Support on root-leaf-tag SAPs requires that the outer VID is overloaded to indicate root and leaf. To support the SR service model for a SAP, the ability to send and receive two different tags on a single SAP has been added. Figure 1 shows the behavior when a root-ac and leaf-ac exchange traffic over a root-leaf-tag SAP. Although the figure shows two SAPs connecting VPLS instances 1 and 2, the CLI shows a single SAP with the format:
sap 2/1/1:25 root-leaf-tag leaf-tag 26 create
The root-leaf-tag SAP performs all of the operations for egress and ingress traffic for both tags (root and leaf):
When receiving a frame, the outer tag VID is compared against the configured root or leaf VIDs and the frame forwarded accordingly.
When transmitting, the system adds a root VLAN (in the outer tag) on frames with an internal indication of Root, and a leaf VLAN on frames with an internal indication of Leaf.