3Com AccessBuilder 500 User Manual - page 64

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There are some issues about which you should be aware when 

implementing an IP bridging solution.

■

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.

■

Some devices and applications (for example autodiscovery programs 

on SNMP managers) poll all devices on a subnet at regular intervals 

and this could lead to frequent ISDN calls if you have entered several IP 

hosts to generate autocalls to the remote site. When combined with 

the problem described above, you could find your ISDN line 

permanently connected.

To avoid this situation you need to be sure that no devices or applications 

exist on your local site that could make unnecessary and costly calls to your 

remote site. You can also configure the AccessBuilder 500 units at both 

ends of the link to implement a Firewall and/or Call Guillotine 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 AccessBuilder 500 to operate 

as a router to prevent this problem occurring altogether. This however 

requires that each site consists of separate subnets. See the next section 

for more information about IP routing.

IP Host on Another IP Network

Most organizations using TCP/IP protocols on their network, choose to 

subnet remote sites or even to have them on different networks. This 

requires that connections to remote sites are routed rather than bridged. 

The advantage of routing over bridging is that calls to the remote site are 

only made when data is specifically addressed to a remote network. 

Bridging passes any data not known to be for the local network to the 

remote network whether that is its destination or not.