This document discusses different types of connecting devices used in computer networks, including repeaters, bridges, routers, and switches. It also describes how to connect local area networks (LANs) using backbone networks in either a bus or star topology. Finally, it introduces the concept of virtual LANs (VLANs), which use software rather than physical wiring to configure network membership and communication between switches, providing advantages such as reduced cost, virtual work groups, security, and separated broadcast domains.

1. Connecting LANs,
Backbone Networks,
and Virtual LANs

2. 15.2
CONNECTING DEVICES
In this section, we divide connecting devices into five
different categories based on the layer in which they
operate in a network.
Passive Hubs
Active Hubs
Bridges
Two-Layer Switches
Routers
Three-Layer Switches
Gateways

3. 15.3
Five categories of connecting devices

4. 15.4
A repeater connecting two segments of a LAN

5. 15.5
A repeater connects segments of a LAN.
Note

6. 15.6
A repeater forwards every frame;
it has no filtering capability.
Note

7. 15.7
A repeater is a regenerator,
not an amplifier.
(same signal strength, bit for bit copy)
Note

8. 15.8
Figure 15.3 Function of a repeater

9. 15.9
Figure 15.4 A hierarchy of hubs

10. 15.10
A bridge checks MAC addresses. It has
a table used in filtering decisions
(Forward? Drop?). The table shows the
map between MAC addresses and ports.
Note

11. 15.11
Figure 15.5 A bridge connecting two LANs

12. 15.12
A bridge does not change the physical
(MAC) addresses in a frame.
Note

13. 15.13
Figure 15.6 A learning bridge and the process of learning

14. 15.14
Figure 15.7 Loop problem in a learning bridge

15. 15.15
Use spanning three in graph theory to
avoid loop topology. Spanning tree is a
graph without loop.
Note

16. 15.16
Figure 15.11 Routers (three-layer switch)
connecting independent LANs and WANs

17. 15.17
15-2 BACKBONE NETWORKS
A backbone network allows several LANs to be
connected. In a backbone network, no station is
directly connected to the backbone; the stations are
part of a LAN, and the backbone connects the LANs.
Bus Backbone
Star Backbone
Connecting Remote LANs
Topics discussed in this section:

18. 15.18
In a bus backbone, the topology
of the backbone is a bus.
Note

19. 15.19
Figure 15.12 Bus backbone

20. 15.20
In a star backbone, the topology of the
backbone is a star;
the backbone is just one switch.
Note

21. 15.21
Figure 15.13 Star backbone

22. 15.22
Figure 15.14 Connecting remote LANs with bridges

23. 15.23
15-3 VIRTUAL LANs
We can roughly define a virtual local area network
(VLAN) as a local area network configured by
software, not by physical wiring.
Membership
Configuration
Communication between Switches
IEEE Standard
Advantages
Topics discussed in this section:

24. 15.24
Figure 15.15 A switch connecting three LANs
Physical wiring makes it hard to
dynamically change group allocation

25. 15.25
Figure 15.16 A switch using VLAN software

26. 15.26
Figure 15.17 Two switches in a backbone using VLAN software
Good for a company with two separate buildings

27. 15.27
VLANs create broadcast domains.
Note

28. VLAN: Membership
 Switch port Numbers
 MAC Address
 IP Address
 Multicast IP Address
 Combination
15.28

29. VLAN Configuration
 Manually
 Automatic
 Semiautomatic
15.29

30. VLAN: Communication between
switches
 Table maintenance
 Frame tagging
 Extra header added to MAC frame
 Time-Division Multiplexing (TDM)
15.30

31. VLAN: Advantages
 Cost and time reduction
 Creating Virtual Work Groups
 Security
 Separation of broadcast messages
15.31