Cisco ASR1002 - ASR 1002 Router Software Configuration Manual page 381

Configuring MPLS Layer 2 VPNs
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
feature.
Figure 2
ATM
CE
LLC (AA-AA)
LLC (03) OU I (00)
CUI (80-C2)
PID (00-007)
PAD (00-00)
Ethertype 81-00
VLAN Tag
CPCS-UU CPI
Length
CRC
shows the protocol stack for the Frame Relay DLCI-to-ATM AAL5SNAP Bridged Interworking
Protocol Stack for Frame Relay DLCI-to-ATM AAL5SNAP Bridged Interworking
ATM
ATM
PE
ATM Header
Tunnel Label
VC Label
Control Word
MAC Header MAC Header
DA DA
SA
SA
Ethertype 81-00
VLAN Tag
Type/Length Type/Length
Remainder of Remainder of
MAC Frame MAC Frame
Cisco ASR 1000 Series Aggregation Services Routers Software Configuration Guide
Frame Relay DLCI-to-ATM AAL5SNAP Bridged Interworking
MPLS
Emulated VC of type 5
Tunnel Label
VC Label
Control Word
MAC Header
DA
SA
Ethertype 81-00
VLAN Tag
Type/Length
Remainder of
MAC Frame
Frame Relay
FR FR
PE 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
FCS
5