PXC traffic passing through the MAC loopback is mapped to a specific source fabric tap that moves the traffic from the local source forwarding complex into the fabric and toward the destination forwarding complex. A fabric tap represents a chip that connects a forwarding complex to the system fabric. Traffic that is on its way from an ingress port (any port, including a PXC port) to the destination port, is always mapped to the same fabric tap (source fabric tap) on the ingress forwarding complex. If the source forwarding complex has two fabric taps, the fabric tap selection plays a role in optimal bandwidth distribution. An example of these forwarding complexes can be found on IOM-s cards in SR-s platforms.
On IOM-s 3.0T, the source tap selection is based on the loopback ID. The mapping scheme is simple; loopbacks with even IDs are mapped to one source tap while loopbacks with odd IDs are mapped to the other. This is shown in Figure: Mapping of internal loopbacks to source taps. On IOM-s 1.5T, the mapping is based on the MDA number.
