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What is the difference between unicast, multicast and broadcast? ​

Short answer

These terms describe how many recipients a packet goes to: unicast is sending to a single recipient, multicast to the recipients that have joined the group (in IPv4, the group addresses in the 224.0.0.0/4 range, RFC 1112), and broadcast to everyone in the same L2 segment (RFC 919). The area a broadcast reaches is called a broadcast domain; VLAN and Bridge draw the boundary of this area, and routers do not forward broadcasts. There is no broadcast in IPv6; multicast does the job. In TR7, the difference is seen most clearly on the VXLAN interface: the Transport Mode field offers the choice of reaching the remote ends with Unicast (remote peer) (default), Unicast (multiple peers) or Multicast Group. In a cluster, while VIP Transition Method = Only VRRP, VRRP advertisements are sent as unicast between the two Interface IPs selected in VIP Communication. ARP requests, on the other hand, are broadcast.

Where It Is Used in TR7 ​

WhereField / labelWhat for
Network → Interfaces → Add Interface → Type = VXLANTransport Mode: Unicast (remote peer) → Remote Peer IP; Unicast (multiple peers) → Peer IPs; Multicast Group → Multicast GroupChoosing how the tunnel reaches the remote ends; the multicast group address must start with 224–239 in IPv4 and have the ff prefix in IPv6
Network → Interfaces → Edit Interface (in a cluster)VIP Communication, VIP Transition Method = Only VRRPSending VRRP advertisements as unicast between the Interface IPs of the two devices
Network → Interfaces → Add InterfaceVLAN, BridgeDividing (VLAN) or joining (Bridge) the broadcast domain
The L4 DDoS Management screen of the route tableSmurf rulesLimiting the per-source ICMP echo rate and TCP reset rate against the Smurf attack, which is amplified with echo requests sent to a broadcast address
WEB-CLIsys-cmd ipcalc (ipcalc)Calculating the broadcast address of a subnet

Comparison ​

UnicastMulticastBroadcast
RecipientSingleThose that have joined the groupEveryone in the segment
Address example192.0.2.10239.1.1.1, ff02::1192.0.2.255 (subnet broadcast), 255.255.255.255
Does it pass through a routerYesIf there is multicast routingNo
In IPv6ExistsExistsDoes not exist
TR7 exampleVXLAN Unicast modes, Only VRRP advertisements, vService trafficVXLAN Multicast Group modeARP request

Common Misconceptions ​

  • "Multicast and broadcast are the same." Multicast goes only to the recipients that have joined the group, broadcast to everyone in the segment.
  • "Broadcast spreads across the whole network." Broadcast is limited to the broadcast domain (VLAN, the segment joined with Bridge); routers do not forward broadcasts.
  • "Any address can be entered in VXLAN Multicast Group mode." The address must really be in the multicast range; otherwise the record is rejected with the Invalid multicast group address error.

How it is done in the documentation

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