Nicholas Morrison - Networking Specialist

flokinet-006 - Layer 2 Broadcast vs Unicast

Broadcast vs Unicast

Broadcast Frames Unicast Frames
- sent to every host - sent to a single host
- processed by every host - ignored by other hosts
- MAC address destination FF:FF:FF:FF:FF:FF - MAC address destination of the host
- eg: ARP, IPv6-NDP, OSPF, DHCP - eg: ping, ssh, http, ftp, traceroute
- all about discovery - transferring data from host to host

Broadcast Domain

  • area of the network in which a broadcast frame is repeated
  • broadcast frames do not leave their own broadcast domain
  • usually this will be:
    • a LAN, or
    • a VLAN
  • but also could be:
    • VXLAN
    • MPLS-L2VPN
    • ZeroTier
    • EVPN-L2VPN

Broadcast example: ARP

  • ARP: Address Resolution Protocol, Layer 2, ethertype 0x0806
  • maps MAC addresses (layer 2 addresses) to IP addresses (layer 3 addresses)
  • example flow:
    • Host A at wants to talk to Host B at
    • Host A and B are in the same network (layer 3)
    • and the same broadcast domain (layer 2)
    • Host A needs to know the MAC address of Host B to send it traffic
    • Host A sends a Broadcast to the network
    • Host B hears the Broadcast
    • Host B replies directly to Host A from its own MAC address
      • IS-AT xx:xx:xx:xx

Broadcast example: ARP

A single broadcast domain shared by 24 hosts.

Single broadcast domain

Broadcast example: ARP (MAC 44:e5:17:00:00:18) wants to send a packet to, so it needs to know’s MAC address. It sends a single layer 2 broadcast to the network, addressed to ff:ff:ff:ff:ff:ff.

ARP packet sent

Broadcast example: ARP

The switch sees the broadcast coming in on port 24, and copies that broadcast to every port in the same VLAN, except port 24. Every host must send this packet to its CPU and take a look at it.

ARP packet copied

Broadcast example: ARP

The host that HAS the address realises that this question is for it, and replies quietly (with a UNICAST frame) to the host who asked. All other hosts stay silent on the matter.

Now the two hosts can communicate directly with one another!

The host replies

Broadcast example: ARP

Broadcast packets are not forwarded by routers into other broadcast domains!

Broadcast packets are not forwarded

Broadcast and (Unknown-)Unicast and Multicast and Switches

  • Layer 2 switches optimise traffic flow by:
    • remembering which MAC address is connected to which switch port (show mac address-table)
    • only copying frames to ports when it’s necessary
  • Broadcast packets must always be sent to every port in the broadcast domain!
    • except the port from which the broadcast packet was received (split-horizon)
  • Unknown-Unicast packets are flooded to every port until the switch learns which port has the MAC address attached.
    • Once they’re learned, they are just Unicast packets.
  • Multicast packets must always be sent to every port in the broadcast domain
    • except in the case that the switch is doing IGMP-snooping
    • IGMP-snooping can “prune” ports from the Multicast group
  • Broadcast, Unknown-Unicast and Multicast packets are known collectively as BUM packets

Layer 2 Loops and Broadcast Storms

  • Creating a loop between switches can break your network
    • broadcast packets will loop between switches as fast as the switches can go
    • each broadcast packet will be copied to every port (except the receiving port)
    • every device must process the broadcast packet
    • … raising CPU on every device in the broadcast domain
  • The Spanning Tree Protocols (STP, MSTP, RSTP, PVSTP et al) are designed to break such loops

Happy network with no loop

Network with no loop

Happy network with two switches and no loop

Two switches, no loop

Sad network a loop between two switches

Somebody disabled spanning tree.

A network with a loop

The beginning of the storm - an innocent “arp who-has”

Calm before the storm

The broadcast who-has is repeated by the switches

The packet is forwarded

.. and the broadcast storm begins ..

The storm begins

.. and continues until devices run out of capacity

The storm destroys everything


  • Since every single device has to process a broadcast packet, all devices on the LAN / Broadcast Domain will be affected by the broadcast storm.
  • Because is often consumed before network switching capacity, unexpected failures can occur, such as routing protocols being unable to send their HELLOs quickly enough
  • Spanning Tree Protocol is used to break layer 2 loops
  • Always* run spanning tree!