GARP: Generic Attribute Registration Protocol

The Generic Attribute Registration Protocol (GARP) provides a generic framework whereby devices in a bridged LAN, e.g. end stations and switches, can register and de-register attribute values, such as VLAN Identifiers, with each other. In doing so, the attributes are propagated to devices in the bridged LAN, and these devices form a ¡°reachability¡± tree that is a subset of an active topology. GARP defines the architecture, rules of operation, state machines and variables for the registration and de-registration of attribute values.

A GARP participation in a switch or an end station consists of a GARP application component, and a GARP Information Declaration (GID) component associated with each port or the switch. The propagation of information between GARP participants for the same application in a bridge is carried out by the GARP Information Propagation (GIP) component. Protocol exchanges take place between GARP participants by means of LLC Type 1 services, using the group MAC address and PDU format defined for the GARP application concerned.  

GARP is part of the IEEE 802.1P extension to its 802.1D (spanning tree) specification. It includes:

• GARP Information Declaration (GID): The part of GARP that generates data.
• GARP Information Propagation (GIP): The part of GARP that distributes data.  

GARP Multicast Registration Protocol (GMRP): Provides a mechanism that allows participants to dynamically register and de-register information with the Media Access Control (MAC) bridges attached to the same local-area network (LAN) segment.

Protocol Structure - GARP: Generic Attribute Registration Protocol

• Protocol ID - Identifies the GARP protocol.
• Identifier - Decimal value which aids in matching requests and replies.
• Attribute type - Defines the attribute. Values may be:  1   Group attribute; 2   Service Requirement attribute.
• Attribute length - Length of the Attribute.
• Attribute event - The values of the attribute event can be:
  • 0 - Leave_all
  • 1 - Join_Empty operator
  • 2 - Join_In operator
  • 3 - Leave_Empty operator
  • 4 - Leave_In operator
  • 5 - Empty operator
• Attribute value - This is encoded in accordance with the specification for the Attribute Type. The number of bytes depends on the specific application.
• End mark - Coded as 0.

GARP standard is defined by IEEE (http://www.ieee.org ) 802.1P.