Filtering; Addressing; Virtual Circuits - 3Com 3C63100-AC-C - PathBuilder S600 Bridge/router Reference Manual

84 C 4: P B
HAPTER ATH UILDER
S600 M A
ODULE AND PPLICATION
(1492) bytes (+ framing bytes) coming off the ATM network. Token ring and FDDI
frames higher than these MTU sizes are dropped and statistics about these frame
drops are reported through VT-100 and SNMP (through bridge MIB).

Filtering

The first bridging operation determines if the packet is to be processed and
transmitted across the bridge or filtered out based on the list of protocols and
addresses input as part of the PathBuilder S600 configuration. Filtering gives you
control over who communicates with whom in the network. The CPU reads the
header of each packet to determine the protocol, source address, and destination
address and then looks in the list to see if the packet should be passed through
the bridge. If it is to be filtered out, it is cleared from memory.

Addressing

The next bridging operation determines 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.
As mentioned above, 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 accomplished
easily. However, packet transmission between units on different LANs requires a
higher-level addressing scheme.

Virtual Circuits

ATM networks are organized into virtual circuits or logical duplex paths between
two ATM unit ports, as shown in Figure 46.
Figure 46 Virtual Circuit Scheme
{
Virtual
VCI 1 (Transmit)
Circuit
VCI 1 (Receive)
{
Virtual
VCI 1 (Transmit)
Circuit
VCI 1 (Receive)
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.
This is then used to steer the cell through an ATM unit and the ATM network.
O
VERVIEW
VPI 1
VPI 2
{
VCIs
VPI 3
{
VCIs
VPI 1
VPI 2
Transmission
Path
Transmission
Path