Overview of Virtual Tie Lines 853 In the Primary DNS, Secondary DNS, and Tertiary DNS text boxes, type theIP addresses of three Domain Name Servers. If you have the IP address ofonly one server, type it in the Primary DNS text box. If you have the IPaddress of only two servers, type them in the Primary and Secondary DNStext boxes. Click OK.Overview ofVirtual Tie LinesA Virtual Tie Line (VTL) provides a way to make calls between NBX systemsites that are separated geographically but tied together by a Wide AreaNetwork (WAN). VTLs are a licensed feature of the NBX systems.■ NBX V3000 — support for 48 simultaneous VTL connections■ SuperStack 3 NBX — support for 48 simultaneous VTL connections■ NBX 100 — support for 8 simultaneous VTL connectionsOn any NBX system, a VTL connection can be used either for an incomingVTL call from any site or for an outgoing VTL call to any site. A VTLconnection is not dedicated in the same way as a physical tie line, whichalways connects the same pair of sites. In the example in Figure 10, theVTLs on the Chicago NBX system can be used for any combination ofincoming and outgoing VTL calls to either Atlanta or Dallas.The NBX system can reroute VTL calls that fail to reach their destinationon the first attempt. For details, see “Call Rerouting for Virtual Tie Lines”on page 99.■ You must configure the system for either IP On-the-Fly or Standard IPin order to use VTL connections to other NBX systems.■ VTL connections cannot be configured to run through firewalls or NATrouters.■ When you calculate the number of devices on an NBX system, do notinclude the number of VTLs.There are two implementation techniques you can use: unique extensionranges, as described next, or site codes, which begins on page 86.VTL ConnectionsUsing UniqueExtension RangesIf you can restrict the extension ranges on each of the NBX systems sothat they do not overlap, you can configure the dial plans to route callsbased only on the extension that is being dialed. The caller does not haveto dial any digits to specify the site.
