On a single VLAN on a single switch all frames are processed and
passed between ports based upon inspection of Layer 2 headers. Once packets
must be passed between VLANs, this requires packet forwarding based upon
inspection of the Layer 3 header, specifically the destination IP address.
In a multilayer switch, Layer 2 and Layer 3 processing occur on the same
device. However, if a switch supports multiple VLANs but has no Layer 3
capability to route packets between those VLANs, the switch must be connected
to a router external to the switch. This can be accomplished by setting up one
router interface for each VLAN on the switch. The router then provides all
Layer 3 routing functionality between VLANs on the switch.
In Figure
, the clients on
VLAN10 need to establish sessions with a server that is in VLAN20. This will
require that traffic be routed between the VLANs as described in Figure
.
External Router with Multiple Interface: Advantages and
Disadvantages
Figure
describes the
advantages and disadvantages of using an external router with multiple
interfaces for the purpose of inter-VLAN routing.