Switch/Router Architectures

1Introduction to Switch/Router Architectures

1.1 Introducing the Multilayer Switch

The term multilayer switch (or equivalently switch/router) in this book refers to a networking device that performs both Open Systems Interconnection (OSI) network reference model Layer 2 and Layer 3 forwarding of packets (Figure 1.1). The Layer 3 forwarding functions are typically based on the Internet Protocol (IP), while the Layer 2 functions are based on Ethernet. The Layer 2 forwarding function is responsible for forwarding packets (Ethernet frames) within a Layer 2 broadcast domain or Virtual Local Area Network (VLAN). The Layer 3 forwarding function is responsible for forwarding an IP packet from one subnetwork, network or VLAN to an another subnetwork, network, or VLAN.

Figure 1.1 Layer 2 forwarding versus Layer 3 forwarding.

The IP subnetwork could be created based on well-known IP subnetworking rules and guidelines or as a VLAN. A VLAN is a logical group of devices that can span one or more physically separate network segments that are configured to intercommunicate as if they were connected to one physical Layer 2 broadcast domain. Even though the devices may be located on a number of different physical or geographically separate network segments, the devices can intercommunicate as if they are all connected to one physical broadcast domain.

For the Layer 3 forwarding functions to ...