Hammer Cordon Usage
Cordon compiling, or compiling with cordon boundaries set will compile only the part of the map you have selected. Say you want to compile one room, or want to test out some lighting; you can compile that small area without compiling the rest of the map, and without waiting for a full or even fast vis compile of the whole map. You can also use cordons to eliminate areas when trying to find the location of a problem.
Use
To use cordon boundaries first load the map you want to compile. Then look on the tool bar for an icon representing a black and yellow box.
Of the two buttons shown circled above, click the active icon, the cordon edit button (the hollowed out yellow and black square on the right). Then go to 2d views and draw the box around the part of the map you want to compile.
Once you have the area you want compiled boxed off, the cordon toggle button (fully colored in black and yellow) will become active.
Note:You must include a start point (an info_player_start or an info_player_deathmatch) inside your cordon boundary or else your map will not work.Now compile, it will take considerably less time then it did with the whole map.
Changing the cordon wall texture
One downside of the cordon tool is the annoying black and purple texture that represents where the cordon boundary is. It can make you sick after moving around a lot. However, this can easily be changed to a different texture by going to the Game Configuration tab shown in the image below and clicking browse by Cordon texture, here you can select a texture that would give you less of a headache, skybox for example.