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In basic terminology, a trunk is a
point-to-point link that supports several VLANs. The purpose of a
trunk is to save ports when creating a link between two devices
implementing VLANs, typically two switches. In the top figure, we
can see two VLANs that we want available on two switches, Sa and Sb.
The first easy method of implementation is to create two physical links
between the devices, each one carrying the traffic for a separate
VLAN.
Of course, this first solution does
not scale very well. If we wanted to add a third VLAN, we would need
to sacrifice two additional ports. This design is also inefficient
in terms of load sharing; the traffic on some VLANs may not justify
a dedicated link. A trunk will bundle virtual links over one
physical link, as shown in the bottom figure.
Here, the unique physical link
between the two switches is able to carry traffic for any VLAN. In
order to achieve this, each frame sent on the link is tagged by Sa
so that Sb knows which VLAN it belongs to. Different tagging schemes
exist. The most common for Ethernet segments follow:
- ISL (this is the original Cisco
proprietary InterSwitch Link protocol)
- 802.1Q (the IEEE standard we focus
on in this section)
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