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Bridges and LAN Switches

Bridges and LAN Switches. Ethernet Backoff revisited. After N collisions, pick a number k between 0 and 2 N -1 Wait for k*51.2 us Send frame if no one has started using the channel. Repeated Collisions. Suppose A, B, and C each have a frame to send, causing a collision

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Bridges and LAN Switches

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  1. Bridges and LAN Switches CS/ECE 438 - UIUC, Fall 2007

  2. Ethernet Backoff revisited • After N collisions, pick a number k between 0 and 2N-1 • Wait for k*51.2 us • Send frame if no one has started using the channel CS/ECE 438 - UIUC, Fall 2007

  3. Repeated Collisions • Suppose A, B, and C each have a frame to send, causing a collision • A picks k=0, B and C pick k=1 • A wins, sends frame • After A is done, B and C both try to send again • Collision again • Increase collision counter CS/ECE 438 - UIUC, Fall 2007

  4. Capture Effect • A and B collide • A picks 0, B picks 1 • A wins, transmits frame • Suppose A has another frame to send • A and B collide again • A’s collision counter is 1, pick k from 0,1 • B’s collision counter is 2, pick k from 0,1,2,3 • A is likely to win again • And keep winning! CS/ECE 438 - UIUC, Fall 2007

  5. Bridges: Building Extended LAN’s • Traditional LAN • Shared medium (e.g., Ethernet) • Cheap, easy to administer • Supports broadcast traffic • Problem • Scale LAN concept • Larger geographic area (> O(1 km)) • More hosts (> O(100)) • But retain LAN-like functionality • Solution • bridges CS/ECE 438 - UIUC, Fall 2007

  6. Bridges • Problem • LANs have physical limitations • Ethernet – 1500m • Solution • Connect two or more LANs with a bridge • Accept and forward • Level 2 connection (no extra packet header) • A collection of LANs connected by bridges is called an extended LAN CS/ECE 438 - UIUC, Fall 2007

  7. Bridges vs. Switches • Switch • Receive frame on input port • Translate address to output port • Forward frame • Bridge • Connect shared media • All ports bidirectional • Repeat subset of traffic • Receive frame on one port • Send on all other ports CS/ECE 438 - UIUC, Fall 2007

  8. Uses and Limitations of Bridges • Bridges • extend LAN concept • Limited scalability • to O(1,000) hosts • not to global networks • Not heterogeneous • some use of address, but • no translation between frame formats CS/ECE 438 - UIUC, Fall 2007

  9. Bridges with Loops • Problem • If there is a loop in the extended LAN, a packet could circulate forever • Side question: Are loops good or bad? • Solution • Select which bridges should actively forward • Create a spanning tree to eliminate unnecessary edges • Adds robustness • Complicates learning/forwarding CS/ECE 438 - UIUC, Fall 2007

  10. Example Extended LAN with LOOPS A B B9 B7 B5 C F D K B2 B1 J E G H B B4 I CS/ECE 438 - UIUC, Fall 2007

  11. Spanning Tree Algorithm • View extended LAN as bipartite graph • LAN’s are graph nodes • Bridges are also graph nodes • Ports are edges connecting LAN’s to bridges • Spanning tree required • Connect all LAN’s • Can leave out bridges CS/ECE 438 - UIUC, Fall 2007

  12. Defining a Spanning Tree • Basic Rules • Bridge with the lowest ID is the root • For a given bridge • A port in the direction of the root bridge is the root port • For a given LAN • The bridge closest to the root (or the bridge with the lowest ID to break ties) is the designated bridge for a LAN • The corresponding port is the designated port • Bridges with no designated ports and ports that are neither a root port nor a designated port are not part of the tree. CS/ECE 438 - UIUC, Fall 2007

  13. D D D R R D D D R D D D R R D D Spanning Tree Algorithm A B Root B9 B7 B5 D – designated port C F D K B2 B1 B1 R – root port J E G H B B4 I CS/ECE 438 - UIUC, Fall 2007

  14. A B B7 B5 C F D K B2 B1 B1 J E G H B4 I Using a Spanning Tree: Forwarding • Forwarding • Each bridge forwards frames over each LAN for which it is the designated bridge or connected by a root port CS/ECE 438 - UIUC, Fall 2007

  15. Finding the Tree by a distributed Algorithm • Bridges run a distributed spanning tree algorithm • Select when bridges should actively forward frames • Developed by Radia Perlman at DEC • Now IEEE 802.1 specification CS/ECE 438 - UIUC, Fall 2007

  16. Distributed Spanning Tree Algorithm • Bridges exchange configuration messages • (Y,d,X) • Y = root node • d = distance to root node • X = originating node • Each bridge records current best configuration message for each port • Initially, each bridge believes it is the root • When a bridge discovers it is not the root, stop generating messages CS/ECE 438 - UIUC, Fall 2007

