Perimeter Router, Internal Router and Firewall

Perimeter Router

Perimeter router's are a  standard router providing a serial connection to the outside world and a LAN connection to the internal network. The perimeter router should provide any filtering of outside traffic to implement basic security for the dirty DMZ and preliminary filtering for the inside network. This device could be running the firewall feature set for additional security options.

Internal Router 

An internal router is a router that has Open Shortest Path First(OSPF) neighbor relationships with interfaces in the same area. Also, an internal router has all its interfaces in a single area.

Firewall

The objective of a router is to control the incoming and outgoing traffic of network by analyzing the data packets and determining whether it should be allowed through or not, based on a predetermined rule set. A network's firewall builds a brigade between an internal network that is assumed to be secure and trusted, and another network, usually an external (inter)network, such as the Internet, that is not assumed to be secure and trusted.

^References

^{http://etutorials.org/Networking/Cisco+Certified+Security+Professional+Certification/Part+II+Securing+the+Network+Perimeter/Chapter+5+Securing+Cisco+Perimeter+Routers/Perimeter+Router+Terms+and+Concepts/}

^{http://en.wikipedia.org/wiki/Open_Shortest_Path_First}

^{http://en.wikipedia.org/wiki/Firewall_%28computing%29}