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 PDU structure
| 2 bytes | |
| Protocol ID | Message |
GARP message structure
| 1 byte | ||||
| Attribute type | Attribute 1 | ...... | Attribute n | End mark |
GARP attribute structure
| 1 byte | ||
| Attribute length | Attribute event | Attribute value |
- 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.
- End mark - Coded as 0.
Related protocols: IEEE 802.1Q, 802.1P, 802.1D, GVRP,
Sponsor Source: GARP standard is defined by IEEE (http://www.ieee.org) 802.1P.
Reference: http://www.alliedtelesyn.co.nz/documentation/at8700/261/pdf/garp.pdf: Overview of Generic Attribute Registration Protocol