Both IP and CLNP packets must be able to enter and leave a dual area, whether or not automatic encapsulation is used. Normally traffic enters and leaves an IS-IS area via level-1,level-2 routers. These are routers that participate both in the level-1 area and in the level-2 subdomain.
The simplest way to build this is to ensure that any level-1,level-2 routers are dual, as shown in Figure III-8.
