LAN Switching - Layer 3 Switching

Layer 3 Switching

The only difference between a layer 3 switch and router is the way the administrator creates the physical implementation. Also, traditional routers use microprocessors to make forwarding decisions, and the switch performs only hardware-based packet switching. However, some traditional routers can have other hardware functions as well in some of the higher-end models. Layer 3 switches can be placed anywhere in the network because they handle high-performance LAN traffic and can cost-effectively replace routers. Layer 3 switching is all hardware-based packet forwarding, and all packet forwarding is handled by hardware ASICs. Layer 3 switches really are no different functionally than a traditional router and perform the same functions, which are listed here

• Determine paths based on logical addressing
• Run layer 3 checksums (on header only)
• Use Time to Live (TTL)
• Process and respond to any option information
• Update Simple Network Management Protocol (SNMP) managers with Management Information Base (MIB) information
• Provide Security

The benefits of layer 3 switching include the following

• Hardware-based packet forwarding
• High-performance packet switching
• High-speed scalability
• Low latency
• Lower per-port cost
• Flow accounting
• Security
• Quality of service (QoS)

1. SWITCHING:

The switching algorithm is relatively simple and is the same for most of the routed protocols: a host would like to send a packet to a host on another network. Having acquired a router's address by some means, the source host sends the packet directly to that router's physical (MAC) address. The protocol (network layer) address is that of the destination host.

The router examines the packet's destination protocol address and determines whether it knows how to forward the packet or not. If the router does not know how to forward the packet, it typically drops the packet. If it knows how to forward packet, it changes the destination physical address to that of the next hop router and transmits the packet.

The next hop may be the destination or the next router, which executes the same switching process. As the packet moves through the internetwork, its physical address changes, but its protocol address remains same.

IEEE has developed the hierarchical terminology that is useful in describing this process. The network devices without capability to forward packets between subnetworks are called end system (ES), whereas network devices with this capabilities are called intermediate systems (IS). IS are further divided into those that can communicate within routing domain (Intradomain ES) and those that communicate both within and between routing domains (Interdomains IS). A routing domain is generally considered as portion of an internetwork under common administrative authority and is regulated by a particular set of administrative guidelines. Routing domains are also called as autonomous systems.