Installation guide

Network Infrastructure for EtherNet/IP™
Designing the Infrastructure
4-53
4.10.1 VLAN Trunking
VLANs can span across multiple switches and you can have more than one VLAN on each switch. For
multiple VLANs on multiple switches to be able to communicate via a single link between the switches,
trunking is needed. Trunking is the technology that allows information from multiple VLANs to be
carried over just one link between switches.
Switches use
VLAN trunking protocols to communicate VLAN configuration information between
them. Standardized protocols such as the
Group VLAN Registration Protocol (GVRP) allow VLAN
trunk connections between switches from different vendors. Proprietary, vendor-specific trunking
protocols are also available.
Figure 4-13 Devices on Three VLANs Communicate via Trunking Links
between Three Switches and a Router.
In Figure 4-13, each switch has two VLANs. On the first switch, VLAN A and VLAN B are sent through
a single port (trunked) to both the router and through another port to the second switch. VLAN C and
VLAN D are trunked from the second switch to the first switch and through it to the router. This trunk
can carry traffic from all four VLANs. The trunk link from the first switch to the router can also carry all
four VLANs. In fact, this one connection to the router actually allows the router to appear on all four
VLANs as if it had four different physical ports connected to the switch.
The VLANs can communicate with each other via the trunking connection between the two switches
using the router. For example, data from a computer on VLAN A that needs to get to a computer on
VLAN B (or VLAN C or VLAN D) must travel from the switch to the router and back again to the
switch. Because of the switches’ transparent bridging algorithm and trunking, both PCs and the router
think they are on the same physical segment.