09-NL-Intro - NetworkLayer: HosttoHostCommunication 1...

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1 Network Layer: Host-to-Host Communication
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2 Network Layer: Motivation Can we built a global network such as Internet by extending  LAN segments using bridges? No! Bridged networks do not scale 4 problems 1. We can only bridge certain link-layer technologies together Link layers to be bridged must have similar MAC address structure 2. Bridge table explosion Bridges use MAC addresses for forwarding and MAC addresses are flat, i.e., not  hierarchical the bridge table needs to have an entry per host in the network  bridge table  explosion!!!
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3 Network Layer: Motivation 3. Robustness Change of network topology requires a new spanning tree computation 4. Link-layer broadcast storms Notice that a bridged network is still a single LAN! A link-layer broadcast packet must still be delivered to ALL hosts in the  network. Can you imagine receiving a link-layer broadcast packet from a host 5000 km  away at your host? Bottom Line:  Bridged/Switched LANs don’t scale!  What’s the solution? --- Next
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4 A B C Bridge E F D Hub Switch L M Hub H I Hub O N Hub K G R1 R2 R3 R4 Network Core Separate LANs Each LAN is a separate  LL Broadcast  Domain Router A collision domain within a LAN Divide the network into  separate  LANs that are NOT part of the same “ LL broadcast ”  domain Connect the LANs using “ routers Notice that we CANNOT use  bridges  to connect separate LANs as bridged LANs form a single  LL broadcast domain, which is what we are trying to avoid to achieve scalability How to achieve scalable growth?
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5 A B C Bridge E F D Hub Switch L M Hub H I Hub O N Hub K G R1 R2 R3 R4 Network Core Separate LANs Each LAN is a separate  LL Broadcast  Domain Router A collision domain within a LAN How do two hosts on separate LANs, e.g., A and E, communicate?
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09-NL-Intro - NetworkLayer: HosttoHostCommunication 1...

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