Perimeter Router, Internal Router and Firewall

A perimeter router is typically a standard router that is connected to the local area network and provides a serial connection to the outside world. Perimeter routers are exposed: They connect networks that can be reached via the Internet, and are often the target of hackers seeking to exploit security vulnerabilities. An unsecured perimeter router is not only ineffective at filtering unwanted network traffic, but can also provide an easy target for denial of service attacks, which can bring a network to a standstill. A secured perimeter route prevents network reconnaissance -- in which hackers attempt to scan networks in readiness for an attack -- and hence the attacks themselves.

An internal router is a router that has OSPF neighbor relationships with interfaces in the same area. An internal router has all its interfaces in a single area.

A firewall is a set of related programs, located at a network gateway server, which protects the resources of a private network from users from other networks. (The term also implies the security policy that is used with the programs.) An enterprise with an intranet that allows its workers access to the wider Internet installs a firewall to prevent outsiders from accessing its own private data resources and for controlling what outside resources its own users have access to.

references:

http://en.wikipedia.org/wiki/Open_Shortest_Path_First#Internal_router

http://www.ehow.com/info_12198351_perimeter-router.html