Ethernet Interface 225Spanning Tree InstancesThe 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 beset aside for other purposes.)For the purpose of Spanning Tree operation, each set of one Ethernet port and itsassociated ATM VCs is treated as one bridge entity. The STAP module runs aseparate instance for each bridge entity. Since the PathBuilder S600 shelf can befilled with a maximum of three Ethernet cards, each with two Ethernet ports, amaximum of six instances of STAP can be running on the management card.Each STAP instance runs independently of the other STAP instances and processesconfiguration packets as if they came from a different physical bridge device. Thisapproach helps segregate Ethernet ports completely for the purposes of bridgingand Spanning Tree and enables the PathBuilder S600 to operate as six separatebridges.VC-VC Bridging operation The Ethernet module supports full-mesh bridgingbetween its Ethernet port and all of its associated VCs. This bridging must bebidirectional and should be conditional upon the operational status of theSpanning Tree. If Spanning Tree operation is disabled, VC-VC bridging is notnecessary since the PathBuilder S600 is not expected to forward traffic comingfrom the ATM network.For instructions on enabling and disabling the Spanning Tree, see “Enabling andDisabling the Spanning Tree”in Chapter 4.AddressingThe next bridging operation is to determine if a packet is addressed to anotherunit on the LAN (in which case it can be rejected) or if it is addressed to a unitacross the bridge.Each Ethernet packet includes a source address and a destination address in itsheader. These are MAC addresses which are unique physical addresses assigned toevery Ethernet interface on every Ethernet LAN. Packet transmission from one unitto another on the same LAN is easily accomplished. However, packet transmissionbetween units on different LANs requires a higher-level addressing scheme.ATM networks are organized into virtual circuits or logical duplex paths betweentwo 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. Thechannels 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.
