ATM Cell Relay Pseudowire VPI/VCI Swapping Overview
In ATM cell-relay mode, one or more ATM cells are bundled together to form a packet that is sent across the packet-switched network (PSN) using MPLS. In this mode, each ATM cell and its header are transported over the MPLS cloud. The ATM header consisting of the VPI and VCI values is transported across the MPLS cloud or the backhaul network. You can configure the ATM MIC to swap the VPI value, the VCI value, or both. You can swap the VPI and VCI values in both directions (ingress and egress). You can also enable swapping only on the egress side. Further, you can disable swapping of the values.
Figure 1 illustrates a sample application based on the mobile backhaul reference model.
In the case of ingress swapping, PE1 (MX Series router with ATM MIC with SFP) swaps the ATM value (7.7 or 7.8) for the configured PSN value (9.9). PE2 transmits the cell without making any change.
In the case of egress swapping, PE1 (MX Series router with ATM MIC with SFP) swaps the PSN value (9.9) for the local ATM value. The local ATM value depends on the ATM pseudowire on which the value arrived. In this example, it can be either 7.7 or 7.8.
This feature is not supported in port-promiscuous mode.
The following guidelines apply to configuring the ATM MIC for swapping:
In the case of promiscuous ports, no swapping is done because the feature is not supported. Values in the ATM cell are inserted into the pseudowire unchanged.
Each locally configured virtual path connection (VPC) or virtual channel connection (VCC) might be assigned a PSN VPI value or a PSN VCI value such that:
In the case of VCCs, the mapping is between the ATM identifier and the PSN identifier.
In the case of VPCs, the mapping is between the ATM identifier and the PSN identifier. No change is made to the VCI.
The following rule applies when the MIC inserts the VPI or VCI values in the outgoing pseudowire cell headers—that is, in case of ingress swapping:
VCCs or promiscuous VPCs—For a given ATM value, the specified PSN identifier is inserted in the outgoing pseudowire cell header.
If no PSN value is specified, then the MIC does not perform ingress swapping. The value inserted in the pseudowire cell header is the one found the in ATM cell.
The following rule applies when the MIC inserts the VPI or VCI values in the outgoing ATM cells—that is, in case of egress swapping:
VCCs or promiscuous VPCs—By default, the MIC performs egress swapping. The ATM identifier is inserted in the outgoing ATM cell irrespective of the value in the pseudowire cell header. If the no-vpivci-swapping statement is present, no swapping is performed and the value in the pseudowire is transmitted as is.