A fully Layer 2 switched network is referred to as a flat network topology.
A flat network is a single broadcast domain in which every connected
device sees every broadcast packet that is transmitted. As the number
of hosts on the network increases, so does the number of broadcasts.
Due to the Layer 2 foundation, flat networks cannot contain redundant
paths for load balancing or fault tolerance. However, a switched network
environment offers the technology to overcome flat network limitations.
Switched networks can be subdivided into virtual LANs (VLANs), each
of which is a single broadcast domain. All devices connected to the
VLAN receive broadcasts from other VLAN members. However, devices connected
to a different VLAN will not receive those same broadcasts because is
made up of defined members communicating as a logical network segment.
A VLAN can have connected members located anywhere in the campus network,
as long as VLAN connectivity is provided between all members. Layer
2 switches are configured with a VLAN mapping and provide the logical
connectivity between the VLAN members.
Collision Domains
A collision domain is a set of network interface cards (NICs) for which a frame sent by one NIC could result in a collision with a frame sent by any other NIC in the same collision domain.
Broadcast Domains
A broadcast domain is a set of NICs for which a broadcast frame sent by one NIC is received by all other NICs in the same broadcast domain.