Participant
On a device, each port running a protocol is considered as a participant. On a device running GVRP, each GVRP-enabled port is considered as a GVRP participant, as shown in Figure 1.
Figure 1 GVRP participant
VLAN Registration and Deregistration
GVRP implements automatic registration and deregistration of VLAN attributes. The functions of VLAN registration and deregistration are:
VLAN registration: adds a port to a VLAN.
VLAN deregistration: removes a port from a VLAN.
GVRP registers and deregisters VLAN attributes through attribute declarations and reclaim declarations as follows:
When a port receives a VLAN attribute declaration, it registers the VLAN specified in the declaration. That is, the port is added to the VLAN.
When a port receives a VLAN attribute reclaim declaration, it deregisters the VLAN specified in the declaration. That is, the port is removed from the VLAN.
A port registers or deregisters VLANs only when it receives GVRP messages.
Figure 2 VLAN registration and deregistration
GARP Messages
GARP participants exchange VLAN information through GARP messages. Major GARP messages are Join messages, Leave messages, and LeaveAll messages.
Join message
When a GARP participant expects other devices to register its attributes, it sends Join messages to other devices. When the GARP participant receives a Join message from another participant or is configured with attributes statically, it also sends Join messages to other devices for the devices to register the new attributes.
Join messages are classified into JoinEmpty messages and JoinIn messages. The difference between the two types of messages is:
JoinEmpty: declares an unregistered attribute.
JoinIn: declares a registered attribute.
Leave message
When a GARP participant expects other devices to deregister its attributes, it sends Leave messages to other devices. When the GARP participant receives a Leave message from another participant or some of its attributes are deregistered statically, it also sends Leave messages to other devices.
Leave messages are classified into LeaveEmpty messages and LeaveIn messages. The difference between the two types of messages is:
LeaveEmpty: deregisters an unregistered attribute.
LeaveIn: deregisters a registered attribute.
LeaveAll message
When a participant starts, it starts the LeaveAll timer. When the LeaveAll timer expires, the participant sends LeaveAll messages to other devices.
A participant sends LeaveAll messages to deregister all attributes so that other participants can re-register attributes of the local participant. LeaveAll messages are used to periodically delete useless attributes on the network. For example, an attribute of a participant is deleted but the participant does not send Leave messages to request other participants to deregister the attribute because of a sudden power failure. Then this attribute becomes useless.
GARP Timers
The GARP protocol defines four timers, which are described as follows:
Join timer
The Join timer controls sending of Join messages including JoinIn messages and JoinEmpty messages.
After sending the first Join message, a participant starts the Join timer. If the participant receives a JoinIn message before the Join timer expires, it does not send the second Join message. If the participant does not receive any JoinIn message, it sends the second Join message when the Join timer expires. This ensures that the Join message can be sent to other participants. Each port maintains an independent Join timer.
Hold timer
The Hold timer controls sending of Join messages (JoinIn messages and JoinEmpty messages) and Leave messages (LeaveIn messages and LeaveEmpty messages).
After a participant is configured with an attribute or receives a message, it does not send the message to other participants before the Hold timer expires. The participant encapsulates messages received within the hold time into a minimum number of packets, reducing the packets sent to other participants. If the participant does not use the Hold timer but forwards a message immediately after receiving one, a large number of packets are transmitted on the network. This makes the network unstable and wastes data fields of packets.
Each port maintains an independent Hold timer. The Hold timer value must be equal to or smaller than half of the Join timer value.
Leave timer
The Leave timer controls attribute deregistration.
A participant starts the Leave timer after receiving a Leave or LeaveAll message. If the participant does not receive any Join message of the corresponding attribute before the Leave timer expires, the participant deregisters the attribute.
A participant sends a Leave message if one of its attributes is deleted, but this attribute may still exist on other participants. Therefore, the participant receiving the Leave message cannot deregister the attribute immediately and needs to wait for messages from other participants.
For example, an attribute has two sources on the network: participant A and participant B. Other participants register the attribute through GARP. If the attribute is deleted from participant A, participant A sends a Leave message to other participants. After receiving the Leave message, participant B sends a Join message to other participants because the attribute still exists on participant B. After receiving the Join message from participant B, other participants retain the attribute. Other participants deregister the attribute only if they do not receive any Join message of the attribute within a period longer than two times the Join timer value. Therefore, the Leave timer value must be greater than two times the Join timer value.
Each port maintains an independent Leave timer.
LeaveAll timer
When a GARP participant starts, it starts the LeaveAll timer. When the LeaveAll timer expires, the participant sends a LeaveAll message and restarts the LeaveAll timer.
After receiving a LeaveAll message, a participant restarts all GARP timers. The participant sends another LeaveAll message when its LeaveAll timer expires. This reduces LeaveAll messages sent in a period of time.
If LeaveAll timers of multiple devices expire at the same time, they send LeaveAll messages at the same time, which causes unnecessary LeaveAll messages. To solve this problem, each device uses a random value between the LeaveAll timer value and 1.5 times the LeaveAll timer value as its LeaveAll timer value. When a LeaveAll event occurs, all attributes on the entire network are deregistered. The LeaveAll event affects the entire network; therefore, you need to set the LeaveAll timer to a proper value, at least greater than the Leave timer value.
Each device maintains a global LeaveAll timer.
Registration Modes
A manually configured VLAN is a static VLAN, and a VLAN created through GVRP is a dynamic VLAN. GVRP provides three registration modes. Static VLANs and dynamic VLANs are processed differently in each registration mode as follows:
Normal mode: Dynamic VLANs can be registered on a port, and the port can send declarations of static VLANs and dynamic VLANs.
Fixed mode: Dynamic VLANs cannot be registered on a port, and the port can send only declarations of static VLANs.
Forbidden mode: Dynamic VLANs cannot be registered on a port. All VLANs except VLAN 1 are deleted from the port, and the port can send only the declaration of VLAN 1.
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