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Chapter 54 Spanning Tree Protocol
XGS2220 Series User’s Guide
379
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.
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.
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.
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.
Once a stable network topology has been established, all bridges listen for Hello BPDUs (Bridge Protocol
Data Units) transmitted from the root bridge. If a bridge does not get a Hello BPDU after a predefined
interval (Max Age), the bridge assumes that the link to the root bridge is down. This bridge then initiates
negotiations with other bridges to reconfigure the network to re-establish a valid network topology.
STP Port States
STP assigns five port states to eliminate packet looping. A bridge port is not allowed to go directly from
Table 185 STP Path Costs
LINK SPEED RECOMMENDED VALUE RECOMMENDED RANGE ALLOWED RANGE
Path Cost 4 Mbps 250 100 to 1000 1 to 65535
Path Cost 10 Mbps 100 50 to 600 1 to 65535
Path Cost 16 Mbps 62 40 to 400 1 to 65535
Path Cost 100 Mbps 19 10 to 60 1 to 65535
Path Cost 1 Gbps 4 3 to 10 1 to 65535
Path Cost 10 Gbps 2 1 to 5 1 to 65535
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