3Com CoreBuilder 7000, Operation Manual

Page: 143 / 264

Emulated LAN Connections 143
Figure 46 VCCs in LAN Emulation Components
Frame Ordering There are two paths for unicast frames between a sending LAN Emulation
Client and a receiving client: one via the BUS and one via a data direct
VCC between them. For a given LAN destination, a sending client is
expected to use only one path at a time, but the choice of paths may
change over time. Switching between those paths introduces the
possibility that frames may be delivered to the receiving client out of
order. Out-of-order delivery of frames between two LAN endpoints is
uncharacteristic of LANs, and undesirable in an ATM ELAN. The flush
protocol is provided to ensure the correct order of delivery of unicast data
frames.
Flush Protocol
When switching between paths, the sender first transmits a flush
message down the old path and suspends further transmission to that
LAN destination. When the flush message is returned by the receiving
client (via control VCCs), the sender knows that all previous messages for
that LAN destination have been processed and it can start using the new
path.
Figure 47 shows the various stages of the flush protocol.