3.5 Layer 3 SwitchRouter

3.5 Layer 3 Switch/Router

Another example used often in this book is the Layer 3 switch/router This is termed the IP Switch (IPS) in this book. The device performs Layer 3 switching (i.e., forwarding based on Layer 3 information). Similar to the Layer 2 switch, the three types of tasks in a Layer 3 switch/router are: drivers, a set of modules for the protocols, and a set of modules/tasks for system operation and management. The drivers perform the same type of functionality as in the Layer 2 switch.

IP forwarding functionality is provided through an IP switching task while the control plane routing protocols are implemented as separate tasks (control tasks). The IP switching task interacts with the hardware for fast forwarding functions similar to the switching task in the Layer 2 switch example.

The other tasks include a Routing Information Protocol (RIP) task, an OSPF task, and a Border Gateway Protocol (BGP) task. The RIP task runs on top of UDP (User Datagram Protocol), which could be a part of the IP end-node function. This function could be a part of the IP switching task or designed as a separate task for performance reasons.

TCP is part of the same end node task. BGP interfaces to TCP since it needs to run on top of TCP to communicate with peer routers for exchanging routing information. The routing protocols serve to build a routing/forwarding table which is used by the IP switching task to forward packets received on any of its Ethernet interfaces. The interfaces between the routing tasks and the IP switching tasks in terms of access to the routing table are depicted in Figure 3.5.

Figure 3.5: Interface between routing and IP switching tasks.

The modules/tasks required for system operation are similar to the Layer 2 switch case. This includes management tasks like an SNMP agent and Health Monitor task.

The Layer 3 switch operations are more complex than a Layer 2 switch. Protocols like OSPF can consume a substantial amount of CPU time for their Shortest Path First (SPF) calculations, especially when the network is large. Depending upon the application in the network, routers are often built as multi-board systems with a control card and multiple line cards (discussed in Chapter 8).

While Layer 2 switches and IP routers can be separate devices, it is also common for Layer 2 switching functionality to be combined with an IP router, forming a combination Layer 2/3 switch. IP forwarding is frequently called IP switching or Layer 3 switching. This book terms the Layer 2/3 switch as the IP Switch (IPS). Here, the number of tasks/modules will be much higher-requiring a careful design of the system, since inter- task communication can cause the system to slow down considerably.