Cisco Nexus 5000 Series Switch CLI Software Configuration Guide, NX-OS 4.0(1a)N1 (OL-16597-01, January 2009)

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12-2
Cisco Nexus 5000 Series Switch CLI Software Configuration Guide
OL-16597-01
Chapter 12 Configuring Access and Trunk Interfaces
Information About Access and Trunk Interfaces
Figure 12-1 Devices in a Trunking Environment
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 (see the “Understanding IEEE 802.1Q Encapsulation” section
on page 12-2 for more information on this subject).
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 it takes the designated port to begin to forward packets.
Note Only an end station can be set as a host port; you will receive an error message if you attempt to configure
other ports as hosts.
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.
Note An Ethernet interface can function as either an access port or a trunk port; it cannot function as both port
types simultaneously.
Understanding IEEE 802.1Q Encapsulation
A trunk is a point-to-point link between the device 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 (tagging) method that uses a tag that is inserted into the frame header. 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 traffic
separation between the VLANs. The encapsulated VLAN tag also allows the trunk to move traffic
end-to-end through the network on the same VLAN.
Switch
Switch
Switch
Switch
Switch
VLAN2
VLAN3VLAN1
VLAN1
VLAN2
VLAN3
Trunk
port
Trunk
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