Chapter 8 Wireless Screen
NWA-3160 Series User’s Guide
131
8.3.5 Spanning Tree Protocol (STP)
STP detects and breaks network loops and provides backup links between
switches, bridges or routers. It allows a bridge to interact with other STP-
compliant bridges in your network to ensure that only one route exists between
any two stations on the network.
8.3.5.1 Rapid STP
The NWA uses IEEE 802.1w RSTP (Rapid Spanning Tree Protocol) that allow faster
convergence of the spanning tree (while also being backwards compatible with
STP-only aware bridges). Using RSTP topology change information does not have
to propagate to the root bridge and unwanted learned addresses are flushed from
the filtering database. In RSTP, the port states are Discarding, Learning, and
Forwarding.
8.3.5.2 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 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 switch with the lowest path cost to the root (the
root path cost). If there is no root port, then this bridge 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.
Table 30 STP Path Costs
LINK SPEED
RECOMMENDE
D VALUE
RECOMMENDE
D 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
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