Computer Networks

1. Computer Networks Data link layer Data link layer -- June 2004

2. Design issues Point-to-point links Local area Networks Data Link layer Switching Multiple LANs? From 802.x to 802.y Transparent bridges Spanning Tree bridges Remote bridges Interconnections <> layers Virtual LANs Overview Data link layer -- June 2004

3. Multiple LANs? • Why different LANs in a single organisation? • Autonomy of owners • Distance too great for a single LAN • Load too high for a single LAN • Reliability: a single defective node will cripple onle a single LAN • Security: most LANs offer a promiscuous mode (receive all packets) • Solution at data link layer: Bridge switch Data link layer -- June 2004

4. From 802.x to 802.y • How do bridges work? Data link layer -- June 2004

5. Bridges: from 802.x to 802.y • Common problems: different • data rates • maximum frame length • security • quality of service • frame formats Data link layer -- June 2004

6. Design issues Point-to-point links Local area Networks Data Link layer Switching Multiple LANs? From 802.x to 802.y Transparent bridges Spanning Tree bridges Remote bridges Interconnections <> layers Virtual LANs Overview Data link layer -- June 2004

7. Transparent bridges • Goal: interconnect LANs with a bridge and everything should work perfectly, instantly • No software changes • No hardware changes • No setting of address switches • No downloading of routing tables • How?Promiscuous mode receive packet… • Source LAN = destination LAN  discard frame • Destination LAN unknown  flooding (on LAN level) • Destination LAN known  forward frame • Learn about LANs?Backward learning: construct routing table • Initially empty • Receive frame from node A via LAN L  A reachable via L • Dynamic topologies: attach timestamp to entries Data link layer -- June 2004

8. C D Transparent bridges A L1 L2 Bridge frame B L3 Data link layer -- June 2004

9. C D Transparent bridges A L1 L2 Bridge B L3 Data link layer -- June 2004

10. C D Transparent bridges A L1 L2 Bridge B L3 Data link layer -- June 2004

11. Spanning Tree bridges • Bridges in parallel • To increase reliability  loops in topology • To increase forwarding capacity Problem • Frame forwarding/flooding forever • Solution:Spanning tree Bridges Data link layer -- June 2004

12. Transparent bridges • Spanning tree bridges: • Solution transparent to hosts • Bridges construct spanning treeone path from every LAN to every other LAN • All forwarding follows spanning tree • Spanning tree construction • Select root of tree: bridge with lowest serial number (unique by construction; broadcast is used to inform all bridges) • Distributed algorithm to add (shortest) paths Data link layer -- June 2004

13. Transparent bridges • Spanning tree bridges Data link layer -- June 2004

14. Interconnect distant LAN Bridge on each LAN Point-to-point line between bridges Routing? No difference Point-to-point line = LAN without hosts Protocol Select point-to-point protocol Inside frame Complete MAC frame Only LLC frame (without MAC header, trailer) Remote bridges Data link layer -- June 2004

15. Design issues Point-to-point links Local area Networks Data Link layer Switching Multiple LANs? From 802.x to 802.y Transparent bridges Spanning Tree bridges Remote bridges Interconnections <> layers Virtual LANs Overview Data link layer -- June 2004

16. Interconnections <> layers • Which device in which layer? • Frames, packets and headers Data link layer -- June 2004

17. Repeater Analog devices to interconnect cable segments Signal on one segment is amplifies and put on other segment Not aware of frames, addresses Hub Frames arriving on one line are sent out on all the other lines A single collision domain Not aware of frames, addresses Interconnections <> layers Physical layer Data link layer -- June 2004

18. Bridge Interconnects LANs Each line has its own collision domain Switch Interconnects hosts cut-through switches Forwarding starts when header is received Interconnections <> layers Data link layer Difference? Data link layer -- June 2004

19. Design issues Point-to-point links Local area Networks Data Link layer Switching Multiple LANs? From 802.x to 802.y Transparent bridges Spanning Tree bridges Remote bridges Interconnections <> layers Virtual LANs Overview Data link layer -- June 2004

20. Virtual LANs • Example of a wiring infrastructure • Logical or physical configuration of LANs? Data link layer -- June 2004

21. Virtual LANs • LANs should to reflect organizational structure • Security • Load • Broadcasting • Solutions? • LAN = hub + rewiring • Rewiring in software  VLAN • Implemented by specially-designed VLAN aware switches/bridges Data link layer -- June 2004

22. Virtual LANs • Example VLANs: White & Gray • How to forward a (broadcast) packet sent out by A? Data link layer -- June 2004

23. Virtual LANs • How to forward a (broadcast) packet sent out by A? • Forwarded by bridge/switch on all Gray ports • What is the color of an incoming frame? Data link layer -- June 2004

24. Virtual LANs • Coloring incoming frames? VLAN color assigned to • Port • Does not work for bridges • MAC address • Use layer 3 • Could be useful • Violates independence of layers • Color frames  IEEE 802.1Q • Changes in Ethernet header to support VLANs Data link layer -- June 2004

25. Bridges/switches fill in the VLAN color Used on lines interconnecting bridges/switches Future ethernet cards will be 802.1Q compliant? Max length increased from 1518 to 1522 bytes IEEE 802.1Q Data link layer -- June 2004

26. VLAN fields: VLAN protocol ID (>max length) Pri: hard real-time <> soft real-time <> time-insensitive CFI: Canonical Format indicator: 802.5 frame IEEE 802.1Q Data link layer -- June 2004

27. IEEE 802.1Q • Configuration of bridges/switches • Autoconfiguration • If only 802.1Q frames arrive • Learning • Manual • For mixed installations Data link layer -- June 2004

28. Computer Networks Data link layer Data link layer -- June 2004