250
A
PPENDIX
B: P
ATH
B
UILDER
S330/S310 M
ODULE
AND
A
PPLICATION
O
VERVIEW
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 S330/S310 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.
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) 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.
Spanning Tree Operation
The Spanning Tree operates as follows:
For more detailed information about Spanning Tree, see IEEE802.1d.
■
An STAP module runs as a task on the management card. This task is
responsible for maintaining all data structures for Spanning Tree operation for
all ports and for sending/receiving Spanning Tree configuration packets.
■
Spanning Tree Bridge configuration Protocol Data Units (BPDUs) are received at
Ethernet port cards (on Ethernet and ATM ports) and are sent to the STAP
module on the management card.
■
The management card STAP module examines the packet, reads the MAC
address of the sender, and analyzes the costs reported by the sender.
■
When the port’s state changes, the STAP notifies the port card of the change.
■
When a port card receives change information for a port, it updates the
operational state of the port so as to change the bridging operation on the
port. For instance, if a port in a forwarding state (normal operation mode of
receiving and forwarding packets) is discovered to be forming a loop, it can be
put in a blocking state. Once the port goes in the blocking state, the port card
will stop receiving frames on the port.
