How does the S2700 implement VLAN mapping and what scenarios need to be concerned

When configuring VLAN mapping on the S2700, pay attention to the following points:

Working principle of VLAN mapping

After VLAN mapping is enabled on an interface, the interface replaces VLAN tags of packets based on the C-VLAN ID and the result of the modulo operation against 128.

For example, if the C-VLAN IDs range from 1 to 10 and the S-VLAN ID is 100, then:

If the VLAN ID of an incoming packet is 130, the modulo result of 128 is 2, which is within the range of 1 to 10. Therefore, the interface changes the VLAN ID of the incoming packet with 100 and then forwards the packet.

If the VLAN ID of an incoming packet is 140, the modulo result is 12, which is out of the range of 1 to 10. According to the initial configuration, VLAN 12 is mapped to VLAN 12. Thus interface changes the VLAN ID of the incoming packet to 12 and forwards the packet, as long as the interface has already been added to VLAN 12.

VLAN translation is configured as follows during initialization:

VLANs 1 to 127 are translated in the format of X -> X, that is, not translated.

VLAN 0 is translated to VLAN 128.

VLAN translation is configured as follows during the VLAN mapping configuration:

When the C-VLAN ID is greater than 128, the system performs the modulo operation on the C-VLAN ID against 128 and uses the modulo result as the C-VLAN ID.

Other VLANs are not translated.

No qinq vlan-translation miss-drop command

The VLAN tags of all packets are translated according to the modulo result. Untagged packets are processed according to whether the default VLAN of the interface is configured by the port default vlan command. If the default VLAN is configured, untagged packets are added the PVID and forwarded. Otherwise, untagged packets are discarded.

Scenarios to be concerned

If C-VLANs 1 to 10 are mapped to S-VLAN 100 in the VLAN mapping configuration, packets of VLAN 130 that should be transmitted transparently are forwarded after their VLAN tags are changed to VLAN 100. In this case, services may be affected.

To enable packets of VLAN 130 to be transmitted transparently, you need to configure the mapping from VLAN 130 to VLAN 130. Therefore, limit the VLAN ID to 128 during the early network planning so that packets from all VLANs can be forwarded normally.

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