ZyXEL Communications IES-612-51A - Manual (Page 175)

IES-612-51A User’s Guide
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H A P T E R
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Spanning Tree Protocol
This chapter introduces the Spanning Tree Protocol (STP) and Rapid Spanning Tree Protocol
(RSTP).
22.1 RSTP and STP
RSTP adds rapid reconfiguration capability to STP. The IES-612-51A 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 Integrated Ethernet Switch 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 Integrated Ethernet Switch with the lowest path cost to the root (the root
path cost). If there is no root port, then this Integrated Ethernet 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.
After a bridge determines the lowest cost-spanning tree with RSTP, it enables the root port and
the ports that are the designated ports for the connected LANs, and disables all other ports that
participate in RSTP. Network packets are therefore only forwarded between enabled ports,
eliminating any possible network loops.
Table 48 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