ZyXEL Communications NETATLAS ENTERPRISE -, User Manual

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NetAtlas Enterprise Ethernet Switch Manager User’s Guide
Chapter 5 View 52
5.2 STP/RSTP
(R)STP detects and breaks network loops and provides backup links between switches,
bridges or routers. It allows a switch to interact with other (R)STP -compliant switches in your
network to ensure that only one path exists between any two stations on the network.
Note: In this user’s guide, “STP” refers to both STP and RSTP.
5.2.1 STP Terminology
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 - see the next table.
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.
5.2.2 How STP Works
After a bridge determines the lowest cost-spanning tree with STP, it enables the root port and
the ports that are the designated ports for connected LANs, and disables all other ports that
participate in STP. Network packets are therefore only forwarded between enabled ports,
eliminating any possible network loops.
STP-aware switches exchange Bridge Protocol Data Units (BPDUs) periodically. When the
bridged LAN topology changes, a new spanning tree is constructed.
Table 17 STP Path Costs
LINK SPEED RECOMMENDED VALUE RECOMMENDED RANGE ALLOWED RANGE
4Mbps 250 100 to 1000 1 to 65535
10Mbps 100 50 to 600 1 to 65535
16Mbps 62 40 to 400 1 to 65535
100Mbps 19 10 to 60 1 to 65535
1Gbps 4 3 to 10 1 to 65535
10Gbps 2 1 to 5 1to 65535