3Com 3C63400-3AC-C - PathBuilder S700 Switch, Reference Manual

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Supported Applications 89
Spanning Tree Operation
The Spanning Tree operates as follows:
For more detailed information about how the Spanning Tree operates, see
IEEE802.1d.
n
An STAP module runs as a task on the management card. This task is
responsible for maintaining all data structures for Spanning Tree operation for
all ports and for sending/receiving Spanning Tree configuration packets.
n
Spanning Tree Bridge configuration Protocol Data Units (BPDUs) are received at
Ethernet port cards (on Ethernet and ATM ports) and are sent to the STAP
module on the management card.
n
The management card STAP module examines the packet, reads the MAC
address of the sender, and analyzes the costs reported by the sender.
n
When state of a port changes, the STAP module notifies the port card of the
change.
n
When a port card receives change information for a port, it updates the
operational state of the port so as to change the bridging operation on the
port. For instance, if a port in a forwarding state (normal operation mode of
receiving and forwarding packets) is discovered to be forming a loop, it can be
put in a blocking state. Once the port goes in the blocking state, the port card
will stop receiving frames on the port.
Spanning Tree Instances
The Spanning Tree logic supports a maximum of 255 physical and virtual ports,
thereby allowing a maximum of 254 ATM VCs. (One Ethernet port is required be
set aside for other purposes.)
For the purpose of Spanning Tree operation, each set of one Ethernet port and its
associated ATM VCs is treated as one bridge entity. The STAP module runs a
separate instance for each bridge entity. Since the PathBuilder S700 shelf can be
filled with a maximum of 14 Ethernet cards, each with two Ethernet ports, a
maximum of 28 instances of STAP can be running on the management card.
Each STAP instance runs independently of the other STAP instances and processes
configuration packets as if they came from a different physical bridge device. This
approach helps segregate Ethernet ports completely for the purposes of bridging
and Spanning Tree and enables the PathBuilder S700 to operate as 28 separate
bridges.
VC-VC Bridging operation The Ethernet module supports full-mesh bridging
between its Ethernet port and all of its associated VCs. This bridging must be
bidirectional and should be conditional upon the operational status of the
Spanning Tree. If Spanning Tree operation is disabled, VC-VC bridging is not
necessary since the PathBuilder S700 is not expected to forward traffic coming
from the ATM network.
For instructions on enabling and disabling the Spanning Tree, see “Enabling and
Disabling the Spanning Tree” in Chapter 5.