Switch Forwarding Logic (Unicast, Multicast, Broadcast)

Switch MAC Learning

• Switches learn source MAC addresses
• MAC addresses are learned dynamically
• Learned MAC addresses are stored in the MAC address table
• Entries eventually age out

Unicast

• Switches uses destination MAC addresses to determine where to send traffic
• If the destination is unicast
  • Switch renews the source MAC entry
  • Switch checks destination MAC against the MAC address table
  • Switch frame to corresponding port
• PC verifies the destination MAC matches its own MAC address.
  • PC discards if it is not its MAC address.

Broadcast

• If the destination is the broadcast MAC

  • FF-FF-FF-FF-FF-FF
  • Switch sends the frame out all ports except the one it came in on

• If the destination is unicast - but the switch doesn't know the destination

  • Switch sends the frame out all ports except the one it came in on
  • This is called "flooding"

• All PCs will open a broadcast frame

• Routers do not broadcast, routers separate broadcast domains

Multicast

• From one to some or one to group
• Group
  • Video server
  • Video chat
  • Routing protocol
• Requires multicast capable switches
  • IGMP
    • A switch listens for which hosts send out an IGMP
  • PIM
• Switches snoop the client multicast frame
• Non-multicast switches flood