3Com AccessBuilder 500 User Manual page 52

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HAPTER ETTING TARTED
Enter at the command prompt to permanently store this
SAVE
configuration in the unit's memory.
Although calls to the remote site are only generated when data is
addressed to a specific IP host or hosts (in our example, 191.000.000.100),
any data that cannot be identified as local is also passed over the link
while it is open. This can prevent the link from closing after the intended
data has been transferred.
To avoid this situation you can also configure the units at both ends of
the link to implement a Firewall and/or set the Maximum Call Duration in
the ISDN parameters screen to reduce the amount of traffic permitted to
pass across the link. See the AccessBuilder ISDN Access Router Software
Reference guide for more information about these features.
Alternatively, you may wish to configure the unit to operate as a router
to prevent this problem occurring altogether. However, this requires
that each site consists of separate subnets. See the next section for
more information.