The forwarding of a labeled packet across an ATM-LSR domain is straightforward and is performed in three distinct steps:
The ingress ATM edge-LSR receives a labeled or unlabeled packet, performs a Forwarding Information Base (FIB) or Label Forwarding Information Base (LFIB) lookup, and finds the outgoing VPI/VCI value, which it uses as the outgoing label. The labeled packet is segmented into ATM cells and is sent toward the next ATM-LSR. The VPI/VCI value found during the label lookup phase is put into the ATM cell header of each cell.
From this moment until the labeled packet exits the ATM-LSR domain, the label lookup is performed purely based on VPI/VCI values, not on the MPLS label header. The MPLS label header is still present in the labeled packet, however, because it is needed to retain additional header fields, such as bottom-of-stack, Time-to-Live (TTL), and experimental bits.
ATM-LSRs switch cells based on the VPI/VCI value in the ATM cell header. The switching mechanism is the same as traditional ATM cell switching, and the MPLS label allocation and distribution mechanisms are responsible for establishing proper inbound/outbound VPI/VCI mappings.
The egress ATM edge-LSR reassembles the cells into a labeled packet, performs the label lookup, and forwards the packet toward its next-hop LSR. The label lookup is based on the VPI/VCI values of the incoming cells, not on the top-of-stack label in the MPLS label header. This is because the ATM-LSRs between the edges of the ATM-LSR domain have changed only the VPI/VCI values, not the labels inside the ATM cells.
Because the top-of-stack label is not used by the egress edge ATM-LSR, it is set to 0 by the ingress ATM edge-LSR before the labeled packet is segmented in ATM cells.
The major differences between frame-based label switching and cell-based label switching are listed here:
Label lookup in frame-based label forwarding is performed based on the top-of-stack label in the MPLS label header. In cell-based forwarding, the lookup is performed on the VPI/VCI values in the ATM cell headers.
The switching mechanism in cell-based label switching is traditional ATM cell switching based on VPI/VCI values in the cell headers. MPLS label stack is completely ignored by ATM-LSRs.
The top-of-stack label in the MPLS label header is set to 0 by the ingress ATM edge-LSR.