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Computer Networks

Internetworking Devices

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Internetworking Devices• Repeaters

• Hubs

• Bridges– Learning algorithms– Problem of closed loops

• Switches

• Routers

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Repeaters

• Repeaters are purely physical layer devices

• Single collision domain

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Functions of a Repeater

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Shared Hubs• Physical Layer devices: essentially repeaters

operating at bit levels: repeat received bits on one interface to all other interfaces

• Hubs can be arranged in a hierarchy with backbone hub at its top

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Shared Hubs Limitations

• Single collision domain results in no increase in maximum throughput– multi-tier throughput same as single segment

throughput• Cannot connect different Ethernet types (e.g.,

10BaseT and 100baseT)

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Bridges

• Bridges are MAC/Link layer devices operating on Ethernet frames, examining frame header and selectively forwarding frame based on its destination

• Bridge isolates collision domains since it buffers frames

• When frame is to be forwarded on segment, bridge uses CSMA/CD to access segment and transmit

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A Bridge in the OSI model

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LAN Bridges• Isolates collision domains resulting in

higher total maximum throughput, and does not limit the number of nodes nor geographical coverage

• Can connect different type Ethernet since it is a store and forward device

• Transparent: no need for any change to hosts LAN adapters. Hosts do not communicate with bridges

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Bridged LAN Configuration

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Bridged LAN with Multiple Segments

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Bridge Modes of Operation• Filtering: Bridges filter frames if source and

destination hosts are on the same segment! Other segments will not get such frames

• Forwarding: Bridges forward frames if source and destination hosts are on different segments and the bridge knows on which segment is the destination host connected to

• Flooding: Bridges flood frames to all interfaces (except the one it received the frame from) if it doesn’t know where the destination host is

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Learning Bridges

• Bridges learn which hosts can be reached through which interfaces by maintain filtering tables– When a frame received, bridge “learns” location of

sender: incoming LAN segment– Records sender location in filtering table

• Filtering table entries – Host MAC address, Bridge interface, Time stamp– Stale entries in filtering table dropped.

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Example of Learning Bridges • Suppose C sends frame to D and D replies

back with frame to C

• C sends frame, bridge has no info about D, so floods to both interfaces 2 & 3– Bridge learns C is on port 1, add it to its table

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Example (Continued)

• D generates reply to C and sends it – Bridge sees frame from D – Bridge learns D is on interface 2, add to table– Bridge knows C on interface 1, so it forwards

frame out via interface 1 and filter it from interface 3

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Closed Loops

Host A sends a frame to Host B Bridge 1 receives the frame Not knowing where host B is it forwards it to segment 2 Frame goes to its destination B, but at the same time is picked up by Bridge 2 Bridge 2 – Erroneously sees a frame on Segment 2 from Host A so he updates his

tables to include Host A in Segment 2 Because it does not know about Host B it forwards the frame to Segment 1 The frame is then received by Bridge 1 again – and the cycle will repeat itself endlessly Solution: Spanning Tree Algorithm (insure that there would be one and only one path between

any two hosts)

Bridge 1 Bridge 2

Host A

Host B

Segment 1

Segment 2

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Ethernet “Layer 2” Switching

• layer 2 (frame) forwarding/ filtering/flooding based on MAC addresses

• Switching: A-to-B and A’-to-B’ simultaneously, no collisions

• Ethernet but no collisions

• Store & Forward v.s Cut- through Switching

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Example of Ethernet Switching

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Routers• Routers are network-layer devices

• Routers implement routing algorithms and maintain routing tables

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Example of an Internetwork• Routers are used to interconnect arbitrary topologies