Cisco Systems OL-4580-01 manual For an 802.1Q trunk, you can use the default-vlan command to

Chapter 1 Configuring Interfaces and Circuits

Interface and Circuit Overview

Using the trunk command, you can assign multiple VLANs to a CSS Ethernet interface port (Fast Ethernet port or Gigabit Ethernet port). A trunk is a point-to-point link carrying the traffic of several VLANs. The advantage of a trunk is to save ports by creating a link between two CSSs implementing VLANs. A trunk bundles virtual links over one physical link. The unique physical link between the two CSSs is able to carry traffic for the specified VLANs.

Note The trunk and vlan commands (and the associated software functionality) comply with the IEEE 802.1Q Standard for Local and Metropolitan Area Networks: Virtual Bridged Local Area Networks.

The CSS forwards VLAN circuit traffic to the IP interface. The IP interface passes the traffic to the IP forwarding function where the CSS compares the destination of each packet to information contained in the routing table. Once the CSS resolves the packet addresses, it forwards the packet to the appropriate VLAN and destination port.

With trunking enabled, the CSS automatically inserts a tag in every frame transmitted over the trunk link to identify the originating VLAN. When the VLAN-aware CSS receives the frame, it reviews the VLAN-tagged packet to identify the transmitting VLAN. If the VLAN is recognized, the frame is routed to the proper port and VLAN destination. If the frame is from a VLAN that is not assigned to the trunk port, the packet is ignored. By default, the CSS discards untagged packets.

For an 802.1Q trunk, you can use the default-vlancommand to:

•Accept packets that arrive untagged on the interface

•Transmit untagged packets

By using this method, the CSS can determine which VLAN transmitted an untagged frame. This capability allows VLAN-aware CSSs and VLAN-unaware CSSs to transmit and receive information on the same cable.

	Cisco Content Services Switch Routing and Bridging Configuration Guide
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