Computer Networks

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