Chapter 5. Inter-VLAN Routing
Networks are constantly evolving, and in the past few years a number of trends have become apparent. First of all, the Internet Protocol (IP) has become the Layer 3 protocol of choice for modern networks, with other Layer 3 protocols such as Internetwork Packet Exchange (IPX) and AppleTalk rapidly being phased out. IP interconnects the Internet. The increasing reliance of organizations on the Internet has promoted IP as the Layer 3 protocol of choice. Secondly, local-area networks (LANs) have seen tremendous advances in terms of performance, bandwidth, and lowering cost. The LAN provides the medium over which users and devices connect to the internal IP network and the Internet hence is an important component of networking. LAN topologies have evolved from traditionally being single, flat broadcast domains into multi-virtual LAN (VLAN) topologies, with inter-VLAN routing required to enable communications between each VLAN. Multiple VLANs increase network efficiency by reducing broadcast domain size, as well as providing a mechanism to allow network layer access control to be applied between VLANs. Using multiple VLANs also means that the resiliency of the network relies less on Layer 2 protocols such as Spanning Tree Protocol (STP), and more upon Layer 3 routing protocols. Modern Layer 3 routing protocols are much more intelligent than STP and as a result can converge much more quickly in the event of a network failure. Finally, segmenting a LAN network into VLANs allows for the isolation of problems to a smaller segment of network, allowed for reduced impact on the network and easier fault finding.
All of the above factors have caused the requirements for inter-VLAN routing within LAN networks to soar over the past few years. Even though this book is primarily about switches, which are traditionally Layer 2-only devices, it is important to understand the basics of inter-VLAN routing. Possessing this knowledge helps you to understand Layer 3 switches, which are becoming cost-effective, high performance alternatives to traditional routers for routing IP traffic between LAN segments. Possessing a fundamental understanding of inter-VLAN is important if you are to design multilayer topologies that are stable, available, and scaleable.
This chapter introduces you to the basic inter-VLAN routing architectures, using both traditional Cisco routers and basic Cisco Layer 3 switching. You also learn about multilayer LAN topologies and how a hierarchical design allows for scalability and a redundant topology that in the event of failure converges quickly and efficiently with minimal disruption to the network.
The following scenarios are presented in this chapter: