Ieee 802.1Q Encapsulation - Cisco Nexus 1000V Configuration Manual

IEEE 802.1Q Encapsulation

• An access port can have only one VLAN configured on that port; it can carry traffic for only one VLAN.
• A trunk port can have two or more VLANs configured on that port; it can carry traffic for several VLANs
By default, all ports on the Cisco Nexus 1000V are Layer 2 ports. You can change the default port mode
(access or trunk). See the Cisco Nexus 1000V Installation and Upgrade Guide for information about setting
the default port mode. The following figure shows how you can use trunk ports in the network. The trunk port
carries traffic for two or more VLANs.
Figure 1: Trunk and Access Ports and VLAN Traffic
In order to correctly deliver the traffic on a trunk port with several VLANs, the device uses the IEEE 802.1Q
encapsulation, or tagging, method.
To optimize the performance on access ports, you can configure the port as a host port. Once the port is
configured as a host port, it is automatically set as an access port and channel grouping is disabled. Use the
host designation to decrease the time that it takes the designated port to begin to forward packets.
If an access port receives a packet with an 802.1Q tag in the header other than the access VLAN value, that
port drops the packet without learning its MAC source address.
A Layer 2 interface can function as either an access port or a trunk port; it cannot function as both port types
simultaneously.
IEEE 802.1Q Encapsulation
A trunk is a point-to-point link between the switch and another networking device. Trunks carry the traffic
of multiple VLANs over a single link and allow you to extend VLANs across an entire network.
To correctly deliver the traffic on a trunk port with several VLANs, the device uses the IEEE 802.1Q
encapsulation, or tagging, method that uses a tag that is inserted into the frame header (see the following
figures). This tag carries information about the specific VLAN to which the frame and packet belong. This
method allows packets that are encapsulated for several different VLANs to traverse the same port and maintain
Cisco Nexus 1000V for VMware vSphere Interface Configuration Guide, Release 5.x
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simultaneously.
Configuring Layer 2 Interfaces