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Computer Networks

Computer Networks. LAN Bridges and Switches. Where are we?. Recall. LANs have physical distance limitations Performance suffers when LAN utilization increases Separate LANs may eventually want to connect to each other. Motivation. Users require arbitrary distance connections

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Computer Networks

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  1. Computer Networks LAN Bridges and Switches

  2. Where are we?

  3. Recall • LANs have physical distance limitations • Performance suffers when LAN utilization increases • Separate LANs may eventually want to connect to each other

  4. Motivation • Users require arbitrary distance connections • Example: 2 computers across a corporate campus are part of one workgroup • May not want to forward all transmissions to all workgroups for performance or security reasons • May want to avoid a single point of failure (redundancy/reliability) The books: Interconnections - Radia Perlman, The Switch Book - Rich Seifert

  5. LAN Bridges/Switches • A hardware device with minimal software • Connects 2 or more LANs together • Generally same LAN types are connected • Forwards frames between connected LANs • Does not forward collisions, noise, beacons, etc. • Examines data link layer information • Allows each LAN to operate independently

  6. Bridge/Switch Operation • Listen to all LANs in promiscuous mode • Only move frames between LANs if necessary • Only act on layer 1/2 information

  7. Connections

  8. Transparent Bridging Illustrated

  9. Transparent Bridging Rules • 1. Watch all frames on each LAN • 2. For each frame, store the source address in a cache along with the associated LAN the frame arrived on (bridge table) • 3. For each frame, the cache is queried for the destination address • a. If found, the frame is forwarded to the LAN associated with the address, unless its the LAN the frame arrived on (filtered) • b. If not found, the frame is forwarded to all LAN interfaces except the one on which the frame arrive (flooding) • Transparent bridges make all the forwarding decisions, end stations don’t even know the bridge is there!

  10. Bridge Address Table

  11. Bridging between dissimiliar LANs • Access methods • Ethernet, Token Ring, FDDI • Frame formats • New fields, non existant fields • MTU • FCS • Bit ordering Don't bother doing this, use routers!

  12. Will This Work?

  13. Introducing Spanning Tree • Allow a path between every LAN without causing loops (loop-free environment) • Bridges communicate with special configuration messages (BPDUs) • Standardized by IEEE 802.1D Note: redundant paths are good, active redundant paths are bad (they cause loops)

  14. Spanning Tree Requirements • Each bridge is assigned a unique identifier • Consists of the MAC address and a priority • A group address for bridges on a LAN • A unique port identifier for all ports on all bridges

  15. Spanning Tree Concepts: Root Bridge • The bridge with the lowest bridge ID value is elected the root bridge • One root bridge chosen among all bridges • Every other bridge calculates a path to this root bridge

  16. Spanning Tree Concepts:Path Cost • Associated with each port on each bridge • The cost associated with transmission onto the LAN connected to the port • Can be manually or automatically assigned • Can be used to alter the path to the root bridge

  17. Spanning Tree Concepts:Root Port • The port on each bridge that is on the path towards the root bridge • The root port is part of the lowest cost path towards the root bridge • If port costs are equal on a bridge, the port with the lowest ID becomes root port

  18. Spanning Tree Concepts:Root Path Cost • The minimum cost path to the root bridge • The cost starts at the root bridge • Each bridge computes root path cost independently based on their view of the network

  19. Spanning Tree Concepts: Designated Bridge • Only one bridge on a LAN at one time is chosen the designated bridge • This bridge provides the minimum cost path to the root bridge for the LAN • Only the designated bridge passes frames towards the root bridge

  20. Spanning Tree Concepts:Illustrated

  21. Spanning Tree Concepts:Illustrated [continued]

  22. Spanning Tree Algorithm:An Overview • 1. Determine the root bridge among all bridges • 2. Each bridge determines its root port • The port in the direction of the root bridge • 3. Determine the designated port on each LAN • The port which accepts frames to forward towards the root bridge

  23. Spanning Tree Algorithm:Selecting Root Bridge • 1. Initially, each bridge considers itself to be the root bridge • 2. Bridges send BDPU frames to its attached LANs • a. The bridge and port ID of the sending bridge • b. The bridge and port ID of the bridge the sending bridge considers root • c. The root path cost for the sending bridge • 3. Best one wins (lowest ID/cost/priority)

  24. Spanning Tree Algorithm:Selecting Root Ports • Each bridge selects one of its ports which has the minimal cost to the root bridge • In case of a tie, the lowest uplink (transmitter) bridge ID is used • In case of another tie, the lowest port ID is used

  25. Spanning Tree Algorithm:Select Designated Bridges • 1. Initially, each bridge considers itself to be the designated bridge • 2. Bridges send BDPU frames to its attached LANs • a. The bridge and port ID of the sending bridge • b. The bridge and port ID of the bridge the sending bridge considers root • c. The root path cost for the sending bridge • 3. Best one wins (lowest ID/cost/priority)

  26. Forwarding/Blocking State • Root and designated ports will forward frames to and from their attached LANs • All other ports are in the blocking state

  27. Configuration Messages

  28. Bridge Encapsulation

  29. Source Route Bridging • Used in token ring environments • Alternative to transparent bridging • Bridge loops can exist • Defined by IBM and standardized by IEEE 802.5 • Intelligence moves from bridges to end stations

  30. Source Routing Bridging

  31. Source Route Destinations • Null - destination on the same LAN • Non-broadcast - includes a route to destination • All routes broadcast - flooded to each LAN, bridges record route along the way • Single route broadcast - only one frame per LAN, spanning tree used

  32. Route Discovery • Transmit "all-route” broadcast to destination • Destination sends non-broadcast response to the first frame received (using that route) • Transmit "single-route” broadcast to destination • Destination sends back an all-route broadcast response • Sender picks the first response received from destination Routes can also be manually configured on stations

  33. Source Route Discovery:Illustrated

  34. Routing Information Field • If bit 0 of byte 0 in the source address is set to 1, then this frame is a source routed frame

  35. Bridge Filters • Useful for controlling LAN traffic • Examines data link layer information • Examples • Do not forward frames from MAC address X • Do not forward Ethernet frames of type X • Do not forward broadcast frames from X • Limit source route hops to 6

  36. Switches • Physically similar to hubs • Logically similar to bridges • Takes advantage of improvements in ASIC technology • Permits full duplex operation • Quickly replacing hub/bridge technology • The name switch is a marketing gimmick

  37. Inside a Switch • Conceptual operation • One LAN segment per host • Bridge interconnects each host/segment

  38. Switches: Final Notes • Store and Forward • Cut-through • Mixing interfaces • Network Management Issues • Port Mirroring • Security

  39. Virtual LANs - An Introduction • Defines a broadcast domain on switches • Only difference from LAN is the packaging • To move between VLANs, you need a route (layer 3 device) • Why have separate VLANs?

  40. VLANs Illustrated

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