require a solution to support AToM with disparate transport at network ends.
This solution must have the capability to translate transport on one customer
edge (CE) device to another transport, for example, Frame relay to Ethernet.
The Cisco ASR 9000 Series SPA Interface Processor-700 and the Cisco ASR 9000
Series Ethernet line cards enable the Cisco ASR 9000 Series Routers to support
multiple legacy services.
IP Interworking is a
solution for transporting Layer 2 traffic over an IP/MPLS backbone. It
accommodates many types of Layer 2 frames such as Ethernet and Frame Relay
using AToM tunnels. It encapsulates packets at the provider edge (PE) router,
transports them over the backbone to the PE router on the other side of the
cloud, removes the encapsulation, and transports them to the destination. The
transport layer can be Ethernet on one end and Frame relay on the other end. IP
interworking occurs between disparate endpoints of the AToM tunnels.
interworking is supported between Ethernet and Frame Relay based networks for
MPLS and Local-connect scenarios.
The following figure
shows the interoperability between an Ethernet attachment VC and a Frame Relay
Figure 8. 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
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
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