 000 000

20 000 000

2 000 000

200 000

1 Gbps

10 Gbps

100 Gbps

20 000

2000

200

1 Tbps

10 Tbps

20

2

RSTP negotiation process

The following section describes the negotiation process between switches that takes place before PCs can exchange data (see the following figure).

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RSTP negotiation process

After turning on, all ports assume the role of designated ports. All ports are in the discarding state except edge ports. Edge ports go directly into the forwarding state without delay.

Unit A port 1 and Unit B port 1 exchange BPDUs. Unit A knows that it is the root and that Unit A port 1 is the designated port. Unit B learns that Unit A has higher priority. Unit B port 1 becomes the root port. Both Unit A port 1 and Unit B port 1 are still in the discarding state.

Unit A starts the negotiation process by sending a BPDU with the proposal bit set.

Unit B receives the proposal BPDU and configures its nonedge ports to discarding state. This operation occurs during the synchronization process.

Unit B sends a BPDU to Unit A with the agreement bit set.

Unit A configures port 1 to the forwarding state, and Unit B configures port 1 to the forwarding state. PC 1 and PC 2 can now communicate. The negotiation process now moves on to Unit B port 3 and its partner port. PC 3 cannot exchange data with either PC 1 or PC 2 until the negotiation process between Unit B and Unit C finishes.

The RSTP convergence time depends on how quickly the switchil recan exchange BPDUs during the negotiation process, and on the number of switches in the network.