Routers control access from every network to every network. They advertise networks and filter who can use them, and they are potentially a hacker's best friend. Router security is a critical element in any security deployment. By their nature, routers provide access and, therefore, you should secure them to reduce the likelihood that they are directly compromised. You can refer to other documents that have been written about router security. These documents provide more detail on the following subjects:
Locking down telnet access to a router
Locking down Simple Network Management Protocol (SNMP) access to a router
Controlling access to a router through the use of Terminal Access Controller Access Control System ...