IP Domain-lookup Commands.
For people coming into Cisco networking for the first time they are always surprised by a few of the default command settings that are present on the Cisco routers and switches.
These default command settings can be a real pain if left at their default settings and unless they are changed the CCNA student is generally going to become increasingly frustrated.
We continually practice the procedure of instructing our CCNA students (and sometimes our CCNP students) that they must change the default settings in order to make their programming via the command line a lot more fluid and without interruption.
The first of our house keeping commands is the “ip domain-lookup” command.
The ip domain-lookup command is enabled by default. The purpose of this command is to assist the administrator resolve names into ip addresses so they may telnet to remote devices. This seems on the face of it a good setting, the issues are that the system does not understand the difference between a valid hostname and a typo. In most cases if not all the system will interpret a typo as a hostname and start to broadcast out to DNS on the address of 255.255.255.255.
The broadcast itself is a minor nuisance on the network but the major issue that our CCNA students find is that in a majority of times the console from which they are configuring the system from be it tera-term, hyper terminal etc will lock up whilst it is broadcasting out for the non-existent IP address for the typo. Being new to Cisco our CCNA students will inevitably make typing mistakes, (although even the most dog eared of network administrators will also making typing mistakes)
To prevent the console from timing out every time a typo is issued it important to turn the “ip domain-lookup” off. At this point it is important to mention that only typos executed at the “user mode” and “privileged exec mode” will cause a time-out to occur, if a typo is issued under any off the other commands prompts the Cisco device will simply return an error message and return the command prompt without any console timeout.
To turn off the default issue the following command:
Note: The prompt is “router” but this command can also be executed on a switch.
Router(config)#no ip domain-lookup
The emphasis is on the “no” argument. Execute the command by pressing the return key. At this point if a typo is executed at the “user mode” and “privileged exec mode” will not cause a time-out to occur but instead the Cisco device will try to interrupt a typo to an IP address using only the local host table held within the “ip host” table.
Using the local “IP host” table is quite common, so rather than having to enter the full IP address of the destination device that you have to telnet to you would simply enter the host name of the device and the Cisco device will lookup the name in the local host table for a matching name and then resolve the name to the associated IP address.