Avaya P332G-ML manual Rapid Spanning Tree Protocol Rstp, About the 802.1w Standard, Port Roles

Chapter 11 Avaya P330 Layer 2 Features

Rapid Spanning Tree Protocol (RSTP)

About the 802.1w Standard

The enhanced feature set of the 802.1w standard includes:

•Bridge Protocol Data Unit (BPDU) type 2

•New port roles: Alternate port, Backup port

•Direct handshaking between adjacent bridges regarding a desired topology change (TC). This eliminates the need to wait for the timer to expire.

•Improvement in the time it takes to propagate TC information. Specifically, TC information does not have to be propagated all the way back to the Root Bridge (and back) to be changed.

•Origination of BPDUs on a port-by-port basis.

Port Roles

At the center of RSTP—specifically as an improvement over STP (802.1d)—are the roles that are assigned to the ports. There are four port roles:

•Root port — port closest to the root bridge

•Designated port — corresponding port on the remote bridge of the local root port

•Alternate port — an alternate route to the root

•Backup port — an alternate route to the network segment

The RSTP algorithm makes it possible to change port roles rapidly through its fast topology change propagation mechanism. For example, a port in the “blocking” state can be assigned the role of “alternate port.” When the backbone of the network fails the port may be rapidly changed to forwarding.

Whereas the STA passively waited for the network to converge before turning a port into the forwarding state, RSTP actively confirms that a port can safely transition to forwarding without relying on any specific, programmed timer configuration.

RSTP provides a means of fast network convergence after a topology change. It does this by assigning different treatments to different port types. The port types and the treatment they receive follow:

•Edge ports — Setting a port to "edge-port" admin state indicates that this port is connected directly to end stations that cannot create bridging loops in the network. These ports transition quickly to forwarding state. However, if BPDUs are received on an Edge port, it’s operational state will be changed to "non- edge-port" and bridging loops will be avoided by the RSTP algorithm. The default admin state of all ports is "edge-port".

Avaya P332G-ML User’s Guide 65