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4
The relative priority of each port.
5
The cost of each port.
After these values are assigned, bridges multicast and process the formatted frames (called
Bridge Protocol Data Units, or BPDUs) to derive a single, loop-free topology throughout the
extended network. The bridges exchange BPDU frames quickly, minimizing the time that
service is unavailable between hosts.
Spanning Tree per Port
The Spanning Tree can take up to 30 seconds to open traffic on a port. This delay can cause
problems on ports carrying time-sensitive traffic. You can therefore enable/disable Spanning
Tree in the W310 on a per-port basis to minimize this effect.
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 network backbone fails, the port may be rapidly
changed to forwarding.
Avaya W310 User's Guide
Chapter 11
W310 WLAN Gateway Layer 2 Features
79
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