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9.1 | |
Trunking | |
| 9.1.3 | |
Trunking operation |
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The switching tables at both ends of the
trunk can be used to make port forwarding decisions based on frame
destination MAC addresses. As the number of VLANs traveling across the
trunk increases, the forwarding decisions become slower and more
difficult to manage . The decision process becomes slower because the
larger switching tables take longer to process.
Trunking protocols were developed to effectively manage the transfer of frames from different VLANs on a single physical line. The trunking protocols establish agreement for the distribution of frames to the associated ports at both ends of the trunk. Currently two types of trunking
mechanisms exist, frame filtering and frame tagging. Frame tagging has
been adopted as the standard trunking mechanism by IEEE.
Trunking protocols that use a frame tagging mechanism assign an identifier to the frames to make their management easier and to achieve a faster delivery of the frames. 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 to identify which VLAN it belongs to. Different tagging schemes exist. The most common tagging schemes for Ethernet segments are listed below:
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