Chapter 13 Spanning Tree Protocol
GS2200-24 User’s Guide
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13.3 What You Need to Know
(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 that then 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.
13.3.1 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.
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.
Table 24 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
Path
Cost
1Gbps 4 3 to 10 1 to 65535
Path
Cost
10Gbps 2 1 to 5 1 to 65535
