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Communication Networks NETW 501

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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


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