What occurs when an untagged frame is received by an 802.1Q trunk port?

Last Updated on August 1, 2021 by Admin 2

What occurs when an untagged frame is received by an 802.1Q trunk port?

  • It discards the frame.
  • It tags the frame with the identified native VLAN value.
  • It forwards the frame out each port of the switch not assigned to a VLAN.
  • It forwards the frame to a port belonging to the same VLAN as the native VLAN.

IEEE 802.1Q supports configuring native VLANs. A native VLAN is the VLAN a port is in when not in trunking mode. Native VLAN packets are sent untagged. If an 802.1Q trunk receives an untagged frame, it will forward that frame to a port that belongs to the same VLAN as the identified native VLAN. The frame is treated as if it were tagged with the same VLAN ID as the native VLAN. Frames received through ports having the same membership as the identified native VLAN of the trunk will be forwarded untagged out of the trunk.

It is important that the native VLAN settings on each end of an 802.1Q trunk match.

The 802.1Q standard specifies support for a maximum 4094 VLANs (IDs 0 and 4095 are reserved). Therefore, ID values of 1-4094 are assignable. In contrast, the valid range of configurable ISL VLANs is 1-1001. The following is a summary of VLAN IDs:

  • 0 and 4095: Reserved
  • 1: Cisco default management
  • 2-1001: Available for Ethernet VLANs
  • 1002-1005: Defaults for FDDI and Token Ring VLANs
  • 1006-4094: Extended range available for Ethernet VLANs (802.1Q only)

Recognizing the difference in supported VLAN ID ranges highlights several issues in constructing a network of both ISL and 802.1Q VLAN networks. Ethernet VLAN IDs above the supported ISL range must be mapped to IDs within the range supported by ISL. Among other limitations, you are limited to eight total mappings. This process of mapping 802.1Q to ISL VLAN IDs will further restrict and define what IDs are actually available to be used.

Untagged frames are not discarded, but are sent to the native VLAN.

Untagged frames are not tagged with the tag of the native VLAN. They are simply forwarded to that VLAN. No packets in the native VLAN have tags.

Untagged frames are not forwarded out all ports not assigned to a VLAN. It will only be forwarded to the switchport where the destination MAC address resides.

Layer 2 Technologies
Configure and verify trunking

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