Spanning Tree Protocol; Rstp And Stp; Table 48 Path Cost - ZyXEL Communications IES-612-51A User Manual

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H A P T E R
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.

Table 48 Path Cost

LINK SPEED
Path Cost 4Mbps
Path Cost 10Mbps
Path Cost 16Mbps
Path Cost 100Mbps
Path Cost 1Gbps
Path Cost 10Gbps
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.
IES-612-51A User's Guide

Spanning Tree Protocol

RECOMMENDED
VALUE
250
100
62
19
4
2
22
RECOMMENDED
ALLOWED RANGE
RANGE
100 to 1000 1 to 65535
50 to 600 1 to 65535
40 to 400 1 to 65535
10 to 60 1 to 65535
3 to 10 1 to 65535
1 to 5 1 to 65535
175