3Com 3C63400-3AC-C - PathBuilder S700 Switch, Reference Manual, Page: 102 / 272

88 C
HAPTER
4: P
ATH
B
UILDER
S700 M
ODULE AND
A
PPLICATION
O
VERVIEW
The cells are assigned to a virtual circuit defined between the incoming and
outgoing ports based on the destination address of the original packet. A 5-byte
header containing the virtual circuit assignment along with other information is
added to each cell. The cells are queued in the output FIFO (First In First Out)
memory of the Ethernet Module based on the bandwidth and quality of service
requirements assigned to the virtual circuit at configuration. The FIFO provides
elastic storage between the Ethernet Module and the STX module which may be
polling and multiplexing up to 14 Ethernet modules.
Reassembly
When the STX Module receives a cell from the DS3 UNI Module (or other ATM
module), it broadcasts it to all Ethernet Module ports. As cells are received by the
reassembler, their header is read by the CPU to determine if they belong to the
port. If so, the header is stripped and the cell is stored in memory appended to the
previous ones for that virtual circuit.
The CPU also looks for the AAL5 trailer that signifies the end of a packet. When it
finds a trailer, it performs the CRC calculation, checks the length of the cells since
the previous trailer to be sure no cells were missed, strips the trailer, appends the
cell to the others to reform the original packet, and sends the packet through the
Ethernet I/F to the LAN.
Spanning Tree Spanning Tree (IEEE 802.1d) is a technique that detects loops in a network and
logically blocks the redundant paths, ensuring that only one route exists between
any two LANs. It eliminates the duplication of packets and provides fault tolerance
for resilient networks.
As the Spanning Tree is being constructed, bridges exchange information, which is
transmitted in packets called Configuration Bridge Protocol Data Units (C-BPDUs).
During this process, the Spanning Tree Algorithm and Protocol (STAP) module
elects a root bridge in order to establish a stable spanning tree topology. The root
bridge determines the spanning tree topology and controls which bridges block
packets and which forward packets.
Once the topology is stable, all STAP bridges listen for special “Hello” C-BPDUs
transmitted from the root bridge at regular intervals (usually every two seconds). If
a STAP bridge timer expires before receiving a “Hello” C-BPDU, it assumes that the
root bridge, or a link between itself and the root bridge, has gone down. It then
initiates a reconfiguration of the Spanning Tree.
When a port goes down (for instance, when an ATM VC is deleted), the port card
notifies the STAP module of such a change. The STAP module then reinitiates the
process of electing a root bridge, and the Spanning Tree calculation process begins
all over again.