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Lecture 13

Lecture 13. Packet Switching: A Network Layer Function. Switches and Hubs. Switches Only forward traffic to a network containing the addressee Hubs Forward traffic to everyone Switched networks scale much better. Broadcast and Multi-cast on Switched Networks.

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Lecture 13

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  1. Lecture 13 Packet Switching: A Network Layer Function

  2. Switches and Hubs • Switches • Only forward traffic to a network containing the addressee • Hubs • Forward traffic to everyone • Switched networks scale much better

  3. Broadcast and Multi-cast on Switched Networks • Broadcast address-how’s it different from ethernet? • Multi-cast address-

  4. LAN Switches and Bridges (Datagram)

  5. Bridges Learn the Port of Each Host! • Hosts must sign in with the bridges to build the routing table—but this is transparent to the users • Routing table starts empty • Addresses not in the table are handled as broadcasts • The first message you send sets up your entry (address and port) in the routing table

  6. Multicast • Bridges have a jointly-held multi-cast address • Members of a multi-cast address send a message using their multicast address to the bridges’ multicast address

  7. An Extended LAN with loops • Broadcast packets loop endlessly

  8. Spanning Tree-Dealing with loops • Idea is to temporarily deselect redundant ports to eliminate loops • Bridges are labeled: B1, B2, B3 etc • Bridge with smallest label becomes the root bridge • Each bridge computes distance to the root for each port—number of hops • Bridge on each LAN with smallest distance to the root becomes a designated bridge-using the port closest to the root

  9. How spanning tree eliminates loops in an extended LAN

  10. Running Spanning Tree • Bridge maintains dynamic values of : • the root id • the minimum distance to the root (in hops) • These numbers are determined by exchanging configuration messages (X,d,Y) • Y=myid • d=minimum distance to root • X=rootid

  11. Initially each bridge thinks itself to be the root and sends----(myid, 0, myid) • Update and resend (incrementing d) if • Receive a message with a smaller rootid • Receive a message with a smaller distance to root • Receive a message with same rootid and distance but smaller sender id

  12. Do not forward traffic from Port A to Port B if a message arrives on Port A with a smaller distance to the root than Port B. (remember ties are broken using smallest ID)

  13. Spanning signaling-an example

  14. Limits of Extended LANs • Spanning tree becomes inoperable for more than 10’s of LANs • Broadcast traffic becomes a burden • VLANs can be used to break up an extended LAN into logical domains • Security

  15. Cell Switching (ATM)

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