Cisco ASR1006 Configuration Guide - Page 381

Configuring MPLS Layer 2 VPNs Frame Relay DLCI-to-ATM AAL5SNAP Bridged Interworking On the ATM PE router with interworking function, when traffic flows from the ATM segment to MPLS cloud, the bridged encapsulation (ATM and SNAP header) is discarded and the Ethernet frame is encapsulated with the labels required to go through the pseudowire using the VC type 5 (Ethernet). In the opposite direction, after the label disposition from the MPLS cloud, the Ethernet frames are encapsulated over AAL5SNAP using bridged encapsulation. On the FR PE router with interworking function, when traffic flows from the FR segment to the MPLS cloud, the bridged encapsulation (Frame Relay and SNAP header) is discarded and the Ethernet frame is encapsulated with the labels required to go through the pseudowire, using the VC type 5 (Ethernet). In the opposite direction, after the label disposition from the MPLS cloud, the Ethernet frames are encapsulated over FR using bridged encapsulation. The PE router automatically supports translation of both Cisco and IETF Frame Relay encapsulation types coming from the Customer edge (CE) router, but translates only to IETF when sending to the CE router. The Cisco CE router can handle the IETF encapsulation on receipt, even if it is configured to send Cisco encapsulation. The following modes are supported: • The ATM permanent virtual circuit (PVC) mode with the AAL5SNAP encapsulation type, and the existing Quality of Service (QoS) functionality for ATM PVCs. • The Frame Relay DLCI mode, and the existing QoS functionality for Frame Relay. PVC status signaling works the same way it does in the like-to-like case. The PE router reports the PVC status to the CE router, based on the availability of the pseudowire. The attachment circuit maximum transmission unit (MTU) on both sides of the pseudowire must match when connected over MPLS. The non-AAL5 traffic (such as OAM cells) is punted to be processed at the RP level. A VC that is configured with OAM cell emulation on the ATM PE router (using the oam-ac emulation-enable command) can send end-to-end F5 loopback cells at configured intervals toward the CE router. When the pseudowire is down, an end-to-end F5 segment alarm indication signal (AIS) and remote defect indication (RDI) is sent from the PE router to the CE router. Figure 2 shows the protocol stack for the Frame Relay DLCI-to-ATM AAL5SNAP Bridged Interworking feature. Figure 2 Protocol Stack for Frame Relay DLCI-to-ATM AAL5SNAP Bridged Interworking ATM ATM CE ATM Header LLC (AA-AA) LLC (03) OU I (00) CUI (80-C2) PID (00-007) PAD (00-00) MAC Header DA SA Ethertype 81-00 VLAN Tag Type/Length Remainder of MAC Frame CPCS-UU CPI Length CRC ATM PE MPLS Emulated VC of type 5 FR PE Tunnel Label VC Label Control Word MAC Header DA SA Tunnel Label VC Label Control Word MAC Header DA SA Frame Relay FR CE Frame Relay Header Q.922 Address Ctrl (03 | Pad(03) NL PD (80) | OUT (00) OUT (80-C2) PID (00-007) MAC Header DA SA Ethertype 81-00 VLAN Tag Type/Length Remainder of MAC Frame Ethertype 81-00 VLAN Tag Type/Length Remainder of MAC Frame Ethertype 81-00 VLAN Tag Type/Length Remainder of MAC Frame FCS 331936 Cisco ASR 1000 Series Aggregation Services Routers Software Configuration Guide 5