GS3700/XGS3700 Series User’s Guide
106
C
H A P T E R
11
Spanning Tree Protocol
The Switch supports Spanning Tree Protocol (STP), Rapid Spanning Tree Protocol (RSTP) and
Multiple Spanning Tree Protocol (MSTP) as defined in the following standards.
• IEEE 802.1D Spanning Tree Protocol
• IEEE 802.1w Rapid Spanning Tree Protocol
• IEEE 802.1s Multiple Spanning Tree Protocol
The Switch also allows you to set up multiple STP configurations (or trees). Ports can then be
assigned to the trees.
11.1 STP/RSTP Overview
(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.
The Switch uses IEEE 802.1w RSTP (Rapid Spanning Tree Protocol) that allows faster convergence
of the spanning tree than STP (while also being backwards compatible with STP-only aware
bridges). In RSTP, topology change information is directly propagated throughout the network from
the device that generates the topology change. In STP, a longer delay is required as the device that
causes a topology change first notifies the root bridge and then the root bridge notifies the
network. Both RSTP and STP flush unwanted learned addresses from the filtering database. In
RSTP, the port states are Discarding, Learning, and Forwarding.
Note: In this user’s guide, “STP” refers to both STP and RSTP.
11.1.1 STP Terminology
The root bridge is the base of the spanning tree.
Path cost is the cost of transmitting a frame onto a LAN through that port. The recommended cost
is assigned according to the speed of the link to which a port is attached. The slower the media, the
higher the cost.
Table 41
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
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