200-301 sybex

200-301 sybex.6.2 Fundamental Elements6.2.1 The Concept of FHRP If the network design includes redundant routers, switches, LAN links, and WAN links, then in some cases it will be necessary to use1.2 Fundamentals 127 with other protocols in order to use redundant devices and prevent the resulting problems. For example, a WAN with many far128 Chapter 6 Creating Redundant First-Hop Routersprogram branches of the office. If each remote branch has two WAN links connecting to the remaining parts of the network, then these routers can use the IP routing protocol to select the best route. When one WAN link fails, this router protocol can be informed of exactly which redundant link's route to use, making good use of the remaining WAN link.To see another example, consider a LAN containing redundant links and switches, as described in Chapters 1 and 2 of this book. STP prevents the problems caused by data frames looping through the LAN's redundant paths.This chapter also describes another protocol that is useful when the network uses some redundancy, this time also including redundant default routers. When two or more routers are connected to the same LAN subnet, they can serve as the default router for hosts on the subnet. However, to make the most of the redundant default routers, we need another protocol. The term "First-Hop Redundancy Protocol" (FHRP) falls into this category, and with it, hosts can take advantage of the redundant routers in the subnet.The first major part of this chapter will discuss the concepts behind how the different FHRPs work. This section starts with a discussion of the network's need for redundancy in general and the need for a redundant default router. It then describes the three available FHRP options that are used to solve the problems that arise when using redundant default routers.The Need for Redundancy in the NetworkNetworks need redundant links to improve the availability of the network. Devices in the network will eventually fail. Router power can go out, cables can break, and switches can lose power. These WAN links are represented as simple lines in most of the diagrams in this book, but they are actually the most complex physical parts of the network, with many separate components that can fail.