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

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

switch with six interfaces 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 switch with six interfaces (1,2,3,4,5,6) A A’ B B’ C C’ 1 2 3 4 5 6 5: DataLink Layer

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

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

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 5: DataLink Layer

Switch example Suppose C sends frame to D Switch receives frame from C address interface switch 1 A B E G 1 2 3 2 3 hub hub hub A I D F B G 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

Switch example Suppose D replies back with frame to C. address interface switch A B E G C 1 2 3 hub hub hub A I D F B G 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

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 hub switch collision domain collision domain collision domain 5: DataLink Layer

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

Summary comparison 5: DataLink Layer