ATM-To-Ethernet Interworking on ATM MICs
ATM-to-Ethernet interworking supports transmission of ATM packets over Ethernet. It specifically provides support for exchange of Layer 2 and Layer 3 Protocol Data Units (PDUs) between ATM and Ethernet domains. On MX Series 5G Universal Routing Platforms with ATM MICs, you can exchange Ethernet frames between ATM and Ethernet domains over a MPLS pseudowire or a Layer 2 cross-connect by using translational cross connect (TCC). For more information about TCC, see Circuit and Translational Cross-Connects Overview.
Consider the following basic ATM-to-Ethernet Interworking topology where the provider edge router PE1 is connected to an ATM domain and the Provider Edge router PE2 is connected to an Ethernet domain (see Figure 1). The customer edge routers CE1 and CE2 are customer-managed devices. The PE routers are connected by means of an MPLS pseudowire. The ATM traffic on the PE1–CE1 link can comprise untagged Ethernet frames over ATM format. The Ethernet traffic on PE2–CE2 link can comprise untagged, single-VLAN or double-VLAN tagged Ethernet frames depending on the configuration of the PE2 router.
For ATM-to-Ethernet Interworking, the virtual path identifier (VPI) and virtual circuit identifier (VCI) values on the ATM link are mapped to the outer and inner VLAN tag on the Ethernet link. Mapping implies that either the same value is copied or a one-to-one translation is performed. If VLAN translation is enabled, instead of copying the value, a one-to-one translation is performed on the Ethernet facing PE, using a lookup table.
ATM cells that are received on the PE1 router are reassembled into ATM Adaptation Layer 5 (AAL5) logical link control (LLC) frames. The router removes the header and footer and adds two VLAN tags (outer and inner) to the untagged Ethernet payload based on the configuration. The VLAN IDs must correspond to the VPI and VCI of the ATM cell. You must add an MPLS label before transmitting the dual-VLAN-tagged Ethernet frame over the MPLS pseudowire. You can also add other optional MPLS tags.
If the AAL5 frame sent by CE1 is not encapsulated with LLC and if the untagged Ethernet payload includes the frame check sequence (FCS), PE1 rejects the AAL5 frame. Also, PE1 can transmit and receive only a dual VLAN-tagged Ethernet frame without FCS. Inclusion of FCS can result in packet drops or data corruption.
On the PE2 router, the MPLS label and optional MPLS tags are removed and the Ethernet frame is transmitted toward the CE2 router. You can modify or remove one or both VLAN tags before forwarding the frame to the CE2 router.
Following are the limitations of the ATM-to-Ethernet interworking feature on MX Series routers with ATM MICs:
The ATM-to-Ethernet interworking feature is not backward compatible or does not interoperate with the ATM-Ethernet interworking feature supported on M Series and T Series Routers. The functionality is the same but the implementation is different.
The total number of VCIs supported is 4000 for the ATM MIC. This is an existing system limit.
If an ATM logical interface is configured with vci-range of N VCIs, then N VCIs are deducted from the available pool of 4000 VCIs.
ATM quality of service (QoS) is not supported with the vlan-vci-ccc encapsulation. If you use the vci-range statement then the vlan-vci-ccc encapsulation supports multiple VCIs on a single logical interface. This is a hardware limitation.