Juniper Networks EX2500 manual Vlan Tagging, Illustrates the default Vlan settings on the switch

Chapter 2: VLANs

VLAN Tagging

EX2500 software supports 802.1Q VLAN tagging, providing standards-based VLAN support for Ethernet systems.

Tagging places the VLAN identifier in the frame header of a packet, allowing each port to belong to multiple VLANs. When you add a port to multiple VLANs, you also must enable tagging on that port.

Since tagging fundamentally changes the format of frames transmitted on a tagged port, you must carefully plan network designs to prevent tagged frames from being transmitted to devices that do not support 802.1Q VLAN tags, or devices where tagging is not enabled.

The following important terms are used with the 802.1Q tagging feature:

„VLAN identifier (VID)—The 12-bit portion of the VLAN tag in the frame header that identifies an explicit VLAN.

„Port VLAN identifier (PVID)—A classification mechanism that associates a port with a specific VLAN. For example, a port with a PVID of 3 (PVID = 3) assigns all untagged frames received on this port to VLAN 3. Any untagged frames received by the switch are classified with the PVID of the receiving port.

„Tagged frame—A frame that carries VLAN tagging information in the header. This VLAN tagging information is a 32-bit field (VLAN tag) in the frame header that identifies the frame as belonging to a specific VLAN. Untagged frames are marked (tagged) with this classification as they leave the switch through a port that is configured as a tagged port.

„Untagged frame— A frame that does not carry any VLAN tagging information in the frame header.

„Untagged member—A port that has been configured as an untagged member of a specific VLAN. When an untagged frame exits the switch through an untagged member port, the frame header remains unchanged. When a tagged frame exits the switch through an untagged member port, the tag is stripped and the tagged frame is changed to an untagged frame.

„Tagged member—A port that has been configured as a tagged member of a specific VLAN. When an untagged frame exits the switch through a tagged member port, the frame header is modified to include the 32-bit tag associated with the PVID. When a tagged frame exits the switch through a tagged member port, the frame header remains unchanged (original VID remains).

NOTE: If an 802.1Q tagged frame is received by a port that has VLAN tagging disabled and the port VLAN ID (PVID) is different from the VLAN ID of the packet, then the frame is dropped at the ingress port.

Figure 1 illustrates the default VLAN settings on the switch.

VLAN Tagging „ 23