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Ethernet Interface
225
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 S600 shelf can be
filled with a maximum of three Ethernet cards, each with two Ethernet ports, a
maximum of six 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 S600 to operate as six 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 S600 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 4.
Addressing
The next bridging operation is to determine if a packet is addressed to another
unit on the LAN (in which case it can be rejected) or if it is addressed to a unit
across the bridge.
Each Ethernet packet includes a source address and a destination address in its
header. These are MAC addresses which are unique physical addresses assigned to
every Ethernet interface on every Ethernet LAN. Packet transmission from one unit
to another on the same LAN is easily accomplished. However, packet transmission
between units on different LANs requires a higher-level addressing scheme.
ATM networks are organized into virtual circuits or logical duplex paths between
two ATM unit ports as shown in Figure 163.
Each transmission direction in a virtual circuit is referred to as a virtual channel.
Virtual channels are then grouped into virtual paths between two ports. The
channels and paths are assigned numbers; VPIs (Virtual Path Indicators) and VCIs
(Virtual Channel Indicators). Each ATM cell (a fixed-length unit of data over ATM)
is assigned to a virtual circuit by including the circuit's VPI/VCI in the cell's header.
These are then used to steer the cell through an ATM unit and the ATM network.
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