An introduction to Layer-3 Network Switches

Do you need Layer-3 Switches in your network? Possibilities are that you might already be having some of them. They are mostly found on the network core (or) distribution level. In this article, we will look at what Layer-3 switches are, and some advantages of using Layer-3 network switches.

In a small network, its possible to have just Layer-2 switches (Edge switches that segment the network based on MAC addresses (or) No segmentation at all). But as the network grows, it is more prudent to segment the network (using IP subnets, etc which limits the broadcast domain) using Layer-3 Switches.

One of the important functions performed by Layer-3 Switches is connecting and routing between the various sub-networks within the corporate LAN (like Inter-VLAN Routing). Though Layer-3 switches perform the functions of a Router, they are designed and optimized to be used in the LAN environment.

Just like Routers, Layer-3 Switches support routing protocols like RIP, OSPF, etc and enable faster convergence in case of link failures within the LAN. When a Layer-3 switch receives a packet, it looks up its routing table to determine the best path that the packet should take to reach its destination within the network.

But with Layer-3 Switches, most of the routing look-up functions are moved from software (as in router) to ASIC chip hardware, and hence they are faster. They have a faster cache-lookup memory for improving the performance, but it can handle limited number of IP addresses – which is sufficient in the case of LAN (Unlike WAN). Layer-3 switches might inspect the routing tables for only the first packet and switch the next ‘n’ packets belonging to the same group at line speed. All these techniques enable a higher aggregate performance, and that is one of the main differences between Layer-3 switches and routers.

Layer-3 switches support IGMP Snooping, preventing IGMP data from being broadcasted (hence consuming lesser bandwidth). Layer-3 Switches do Inter-VLAN routing faster (than routers, if they were employed). They also allow the servers to be on their own domain with no performance penalty,   while still achieving their centralization and higher security. Layer-3 Switches help perform QoS and traffic prioritization more effectively across the network to enable better performance for voice, video and multimedia data while still preserving the subnets.

Many Layer-3 Switches however, lack the range of WAN interfaces to connect to the outside world (MPLS Links, Internet Leased Lines, etc). A router might be better for that purpose. Routers also employ techniques like delay bandwidth buffering, congestion buffering to queue up excessive bandwidth going through a narrow WAN link, hence avoiding congestion and making the WAN connectivity more reliable. Routers can also look-up a higher range of IP addresses, which is typically needed in huge WAN networks like the Internet.

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