Summary

Routing is a fundamental component of modern networking. A solid knowledge of routing is required no matter what area of network you specialize in.

In LAN networks, routing is used to allow VLANs to communicate with each othersuch a process is referred to as inter-VLAN routing. Two main inter-VLAN routing architectures exist. The most simple is called the router-on-a-stick, where essentially all inter-VLAN packets in the source VLAN are sent to a router that routes the packet and forwards the packet back to the LAN network on another VLAN. The router-on-a-stick architecture requires that the router have an interface attached to each VLAN for which inter-VLAN routing is required. These interfaces can be multiple physical interfaces or can be multiple virtual interfaces operating over a single physical trunk interface (a trunk interface can also be operating over multiple physical links in an EtherChannel bundle). The second inter-VLAN routing architecture is called Layer 3 switching. L3 switching refers to what you might consider as a LAN switch being capable of routing packets. L3 switching increases the intelligence of the LAN network, allowing for high-performance inter-VLAN routing at wire speed and increasing the efficiency of traffic flows in the LAN network.

L3 switching has advanced as a technology to a point where it is cost effective to implement and provides the same level of performance as L2 switching. This is driving designers of LANs to design and recommend multilayer LAN topologies over traditional collapsed backbone architectures. A collapsed backbone architecture relies on a flat Layer 2 network consisting of Layer 2 switches, with a router-on-a-stick located at the edge of the network providing inter-VLAN routing. This architecture has issues in terms of availability because of the effect that spanning tree has on the convergence of large Layer 2 topologies. Performance is also affected for larger networks as the size of each broadcast domain (VLAN) increases. A multilayer design mitigates these issues by segmenting the LAN topology into smaller Layer 2 domains, with routing used to enable communications between each Layer 2 domain.

An important technology used with inter-VLAN routing architectures is Cisco's Hot Standby Router Protocol (HSRP). HSRP provides redundancy for inter-VLAN routing devices, where an active router services all inter-VLAN routing during normal operation, while a backup (standby) router provides inter-VLAN routing in the event the active router fails. HSRP is most commonly used at the edge of the IP network, where end devices are attached. End devices, such as servers and workstations, are not designed to be routers and thus are typically configured with a single next-hop IP address (default gateway) to which all inter-VLAN traffic is sent for routing. This provides a simple method of connecting to the rest of the network but does not offer any means of dynamically sending inter-VLAN traffic through another path should the configured path fail.