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Ethernet Interface (10BASE-T or 10/100BASE-T)
251
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, the Ethernet port and its associated
ATM VCs is treated as one bridge entity. The spanning tree Algorithm and Protocol
(STAP) module runs a separate instance for each bridge entity.
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 S330/S310 switch is not expected to forward
traffic coming from the ATM network.
For instructions on enabling and disabling the Spanning Tree, see "Configuring
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 that 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 179.
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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