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.

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.

Section 2.1: VLAN Membership

When a VLAN is provided at an access layer switch, an end user must be able to gain membership to it. Two membership methods exist on Cisco Catalyst switches: static VLANs and dynamic VLANs.

  • Static VLANs offer port-based membership, where switch ports are assigned to specific VLANs. End user devices become members in a VLAN based on which physical switch port they are connected to. No handshaking or unique VLAN membership protocol is needed for the end devices; they automatically assume VLAN connectivity when they connect to a port. The static port-to-VLAN membership is normally handled in hardware with application specific integrated circuits (ASICs) in the switch. This membership provides good performance because all port mappings are done at the hardware level with no complex table lookups needed.
  • Dynamic VLANs are used to provide membership based on the MAC address of an end user device. When a device is connected to a switch port, the switch must query a database to establish VLAN membership. A network administrator must assign the user's MAC address to a VLAN in the database of a VLAN Membership Policy Server (VMPS). With Cisco switches, dynamic VLANs are created and managed through the use of network management tools like CiscoWorks 2000 or CiscoWorks for Switched Internetworks (CWSI). Dynamic VLANs allow a great deal of flexibility and mobility for end users, but require more administrative overhead.