STP Operation
Fixed Switch Configuration Guide 15-3
STP Operation
Enterasys switch devices support the Spanning Tree Protocol (STP), Rapid Spanning Tree Protocol
(RSTP), and Multiple Spanning Tree Protocol (MSTP) as defined in the following standards and
described in IEEE 802.1Q:
• IEEE 802.1D (Spanning Tree Protocol)
• IEEE 802.1w (Rapid Spanning Tree Protocol)
• IEEE 802.1s (Multiple Spanning Tree Protocol)
• IEEE 802.1t (Update to 802.1D)
STP forms a network of bridges connected by LANs into a tree that is:
• Predictable – A given set of configured bridges always yields the same topology when the
network reaches steady state
• Optimized – STP selects the best path to the root bridge
• Fully connected – Each bridge communicates with every other bridge in the network
• Free from data loops – One root port is chosen in a bridge and the remaining ports with paths
to the root bridge are put into blocking mode
The root bridge is the bridge with the lowest bridge ID in the network and functions as the logical
center of the STP network. Each bridge calculates its best path to the root using the information
contained in BPDUs received from its neighbor bridges. Non-root bridges select the root port
among all the ports receiving BPDUs. BPDUs advertise a bridge’s cost to the root bridge. The root
port is chosen from the ports with received BPDUs indicating a path to the root. The root port will
have the lowest cost path to the root. In the case of multiple ports offering identical costs, tie
breaking is based upon the transmitting bridge ID, transmitting port ID, and receive port ID. For
MSTP there are additional fields to consider – internal path cost and regional root ID. These are all
discussed in more detail below.
Once the root port has been established, STP determines the other port roles. Ports providing a
path to root but are not the root port become alternate ports because they provide an alternate
path to the root. Other operational ports that provide a path to the root for attached bridges have
the designated role. There is another type of port known as a backup port. A backup port attaches
to a LAN where another port of the same bridge is a designated port. A backup port does not
become part of the active topology unless the LAN’s designated port is disabled and the backup
port takes over the designated role.
The alternate and backup ports are set to blocking state while the root and designated ports move
to the forwarding state.
Bridge priority, port path cost, and port priority are configurable parameters that are part of the
port role calculation and may be modified to create the desired topology.
Bridge Priority – A typical network configuration would place two or more bridges in the core. To
preserve root in the core, the core bridges would each have their bridge priority set to a lower
value than bridges you do not desire to be root. The default bridge priority value is 32768. If you
desire a particular bridge to be root, set its bridge priority to a lower value than bridges that
should not be root. Otherwise the bridge with the lowest MAC address is set to root.
Port Path Cost – If it is desired for a bridge to use one link over another, the administrative port
path cost may be modified. The default of zero ensures that the link with the highest speed gets
chosen.
Port Priority – Port Priority may be set but is not typically modified, as Link Aggregation is
usually run on multiple links between two bridges.
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