1. Communication Networks NETW 501
Lecture 10
Interconnecting Networks: Hubs and Bridges
Course Instructor: Dr.-Ing. Maggie Mashaly
C3.220

2. Interconnecting Networks
There are several ways of interconnecting networks:
Hubs/Repeaters
Interconnecting networks at the physical layer
Bridges
Interconnecting networks at the MAC or data link layer
Routers
Interconnecting networks at the Network layer
Gateway
Interconnecting networks at Higher layers

3. Hubs
LAN
Physical layers devices (essentially Repeaters) operating at the bit level
A hub repeats received bits on one interface to all other interfaces
Hubs can be arranged in a hierarchy
Each connected LAN is referred to as a LAN segment
Hubs do not isolate collision domains (i.e., A station may collide with any node residing at any segment in LAN)
Hub

4. Hub Advantages and Limitations
Simple, inexpensive device
Multi-tier provide graceful degradation (i.e., portions of the LAN continue to operate if one hub malfunctions
Extends minimum distance between node pairs (100 meters per hub)
Single collision domain results in no increase in maximum throughput
Individual LAN restrictions pose limits on number of nodes in same collision domain and on total allowed geographical coverage
Cannot connect different LAN technologies

5. Bridges Link Layer devices operate on data link layer frames
A bridge examines frame header and selectively forwards frame based on its destination
Bridges filter packets
Same-LAN -segment frames not forwarded onto other LAN segments
When frame is to be forwarded on segment, bridge uses CSMA/CD to access segment and transmit
If most traffic is local the load on each LAN will be reduced. If a repeater is used both LANs will be busy at transmission intervals of any given node

6. Bridge Advantages LAN Extension Isolates collision domains
Example: Bridges may address Interconnecting departmental LANs problems
The departments use different LAN technologies
Different buildings
Different network layers
Isolates collision domains
Resulting in higher total maximum throughput
Does not limit the number of nodes nor geographical coverage

7. Transparent Bridges
Stations are completely unaware about the existence of the bridge
Transparent bridge functions:
Forwards frames from one LAN to another
Learns where stations are attached to bridges
When a bridge receives a frame it has to make a decision to forward or not:
Each bridge has a forwarding table (forwarding data base) to determine towards the direction of which bridge port is the required destination attached.
How to fill forwarding table?
Network administrator
Not desirable as it requires manual intervention of administrator when a station is moved from one LAN to another or if a new station or even a new LAN is added
Bridge Learning

8. Bridge Learning Bridge receives a frame Is Source Address
in forwarding table No
Update forwarding table with source address and port on which the frame was received
Yes
Is destination Address
in forwarding table No
Yes
Flood on all ports except port of arrival
Forward frame on port indicated unless it is port of the arrival

9. Bridge Learning
Bridge B1
Address
Port
Bridge B2
Address
Port

10. Bridge Learning: S1  S5 Bridge B1 Address Port S1 1 Bridge B2 Address

11. Bridge Learning: S3  S2 Bridge B1 Address Port S1 1 S3 2 Bridge B2

12. Bridge Learning: S4  S3 Bridge B1 Address Port S1 1 S3 2 S4 Bridge B2

13. Bridge Learning: S2  S1 Bridge B1 Address Port S1 1 S3 2 S4 S2

14. Adapting to LAN Dynamics
Stations may be added to a LAN or moved from one LAN to another
How do bridges adapt to network dynamics?
When a frame is received on a different port than that indicated for the source address in the forwarding table, the corresponding entry is updated
A timer is associated with each entry
When timer expires the entry is erased
If a frame is received with the source address already in the table, the entry is refreshed

15. Why allow network with loops?
Bridging Loops
Bridging loops can result in inaccurate forwarding and learning in transparent bridging environments
Example: Host A sends a frame to Host B
Host B receive two copies of the frame
Each bridge now believes that Host A resides on the same segment as Host B.
When Host B replies to Host A's frame, both bridges will receive and subsequently filter the replies because the bridge table will indicate that the destination (Host A) is on the same network segment as the frame's source (i.e., Host B).
Host A
LAN 2
Bridge 1
Bridge 2
LAN 1
Host B
Why allow network with loops?
Redundancy can help increase network fault tolerance

16. Solution: Spanning Tree Algorithm
Is an algorithm that builds a loop-free logical topology for networks by defining the following entities:
Root Bridge: Let the root bridge be the bridge with the lowest bridge ID
Root Port: The port with the least cost path to the root bridge. In case of ties the root port is the minimum port number
Designated bridge: is the bridge that offers the least-cost path from LAN to root bridge. In case of ties the designated bridge is the minimum bridge ID. The port that connects to the LAN is the designated port

17. Spanning Tree Algorithm (Example)
Bridge B1 is the root bridge
LAN 1
(1)
(1) B1 B2
(1)
(2)
LAN 2
(2)
(3) B3
(1)
(2) B4
(2)
LAN 3
(1) B5
(2)
LAN 4

18. Spanning Tree Algorithm (Example)
Root Ports (Cost is Number of LANs in the path) B1 B2 B4 B3 B5
LAN 1
LAN 2
LAN 3
LAN 4
(1)
(2)
(3) R

19. Designated Bridges and Ports (Cost is Number of LANs in the path)
Spanning Tree Algorithm (Example)
Designated Bridges and Ports (Cost is Number of LANs in the path)
LAN 1
(1) D R
(1) B1 B2 R
(1)
(2) D
LAN 2
(2)
(3) B3 D R
(1) D
(2) B4
(2)
LAN 3 R
(1) B5
(2)
LAN 4

20. Set the root and designated ports only in forwarding state
Spanning Tree Algorithm (Example)
Set the root and designated ports only in forwarding state
LAN 1
(1) D R
(1) B1 B2 R
(1)
(2) D
LAN 2
(2)
(3) B3 D R
(1) D
(2) B4
(2)
LAN 3 R
(1) B5
(2)
LAN 4

21. Spanning Tree Algorithm (Example)
After spanning tree the network effectively follows the topology shown below B1
LAN 2
LAN 1 B3
LAN 3
LAN 4

22. References NETW 501 Lectures slides by Assoc. Prof. Tallal El-Shabrawy
“Communication Networks 2nd Edition”, A. Leon-Garcia and I. Widjaja, McGraw Hill, 2013
“Computer Networks 4th Edition”, A. S. Tanenbaum, Pearson International