What is Layer 3 Switching

Layer-3 Switching

In addition to performing Layer-2 switching functions, a Layer-3 switch
must also meet the following criteria:
• The switch must be capable of making Layer-3 forwarding decisions
(traditionally referred to as routing).
• The switch must cache network traffic flows, so that Layer-3
forwarding can occur in hardware.

Many older modular switches support Layer-3 route processors – this alone
does not qualify as Layer-3 switching. Layer-2 and Layer-3 processors can
act independently within a single switch chassis, with each packet requiring
a route-table lookup on the route processor.

Layer-3 switches leverage ASICs to perform Layer-3 forwarding in
hardware. For the first packet of a particular traffic flow, the Layer-3 switch
will perform a standard route-table lookup. This flow is then cached in
hardware – which preserves required routing information, such as the
destination network and the MAC address of the corresponding next-hop.

Subsequent packets of that flow will bypass the route-table lookup, and will
be forwarded based on the cached information, reducing latency. This
concept is known as route once, switch many.

Layer-3 switches are predominantly used to route between VLANs:

Traffic between devices within the same VLAN, such as ComputerA and
ComputerB, is switched at Layer-2 as normal. The first packet between
devices in different VLANs, such as ComputerA and ComputerD, is routed.
The switch will then cache that IP traffic flow, and subsequent packets in
that flow will be switched in hardware
.
Layer-3 Switching Vs. Routing