H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 79)

9-2
2) GARP timers
The interval of sending of GARP messages is controlled by the following four timers:
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Hold timer –– A GARP participant usually does not forwards a received registration request
immediately after it receives a registration request, instead, it waits for the expiration of the hold
timer. That is, a GARP participant sends Join messages when the hold timer expires. The Join
message contains all the registration information received during the latest Hold timer cycle. Such
a mechanism saves the bandwidth.
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Join timer –– Each GARP participant sends a Join message twice for reliability sake and uses a
join timer to set the sending interval. If the first Join message is not acknowledged after the interval
defined by the Join timer, the GARP participant sends the second Join message.
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Leave timer –– Starts upon receipt of a Leave message sent for deregistering some attribute
information. If no Join message is received before this timer expires, the GARP participant
removes the attribute information as requested.
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LeaveAll timer –– Starts when a GARP participant starts. When this timer expires, the entity sends
a LeaveAll message so that other participants can re-register its attribute information. Then, a
LeaveAll timer starts again.
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The settings of GARP timers apply to all GARP applications, such as GVRP, on a LAN.
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Unlike other three timers, which are set on a port basis, the LeaveAll timer is set in system view and
takes effect globally.
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A GARP participant may send LeaveAll messages at the interval set by its LeaveAll timer or the
LeaveAll timer on another device on the network, whichever is smaller. This is because each time a
device on the network receives a LeaveAll message it resets its LeaveAll timer.
Operating mechanism of GARP
The GARP mechanism allows the configuration of a GARP participant to propagate throughout a LAN
quickly. In GARP, a GARP participant registers or deregisters its attributes with other participants by
making or withdrawing declarations of attributes and at the same time, based on received declarations
or withdrawals, handles attributes of other participants. When a port receives an attribute declaration, it
registers the attribute; when a port receives an attribute withdrawal, it deregisters the attribute.
GARP participants send protocol data units (PDU) with a particular multicast MAC address as
destination. Based on this address, a device can identify to which GVRP application, GVRP for example,
should a GARP PDU be delivered.
GARP message format
The following figure illustrates the GARP message format.