1. Chapter 15. Connecting Devices
Backbone networks
Virtual LANs
Spring 2010
Communication Networks, KWU

2. Communication Networks, KWU
Connecting Devices
number of ports
layer
repeater 2 1
hub n
bridge
L2 switch
router or
L3 switch 3
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Communication Networks, KWU

3. Communication Networks, KWU
Repeater
A repeater connects segments of a LAN
A repeater forwards every frame; it has no filtering capability
The repeater is a two-port device
Spring 2010
Communication Networks, KWU

4. Communication Networks, KWU
Function of a Repeater
A repeater connects segments of a LAN
A repeater is a regenerator, not an amplifier
A repeater forwards every frame; it has no filtering capability.
Spring 2010
Communication Networks, KWU

5. Communication Networks, KWU
Hubs
A hub is a multiport repeater
It is normally used to create connections between stations in a star topology
Hubs can also be used to create multiple levels of hierarchy
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Communication Networks, KWU

6. Communication Networks, KWU
Repeaters/Hubs
Spring 2010
Communication Networks, KWU

7. Communication Networks, KWU
Bridges
A bridge has a table used in filtering decisions
A bridge does not change the physical (MAC) address in a frame
Spring 2010
Communication Networks, KWU

8. Communication Networks, KWU
Bridges
Transparent bridges
A bridge in which the stations are completely unaware of the bridge’s existence
Three criteria for a transparent bridge
Frames must forward from one station to another
The forwarding table is automatically made by learning frame movements in the network
Loops in the system must be prevented
Source routing bridges
A sending station defines the bridges that the frame must visit
Not very common today
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Communication Networks, KWU

9. Transparent Bridges: Learning
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Communication Networks, KWU

10. Transparent Bridges: Loop Problem
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Communication Networks, KWU

11. Transparent Bridges: Spanning Tree
Spanning tree is a graph in which there is no loop
To solve the looping problem, IEEE spec requires that bridges use the spanning tree algorithm
Select the root bridge
The one with the smallest ID
Select the root port of each bridge
The port with the least-cost path from the bridge to the root bridge
Choose a designated port for each LAN
The bridge with the least-cost path from the LAN to the root bridge
The corresponding port is the designated port
Spring 2010
Communication Networks, KWU

12. Spanning Tree Algorithm: Example
★ root port
★★ designated port
Spring 2010
Communication Networks, KWU

13. Layer 2 (Two-layer) Switch
Layer 2 switch is an N-port bridge
Ethernet switch or LAN switch
Switched Ethernet (←)
Full-duplex switched Ethernet (↓)
Spring 2010
Communication Networks, KWU

14. Communication Networks, KWU
Layer 2 and Layer 3 Switch
Layer 2 Switching
Switching based on MAC destination address
Builds switching table by “learning” host addresses from source addresses of incoming packets
Unknown destination addresses are flooded out other ports
Broadcast frames are flooded out other ports
Routing
Switching based on IP (network layer) destination address.
Builds routing table by neighbor routers using routing protocols
Unknown IP packets are discarded
Broadcast frames are discarded
Layer 3 switching
Hardware-based routing
Spring 2010
Communication Networks, KWU

15. Broadcast and Collision Domains
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Communication Networks, KWU

16. Communication Networks, KWU
LAN Segmentation
Spring 2010
Communication Networks, KWU