Supermicro SuperBlade SBM-GEM-X2C, Руководство пользователя

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Superblade Network Modules User’s Manual
20
6-10 Spanning Tree
The Spanning Tree Protocol (STP) helps to detect and prevents loops from occurring on
a switched or bridged network. When multiple paths exist on a network, STP will
configure the network to use the most efficient path between network devices. All other
paths are forced into a blocked standby state. If the active path fails, then STP will
automatically select another path to become the active path on the network to sustain
normal network operations. An active path is selected by comparing path costs defined
on each path. The path with the lowest cost will be selected.
The switch supports IEEE802.1d Spanning Tree Protocol and IEEE802.1w Rapid
Spanning Tree Protocol (RSTP). The Rapid Spanning Tree Protocol significantly
reduces the convergence time by assigning port roles and by determining the active
topology. A reconfiguration of the spanning tree can occur in less than one second. The
RSTP is backward compatible with legacy devices running IEEE802.1d STP and serves
as an STP device when an STP device is present in the network.
Bridge Protocol Data Unit (BPDU)
The spanning tree is built by obtaining switch information by exchanging Bridge Protocol
Data Unit (BPDU) packets among the participating switches. When RSTP is enabled for
a switch, it will generate a BPDU and periodically forward it out through each port on the
switch. The interval is configurable through the Hello Time, which is set to a two second
default. This enables the switch to keep track of network topology changes and enable
or disable ports as required.
The BPDU contains the information about the transmitting switch and its ports including
MAC address, bridge priority, port priority and port path cost. The BPDU packet is sent
out by using the unique MAC address of the port itself as a source address, and the
destination address of the STP multicast address 01:80:C2:00:00:00.
There are three types of BPDUs:
• Configuration BPDU – for spanning tree computation
• Topology Change Notification (TCN) BPDU – announces changes in network
topology.
• Topology Change Notification Acknowledge (TCA) BPDU
The major operation of the spanning tree protocol includes a root bridge election, finding
paths to a root bridge, determining the least cost path to root and disabling all other root
paths. When a RSTP enabled switch is turned on, it automatically assumes that it is the
root bridge in the spanning tree. The software in the switch will elect a switch as the root
bridge based on the Bridge ID in the received BPDU. The Bridge ID is an 8-byte field
which combines a high order two-byte bridge priority number and a lower order six-byte
switch MAC address. The switch with the lowest Bridge ID will be elected as the root
bridge.
All RSTP participating switches will use an algorithm to determine how close they are to
the root bridge, which is known as Path Cost. The path with lowest cost will be selected
as the active path. All others will be blocked (standby). TCN packets are injected into the
network by a non-root switch and propagated to the root. Upon receipt of the TCN, the