Symmetry/Asymmetry Mode

QinQ termination sub-interfaces can access the L2VPN in symmetry mode or asymmetry mode.

In symmetric mode, when sub-interfaces for QinQ VLAN tag termination are used to access an L2VPN, packets received by the edge devices on the two ends of the public network must carry the same VLAN tags.

In symmetry mode, the VLAN planning at each site must be consistent, and only users in the same VLAN at different sites can communicate with each other. In this mode, user VLANs can be isolated according to inner tags. MAC address learning is based only on outer tags, and inner tags are transparently transmitted to the remote end.
In asymmetric mode, when sub-interfaces for QinQ VLAN tag termination are used to access an L2VPN, packets received by the edge devices on the two ends of the public network may carry different VLAN tags.

In asymmetrical mode, the VLANs planning at each site can be different, and users in VLANs at any sites can communicate with each other. In this mode, user VLANs cannot be isolated, and MAC address learning is based on both inner and outer tags.

Table 1 and Table 2 describe how a PE processes user packets that arrive at an L2VPN in different ways.

**Table 1** Packet processing on an inbound interface
Type of the Inbound Interface	VPWS/VPLS Ethernet Encapsulation	VPWS/VPLS VLAN Encapsulation
Symmetry mode	Removes the outer tag.	No action is required.
Asymmetry mode	Removes both the inner and outer tags.	Removes both inner and outer tags and adds another tag.

**Table 2** Packet processing on an outbound interface
Type of the Outbound Interface	VPWS/VPLS Ethernet Encapsulation	VPWS/VPLS VLAN Encapsulation
Symmetry mode	Adds an outer tag.	Replaces the outer tag.
Asymmetry mode	Adds two tags.	Removes one tag and adds another double tags.