1-44 CHAPTER 1: GETTING STARTED
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.