  17. Distributed Spanning Tree Algorithm • Bridges forward configuration messages • Outward from root bridge • i.e., on all designated ports • Bridge assumes • It is designated bridge for a LAN • Until it learns otherwise • Steady State • root periodically send configuration messages • A timeout is used to restart the algorithm CS/ECE 438 - UIUC, Fall 2007

  18. Spanning Tree Algorithm (5,1,1) A B (5,0,5) (9,0,9) B9 (7,0,7) B7 B5 (9,1,2) (7,1,1) (9,2,1) C F D (2,0,2) K B2 (2,1,1) B1 J (1,0,1) E G H (6,0,6) (4,0,4) B6 B4 (4,1,1) (6,1,1) I CS/ECE 438 - UIUC, Fall 2007

  19. Bridges: Limitations • Does not scale • Spanning tree algorithm scales linearly • Broadcast does not scale • Virtual LANs (VLAN) • An extended LAN that is partitioned into several networks • Each network appears separate • Limits effect of broadcast • Simple to change virtual topology CS/ECE 438 - UIUC, Fall 2007

  20. Bridges: Limitations • Does not accommodate heterogeneity • Networks must have the same address format • e.g. Ethernet-to-Ethernet • Caution • Beware of transparency • May break assumptions of the point-to-point protocols • Frames may get dropped • Variable latency • Reordering • Bridges happen! CS/ECE 438 - UIUC, Fall 2007

  21. Switch • Link layer device • stores and forwards Ethernet frames • examines frame header and selectively forwards frame based on MAC dest address • when frame is to be forwarded on segment, uses CSMA/CD to access segment • transparent • hosts are unaware of presence of switches • plug-and-play, self-learning • switches do not need to be configured CS/ECE 438 - UIUC, Fall 2007

  22. switch hub hub hub Forwarding 1 2 3 • How do determine onto which LAN segment to forward frame? • Looks like a routing problem... CS/ECE 438 - UIUC, Fall 2007

  23. Self learning • A switch has a switch table • entry in switch table: • (MAC Address, Interface, Time Stamp) • stale entries in table dropped (TTL can be 60 min) • switch learns which hosts can be reached through which interfaces • when frame received, switch “learns” location of sender: incoming LAN segment • records sender/location pair in switch table CS/ECE 438 - UIUC, Fall 2007

  24. Filtering/Forwarding • When switch receives a frame: index switch table using MAC dest address if entry found for destination then { if dest on segment from which frame arrived then drop the frame else forward the frame on interface indicated } else flood forward on all but the interface on which the frame arrived CS/ECE 438 - UIUC, Fall 2007

  25. Switch example address interface Suppose C sends frame to D switch 1 1 1 2 3 A B E G 3 2 hub hub hub A I F D G B C H E • Switch receives frame from from C • notes in bridge table that C is on interface 1 • because D is not in table, switch forwards frame into interfaces 2 and 3 • frame received by D CS/ECE 438 - UIUC, Fall 2007

  26. Switch example interface address Suppose D replies back with frame to C. switch 1 1 2 3 1 A B E G C hub hub hub A I F D G B C H E • Switch receives frame from from D • notes in bridge table that D is on interface 2 • because C is in table, switch forwards frame only to interface 1 • frame received by C CS/ECE 438 - UIUC, Fall 2007

  27. switch hub hub hub Switch: traffic isolation • switch installation breaks subnet into LAN segments • switch filters packets: • same-LAN-segment frames not usually forwarded onto other LAN segments • segments become separate collision domains collision domain CS/ECE 438 - UIUC, Fall 2007 collision domain collision domain

  28. Switches: dedicated access A • Switch with many interfaces • Hosts have direct connection to switch • No collisions; full duplex Switching: A-to-A’ and B-to-B’ simultaneously, no collisions C’ B switch C B’ A’ CS/ECE 438 - UIUC, Fall 2007

  29. More on Switches • cut-through switching: frame forwarded from input to output port without first collecting entire frame • slight reduction in latency • combinations of shared/dedicated, 10/100/1000 Mbps interfaces CS/ECE 438 - UIUC, Fall 2007

  30. Institutional network mail server to external network web server router switch IP subnet hub hub hub CS/ECE 438 - UIUC, Fall 2007

  31. Switches vs. Routers • both store-and-forward devices • routers: network layer devices (examine network layer headers) • switches are link layer devices • routers maintain routing tables, implement routing algorithms • switches maintain switch tables, implement filtering, learning algorithms CS/ECE 438 - UIUC, Fall 2007

  32. Summary comparison CS/ECE 438 - UIUC, Fall 2007

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