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MSAP2000 AAMS User’s Guide

C HAPTER 13

Spanning Tree Protocol
This chapter introduces the Rapid Spanning Tree Protocol (RSTP).
13.1 RSTP (Rapid Spanning Tree Protocol) and STP

(Spanning Tree Protocol)

RSTP adds rapid reconfiguration capability to STP. The switch supports RSTP
and the earlier STP. RSTP and STP detect and break network loops and provide
backup links between switches, bridges or routers. They allow a device to
interact with other RSTP or STP-aware devices in your network to ensure that
only one path exists between any two stations on the ne twork. The switch uses
RSTP by default but can still interoperate with STP switches
(although without RSTP's benefits).
The root bridge is the base of the spanning tree; it is the bridg e with the lowest
identifier value
(MAC address). Path cost is the cost of transmitting a frame onto a LAN through
that port. It is assigned according to the speed of the link to which a port is
attached. The slower the media, the higher the cost - see the following table.
Table 33 STP Path Costs
LINK SPEED RECOMMENDED VALUE RECOMMENDED RANGE ALLOWED
RANGE
Path Cost 4Mbps 250 100 to 1000 1 to 65535
Path Cost 10Mbps 100 50 to 600 1 to 65535
Path Cost 16Mbps 62 40 to 400 1 to 65535
Path Cost 100Mbps 19 10 to 60 1 to 65535
Path Cost 1Gbps 4 3 to 10 1 to 65535
Path Cost 10Gbps 2 1 to 5 1 to 65535
On each bridge, the root port is the port through which this bridge communicates
with the root. It is the port on this switch with the lowest path cost to the root (the
root path cost). If there is no root port, then this switch has been accepted as the
root bridge of the spanning tree network.

For each LAN segment, a designated bridge is selected. This bridge has the
lowest cost to the root among the bridges connected to the LAN.