When would you need this: When your ISP gives you dynamic IP address upon each connection or you need to configure the router to obtain its interface IP address automatically.

Special Requirements: None.

When would you need this: When you want to create a secure tunnel to transfer data between two sites without the use of VPN concentrator or other security devices.

Special Requirements: The routers used must support IPSec. Most of Cisco routers do. Another need is that both sides use a static public IP address to connect to the Internet.

When would you need this: When you are creating a WAN link. And also when the other end of a WAN link is NOT a Cisco router. Point-to-Point Protocol can be used in synchronous, asynchoronous, HSSI, and ISDN links.

Special Requirements: None.

When would you need this: When you are using ISDN links between more than two nodes.

Special Requirements: The router should have ISDN interface(s).

When would you need this: When you have ISDN WAN link and you want the router to use it.

Special Requirements: The router should have BRI interface(s).

When would you need this: When you want to perform routing between different VLANs.

Special Requirements: You have to make sure that your router support the frame tagging technology used between the switches.

When would you need this: When you are implementing a routing protocol on a large Internetwork and all the networking devices involved are Cisco devices.

Special Requirements: EIGRP is a Cisco proprietary protocol. So, either all the routers in the Internetwork must be Cisco routers, or the routers should be EIGRP capable.

When would you need this: When you want to copy IOS file from one router to another for the purposes of upgrade or install. This is usually required when you do not have a TFTP server around.

Special Requirements: The flash size of the destination router should be adequate for the new IOS file size.