Cisco IOS Wide-Area Networking Configuration Guide - Free Books

Configuring DNS-Based X.25 RoutingConfiguring X.25 and LAPBAddress ResolutionWith DNS-based X.25 routing, managing the X.121-to-IP addressing correlation and themnemonic-to-X.121 addressing correlation is easy. Instead of supplying the router multiple routestatements to all destinations, it may be enough to use a single wildcard route statement that covers alladdresses in the DNS.The x25 route disposition xot command option has been modified to include the dns pattern argumentafter the xot keyword, where pattern is a rewrite element that works in the same way that addresssubstitution utilities work (see the Cisco IOS Wide-Area Networking Command Reference for furtherdetails).The wildcard ^.* characters and \0 pattern of the modified x25 route ^.* xot dns \0 command give thecommand more universality and effectiveness and make DNS-based X.25 routing simple and easy to use.These characters and pattern already exist and are explained in detail under the x25 route command inthe Cisco IOS Wide-Area Networking Command Reference. This command functions only if the DNSroute table mapping has been configured in a method recognized and understood by X.25 and the DNSserver.The following example is a setup from a DNS route table showing which X.121 address relates to whichIP address:222 IN A 172.18.79.60444 IN A 10.1.1.3555 IN A 10.1.1.2 10.1.2.2 10.1.3.2 10.1.4.2 10.1.5.7 10.1.6.3The command line x25 route 444 xot dns \0 shown in the DNS-based X.25 routing configurationexample is what extracts the IP address from the DNS. The \0 pattern replaces itself with 444. The 444is then used as the index into the DNS route table to generate the IP address 10.1.1.3. Other characterscan be combined with the pattern; for example, A-\0. In the DNS database, the index would appear asA-444.As the example in Figure 42 shows, a call sent by the router goes to the DNS. The DNS checks its routetable and identifies the X.121 address 444 and its related IP address 10.1.1.3. The DNS returns the IPaddress to the host router, which then creates a route statement and forwards the data to the IP addressof the destination router (10.1.1.3).If the DNS-based X.25 routing configuration example included the command x25 route 555 xot dns \0,then a call to the X.121 address 555 would also go to the DNS. Since multiple IP addresses have beenconfigured in the domain name space records, all of the IP addresses for that domain name would bereturned to the router. Each address would be tried in sequence, just as if the X.25 routing configurationhad been x25 route 555 xot 10.1.1.2 10.1.2.2 10.1.3.2 10.1.4.2 10.1.5.7 10.1.6.3. The router will acceptup to 6 IP addresses from DNS for the domain name. If there are more than six, there will be an errormessage, and the list will be trucated to the first six received.40