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Chapter 5 Data Link Layer – Hub, Switch

CMPT 371 Data Communications and Networking. Chapter 5 Data Link Layer – Hub, Switch. Interconnecting with hubs. Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large collision domain. hub. hub. hub. hub.

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Chapter 5 Data Link Layer – Hub, Switch

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  1. CMPT 371Data Communications and Networking Chapter 5Data Link Layer – Hub, Switch 5: DataLink Layer

  2. Interconnecting with hubs • Backbone hub interconnects LAN segments • Extends max distance between nodes • But individual segment collision domains become one large collision domain hub hub hub hub 5: DataLink Layer

  3. Switch • hosts have dedicated, direct connection to switch • switches buffer packets • Ethernet protocol used on each incoming link, but no collisions; full duplex • each link is its own collision domain • switching:A-to-A’ and B-to-B’ can transmit simultaneously, without collisions A B C’ 1 2 6 4 5 3 B’ C A’ switch with six interfaces (1,2,3,4,5,6) 5: DataLink Layer

  4. Forwarding Q:how does switch know A’ reachable via interface 4, B’ reachable via interface 5? A B C’ • A:each switch has a switch table,each entry: • (MAC address of host, interface to reach host, time stamp) • looks like a routing table! 1 2 6 4 5 3 B’ C Q:how are entries created, maintained in switch table? • something like a routing protocol? A’ switch with six interfaces (1,2,3,4,5,6) 5: DataLink Layer

  5. MAC addr interface TTL 60 1 A Self learning • A switch has a switch (forwarding) table • switchlearns 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 Switch table (initially empty) 5: DataLink Layer

  6. Filtering/Forwarding When switch receives a frame: index switch table using MAC dest address if entry found for destinationthen{ if dest on segment from which frame arrivedthen drop the frame else forward the frame on interface indicated } else flood forward on all but the interface on which the frame arrived 5: DataLink Layer

  7. Switch example Suppose C sends frame to D address interface 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 C • notes in switch 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 5: DataLink Layer

  8. Switch example Suppose D replies back with frame to C. address interface 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 switch table that D is on interface 2 • because C is in table, switch forwards frame only to interface 1 • frame received by C 5: DataLink Layer

  9. 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 collision domain collision domain 5: DataLink Layer

  10. Institutional network mail server to external network web server router switch IP subnet hub hub hub 5: DataLink Layer

  11. Summary comparison 5: DataLink Layer

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