Switching · VLANs and Trunking

Broadcast domains

Follow one broadcast to identify its Layer 2 boundary.

Independent lab examples; selected output and frame summaries are illustrative. Port naming and command support vary by switch and software release.

Network and objective

Normal VLAN forwarding is assumed, with no isolation or storm suppression.

Three access hosts use VLAN 10 on SW1. A routed interface provides access to a different subnet.

Physical links are solid; device roles and relevant links are labeled.

How it works

An Ethernet broadcast has destination ff:ff:ff:ff:ff:ff. It is replicated to eligible forwarding ports in the ingress VLAN except the receiving port. ARP requests are a familiar example; unknown unicast may use the same egress set but retains its original unicast destination.

A VLAN defines a Layer 2 broadcast domain across the trunks carrying it. Ordinary routing does not forward the received Ethernet broadcast frame into a second VLAN. A configured application relay, such as DHCP relay, can create a separate routed message; this does not merge the broadcast domains.

Configure or inspect

vlan 10
interface range GigabitEthernet1/0/1 - 3
 switchport mode access
 switchport access vlan 10

Verify the result

Illustrative output · selected lines

SW1#show vlan brief
VLAN Name                             Status    Ports
10   VLAN0010                         active    Gi1/0/1, Gi1/0/2, Gi1/0/3
Reconstructed frame on Gi1/0/1:
  Destination ff:ff:ff:ff:ff:ff, EtherType 0x0806 (ARP)

An ARP request received from H1 is eligible for Gi1/0/2 and Gi1/0/3, not a port in another VLAN. The frame summary is illustrative, not a capture.

Failure and recovery

If an expected listener misses the broadcast, inspect VLAN membership, trunk carriage, spanning-tree state and policy. Do not conclude that clearing the sender’s ARP cache changes the Layer 2 boundary.

Restore the intended VLAN membership or trunk path, then test with one known broadcast and a matching observation point.

Key Takeaways

Broadcast is identified by its destination MAC.
A VLAN bounds ordinary Layer 2 broadcast flooding.
Trunks can extend that VLAN across switches.
Routing separates broadcast domains.
Unknown unicast flooding is not address rewriting.