Rstp Bpdu Flag Byte





Port Role-Proposal Topology Change

Topology Change ACK Agreement Forwarding Learning f 00 Unknown \ _ 01 Alternative/Backup 10 Root

.11 Designated::

RSTP (802.1w) uses type 2, version 2 BPDUs, so an RSTP bridge can communicate with

802.1D on any shared link or with any switch running 802.1D. RSTP sends BPDUs and populates the flag byte in a slightly different manner than the manner used by 802.1D.

■ An RSTP bridge sends a BPDU with its current information every hellotime period (2 seconds by default), even if it does not receive any BPDUs from the root bridge.

■ Protocol information can be immediately aged on a port if hellos are not received for three consecutive hellotimes or if the max age timer expires.

■ Because BPDUs are now used as a "keepalive" mechanism, three consecutively missed BPDUs indicate lost connectivity between a bridge and its neighboring root or designated bridge. This fast aging of the information allows quick failure detection.

RSTP uses the flag byte of version 2 BPDU as shown in the figure.

■ Bits 0 and 7 are used for TCN and acknowledgement (ACK), as they are in 802.1D.

■ Bits 1 and 6 are used for the proposal and agreement process.

■ Bits 2-5 encode the role and state of the port originating the BPDU.

© 2006 Cisco Systems, Inc. Implementing Spanning Tree 3-41

The Flag field in the STP BPDU packet contained TCN and TCA. In RSTP, the Flag field, 1 byte long, has been modified to accommodate port designations and proposal/agreement between adjacent switches. BPDUs are sent every 2 seconds. Unlike in legacy STP, in RSTP, each switch generates its own BPDUs, regardless of whether or not it hears BPDUs from the root.

In legacy STP, BPDUs were generated by only the root and propagated throughout the spanning tree domain. As a result, when a switch did not receive a configuration BPDU, it did not know where the failure occurred.

In RSTP mode, the switch needs direct interactions with only its immediate neighbors. Hence, BPDUs also serve as keepalive mechanisms between adjacent switches. If the switch does not hear three consecutive BPDUs from its downstream neighbor, it will transition appropriate ports to converge the network.

3-42 Building Cisco Multilayer Switched Networks (BCMSN) v3.0 © 2006 Cisco Systems, Inc.

Identifying the RSTP Proposal and Agreement Process

This topic describes the stages of the RSTP proposal and agreement process.

In 802.1D, when a port has been selected by spanning tree to become a designated port, it must wait two times the forward delay before transitioning the port to a forwarding state. RSTP significantly speeds up the recalculation process after a TC in the network because it converges on a link-by-link basis and does not rely on timers expiring before ports can transition. Rapid transition to a forwarding state can be achieved on only edge ports and point-to-point links. In RSTP, this condition corresponds to a port with a designated role that is in a blocking state. The figure illustrates, step by step, how rapid transition is achieved.

1. Switch A had a path to the root via switch B and switch C. A new link is then created between the root and switch A, and both ports are in designated blocking state until they receive a BPDU from their counterparts. 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 happens for port P0 of the root bridge.

2. Switch A sees the proposal BPDU with a superior path cost. It blocks all nonedge designated ports other than the one over which the proposal and agreement process are occurring. This operation, called "sync," prevents switches below A from causing a loop during the proposal and agreement process. Edge ports need not be blocked and remain unchanged during sync.

3. Bridge A explicitly sends an agreement that allows the root bridge to put the root port P0 in forwarding state. Port P1 becomes the root port for A.

© 2006 Cisco Systems, Inc. Implementing Spanning Tree 3-43

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