Network Devices Repeaters, hubs, bridges, switches, routers, NICs.
1 Computer Networks Internetworking Devices. 2 Repeaters Hubs Bridges –Learning algorithms...
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Transcript of 1 Computer Networks Internetworking Devices. 2 Repeaters Hubs Bridges –Learning algorithms...
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Internetworking Devices• Repeaters
• Hubs
• Bridges– Learning algorithms– Problem of closed loops
• Switches
• Routers
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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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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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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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Routers• Routers are network-layer devices
• Routers implement routing algorithms and maintain routing tables