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Data link layer -- June 20041 Data link layer Computer Networks.

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Presentation on theme: "Data link layer -- June 20041 Data link layer Computer Networks."— Presentation transcript:

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

2 Data link layer -- June Overview  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

3 Data link layer -- June Multiple LANs?  Why different LANs in a single organisation? oAutonomy of owners oDistance too great for a single LAN oLoad too high for a single LAN oReliability: a single defective node will cripple onle a single LAN oSecurity: most LANs offer a promiscuous mode (receive all packets)  Solution at data link layer: Bridge switch

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

5 Data link layer -- June Bridges: from 802.x to 802.y  Common problems: different odata rates omaximum frame length osecurity oquality of service oframe formats typeData Rate Mbps Max length bytes Support for encrypt QoS g x x

6 Data link layer -- June Overview  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

7 Data link layer -- June Transparent bridges  Goal: interconnect LANs with a bridge and everything should work perfectly, instantly oNo software changes oNo hardware changes oNo setting of address switches oNo downloading of routing tables  How? Promiscuous mode receive packet… oSource LAN = destination LAN  discard frame oDestination LAN unknown  flooding (on LAN level) oDestination LAN known  forward frame  Learn about LANs? Backward learning: construct routing table oInitially empty oReceive frame from node A via LAN L  A reachable via L oDynamic topologies: attach timestamp to entries

8 Data link layer -- June Transparent bridges A B C Bridge L1L2 fromtodata frame D L3 packetactionTable host entries LAN initiallyempty A  B flooding to L2 & L3AL1

9 Data link layer -- June Transparent bridges A B C Bridge L1L2 D L3 packetactionTable host entries LAN AL1 C  A forward to L1ACAC L1 L2

10 Data link layer -- June Transparent bridges A B C Bridge L1L2 D L3 packetactionTable host entries LAN ACAC L1 L2 D  B flooding to L1 & L2ACDACD L1 L2 L3

11 Data link layer -- June Spanning Tree bridges  Bridges in parallel oTo increase reliability  loops in topology oTo increase forwarding capacity Problem oFrame forwarding/flooding forever  Solution: Spanning tree Bridges

12 Data link layer -- June Transparent bridges  Spanning tree bridges: oSolution transparent to hosts oBridges construct spanning tree one path from every LAN to every other LAN oAll forwarding follows spanning tree oSpanning 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

13 Data link layer -- June Transparent bridges  Spanning tree bridges

14 Data link layer -- June Remote bridges  Interconnect distant LAN oBridge on each LAN oPoint-to-point line between bridges  Routing? oNo difference oPoint-to-point line = LAN without hosts  Protocol oSelect point-to-point protocol oInside frame Complete MAC frame Only LLC frame (without MAC header, trailer)

15 Data link layer -- June Overview  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

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

17 Data link layer -- June Interconnections <> layers  Repeater oAnalog devices to interconnect cable segments oSignal on one segment is amplifies and put on other segment oNot aware of frames, addresses  Hub oFrames arriving on one line are sent out on all the other lines oA single collision domain oNot aware of frames, addresses Physical layer

18 Data link layer -- June Interconnections <> layers  Bridge oInterconnects LANs oEach line has its own collision domain  Switch oInterconnects hosts  cut-through switches oForwarding starts when header is received Difference? Data link layer

19 Data link layer -- June Overview  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

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

21 Data link layer -- June Virtual LANs  LANs should to reflect organizational structure oSecurity oLoad oBroadcasting  Solutions? oLAN = hub + rewiring oRewiring in software  VLAN Implemented by specially-designed VLAN aware switches/bridges

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

23 Data link layer -- June 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?

24 Data link layer -- June Virtual LANs  Coloring incoming frames? VLAN color assigned to oPort Does not work for bridges oMAC address oUse layer 3 Could be useful Violates independence of layers oColor frames  IEEE 802.1Q Changes in Ethernet header to support VLANs

25 Data link layer -- June IEEE 802.1Q  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

26 Data link layer -- June IEEE 802.1Q  VLAN fields: oVLAN protocol ID (>max length) oPri: hard real-time <> soft real-time <> time-insensitive oCFI: Canonical Format indicator: frame

27 Data link layer -- June IEEE 802.1Q  Configuration of bridges/switches oAutoconfiguration If only 802.1Q frames arrive Learning oManual For mixed installations

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


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