Application Overview
85
Figure 47 illustrates virtual circuits on a simple network using PathBuilder S600s.
Figure 47
Virtual Circuits in a Simple Network
Suppose workstation 1 on LAN AA wants to send data to workstation 2 on LAN
BB. It transmits Ethernet packets which include its MAC address and that of
workstation 2. PathBuilder S600 A groups the packets into cells and, since it does
not know where workstation 2 is, broadcasts the cells on all VPI/VCIs.
PathBuilder S600 B learns that workstation 1 transmits to its slot 4 port 2 over
VPI/VCI 1/33 and to slot 5 port 2 over VPI/VCI 2/35. This information is then stored
in forwarding tables in PathBuilder S600 B.
When workstation 2 responds, it includes its MAC address and that of workstation
1 in the packets. PathBuilder S600 B looks in the appropriate forwarding table and
finds that it can reach workstation 1 on VPI/VCI 2/5. When the first response cell
on 2/35 is formatted back into packets at PathBuilder S600 A, PathBuilder S600 B
learns how to reach workstation 2 without broadcasting.
Learning Bridge
The aggregate of VPIs/VCIs assigned to a slot and port (Ethernet connection) of
the PathBuilder S600 is referred to as a bridge. Since the PathBuilder S600 reads
and stores MAC addresses and associated VPIs/VCIs as described above, the bridge
is called a learning bridge. Each learning bridge of the PathBuilder S600 has a
separate forwarding table containing the MAC addresses to VPI/VCI associations.
When a learned entry is stored in a forwarding table, it is time tagged. An aging
timer in the PathBuilder S600, which you can set, purges entries from the
forwarding tables after the specified time. The bridges must then relearn purged
addresses. This controls the size of the forwarding tables, particularly in large
networks.
In the PathBuilder S600, each bridge also has a static table associated with it. You
can enter MAC addresses and corresponding PVCs into the static table that you do
not want to be affected by the aging timer. These entries might be for LAN units
LAN
AA
LAN
AB
LAN
AA
LAN
BB
VPI/VCI
2/35
VPI/VCI 1/33
S4P2
S5P2
PathBuilder S600
A
S4P1
S5P1
DS3 UNI
Workstation 1
PathBuilder S600
PathBuilder
S600
Workstation 2
Summary of Contents for 3C63100-AC-C - PathBuilder S600...
Page 8: ...INDEX 3COM CORPORATION LIMITED WARRANTY ...
Page 14: ...xiv CHAPTER SUPPLEMENTARY REGULATORY INFORMATION ...
Page 18: ...18 ABOUT THIS GUIDE ...
Page 28: ...28 CHAPTER 1 SYSTEM DESCRIPTION ...
Page 74: ...74 CHAPTER 3 GETTING STARTED ...
Page 230: ...230 INDEX ...