Network redundancy is a key to maintaining network reliability. Multiple physical links between devices provide redundant paths. The network can then continue to operate when a single link or port has failed. Redundant links can also share the traffic load and increase capacity.
Multiple paths need to be managed so that Layer 2 loops are not created. The best paths are chosen, and an alternate path is immediately available should a primary path fail. The Spanning Tree Protocols are used to manage Layer 2 redundancy.
Redundant devices, such as multilayer switches or routers, provide the capability for a client to use an alternate default gateway should the primary default gateway fail. A client may now have multiple paths to more than one possible default gateway. First Hop Redundancy Protocols are used to manage how a client is assigned a default gateway, and to be able to use an alternate default gateway should the primary default gateway fail.
This chapter focuses on the protocols used to manage these forms of redundancy. It also covers some of the potential redundancy problems and their symptoms.