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Layer 3 Switching

Layer 3 Switching. Routers vs Layer 3 Switches. Both forward on the basis of IP addresses But Layer 3 switches are faster and cheaper However, Layer 3 switches are more limited Usually do not support WAN connections and only Ethernet for LANs

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Layer 3 Switching

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  1. Layer 3 Switching

  2. Routers vs Layer 3 Switches • Both forward on the basis of IP addresses • But Layer 3 switches are faster and cheaper • However, Layer 3 switches are more limited • Usually do not support WAN connections and only Ethernet for LANs • Usually support only TCP/IP and sometimes IPX/SPX, not SNA, AppleTalk, etc. • There are other differences as well

  3. Routers • Forward each packet independently • Ignore previous IP packets to the same destination • Make a full router forwarding decision (Ch. 3) for each arriving IP packet • This takes a great deal of processing time • This makes routers slow and inexpensive • However, this allows routers to adjust rapidly to failures

  4. Layer 3 Switches • Do NOT forward each packet independently • For instance decision caching; Remember previous decisions based on IP address • No need to make a full router forwarding decision (Ch. 3) for each arriving IP packet • This saves a great deal of processing time • This makes Layer 3 switches fast and inexpensive • However, this may make failure recovery slow

  5. Routers • IP being extended with MultiProtocol Label Switching (MPLS) • A label is added to each packet • Packets that should follow the same route get the same label value • Router learns output port for each label value • When packet arrives, send it back out the correct port without a full router forwarding decision using routing tables

  6. Interface Interface Routers • Traditionally used a Bus Architecture • Interfaces are connected to the processor via a serial bus like the bus in a PC • The processor and the bus can become bottlenecks Processor Bus

  7. Interface Interface Routers • Traditionally used a Bus Architecture • Busses only handle one transmission at a time • Processors only handle one packet at a time • Problem if there are several interfaces Processor Bus

  8. Interface Interface Routers • Traditionally used a Bus Architecture • Each packet has to pass over the bus twice • From the incoming interface to the processor • From the processor to the outgoing interface • This ties up the router for two bus cycles Processor Bus

  9. Layer 3 Switches • Interfaces are intelligent • Make forwarding decisions independently • So no single processor bottleneck Decision Interface Interface IP Packet Switching Matrix Interface Interface

  10. Layer 3 Switches • Interfaces are Intelligent • Can handle forwarding decisions simultaneously, reducing processing delays Decision Interface Interface IP Packet Switching Matrix Decision Interface Interface

  11. Layer 3 Switches • Interfaces are Connected by a Switching Matrix • Forward packet within frame directly to outgoing interface • No bus Interface Interface IP Packet Switching Matrix Interface Interface

  12. Layer 3 Switches • Interfaces are Connected by a Switching Matrix • Can handle multiple frame forwardings simultaneously Interface Interface Switching Matrix IP Packets Interface Interface

  13. Routers • Traditionally Did all Processing in Software • Software is slow • Many commands may be needed to execute router forwarding decisions • Must be executed one at a time • These will take a long time to execute • In contrast, hardware is fast • Many processes can be executed in parallel • Reduces number of cycles

  14. Layer 3 Switches • In some Layer 3 switches, ASICs handle some or all processing • Application-Specific Integrated Circuits • Designed for particular applications, such as routing • Much faster than software • Can only handle processes of medium complexity • Layer 3 switching’s reduced protocol support allows ASICs to be used there

  15. Layer 3 Switches • ASIC Technology is Receiving Competition from Network Processors • ASICS are purely hardware • Network processors are programmable but have hardware optimized for network functions • Network processors are slower than ASICs but much faster than software processes on general routers • Network processors can be programmed for specific functions less expensively than ASICs can be built

  16. Routers • Adopting Many Speed Switch Innovations • Some use decision caching • MPLS is already being used somewhat • Switching matrices • Uses some ASICs in processing; Will use more as ASIC technology becomes able to support more processing • Limited protocol support may become the only enduring Layer 3 switch distinction

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