ZyXEL Communications IES-708-22A - Manual (Page 169)

IES-708-22A User’s Guide
169
C
H A P T E R
23
Spanning Tree Protocol
This feature is available in STU-C mode only.
This chapter introduces the Spanning Tree Protocol (STP) and Rapid Spanning Tree Protocol
(RSTP).
23.1 RSTP and STP
RSTP adds rapid reconfiguration capability to STP. The IES-708-22A 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
network. The IES-708-22A uses RSTP by default but can still operate with STP switches
(although without RSTP’s benefits).
The root bridge is the base of the spanning tree; it is the bridge 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, as illustrated in the following table.
On each bridge, the root port is the port through which this bridge communicates with the root.
It is the port on this IES-708-22A with the lowest path cost to the root (the root path cost). If
there is no root port, then this IES-708-22A 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.
Table 49 Path Cost
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