Atom Imsg - Cisco ASR 9000 Series Configuration Manuallines

Implementing Point to Point Layer 2 Services
Figure 13: IP Interworking over MPLS Core
An attachment circuit (AC) is a physical or logical port or circuit that connects a CE device to a PE device.
A pseudowire (PW) is a bidirectional virtual connection (VC) connecting two ACs. In an MPLS network,
PWs are carried inside an LSP tunnel. The core facing line card on the PE1 and PE2 could be a Cisco ASR
9000 Series SPA Interface Processor-700 or a Cisco ASR 9000 Series Ethernet line card.
In the IP Interworking mode, the Layer 2 (L2) header is removed from the packets received on an ingress PE,
and only the IP payload is transmitted to the egress PE. On the egress PE, an L2 header is appended before
the packet is transmitted out of the egress port.
In Figure above , CE1 and CE2 could be a Frame Relay (FR) interface or a GigabitEthernet (GigE) interface.
Assuming CE1 is a FR and CE2 is either a GigE or dot1q, or QinQ. For packets arriving from an Ethernet
CE (CE2), ingress LC on the PE (PE2) facing the CE removes L2 framing and forwards the packet to egress
PE (PE1) using IPoMPLS encapsulation over a pseudowire. The core facing line card on egress PE removes
the MPLS labels but preserves the control word and transmits it to the egress line card facing FR CE (CE1).
At the FR PE, after label disposition, the Layer 3 (L3) packets are encapsulated over FR.
Similarly, IP packets arriving from the FR CE are translated into IPoMPLS encapsulation over the pseudowire.
At the Ethernet PE side, after label disposition, the PE adds L2 Ethernet packet header back to the packet
before transmitting it to the CE, as the packets coming out from the core carry only the IP payload.
These modes support IP Interworking on AToM:
• Ethernet to Frame Relay
• Ethernet to Ethernet

AToM iMSG

This feature enables an interworking layer in the access network(s) to terminate all non-Ethernet functionality
and translate these connections to a Ethernet centric service which can be terminated on the Layer 3 edge
routers. Currently, the time-division multiplexing (TDM) based services terminate on the Layer 3 edge routers
directly. A simplified and more cost optimized model for the L3 networks is enabled by moving the TDM
complexity into the access layer.
The Layer 2 encapsulation is removed from an IP packet by the ingress PE's attachment circuit facing ingress
line card. The MPLS encapsulated IP packet payload is then sent across the fabric to the core facing egress
Packets arriving from the Ethernet CE device have MAC (port-mode, untagged, single, double tag), IPv4
header and data. The Ethernet line card removes the L2 framing and then forwards the L3 packet to the
egress line card. The egress line card adds the FR L2 header before transmitting it from the egress port.
Both the CE devices are Ethernet. Each ethernet interface can be port-mode, untagged, single, or double
tag, although this is not a typical scenario for IP interworking.
L2VPN and Ethernet Services Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 6.3.x
AToM iMSG
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