3Com MSR 50 Series Configuration Manual page 273

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The settings of GARP timers apply to all GARP applications, such as GVRP, on a
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LAN.
Unlike other three timers, which are set on a port basis, the LeaveAll timer is
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set in system view and takes effect globally.
A GARP application entity may send LeaveAll messages at the interval set by its
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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 application entities 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.
Figure 52 GARP message format
DA DA length
1 3
Protocol ID
1 2
Attribute Type
1
Attribute 1
1
Attribute Length
The following table describes the GARP message fields.
Table 2 Description on the GARP message fields
Field
Protocol ID
DSAP SSAP Ctrl
Message 1 ... Message N
N
Attribute List
... Attribute N End Mark
2 3
Attribute Event Attribute Vlaue
Description
Protocol identifier for GARP
Introduction to GVRP
PDU
N
End Mark
N
N
Value
1
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Ethernet Frame
GARP PDU structure
Message structure
Attribute List structure
Attribute structure