• The IP address of the loopback interface of the peer PE router, which enables the PE routers to
communicate.
• A unique combination of peer PE IP address and VC ID that identifies the pseudowire.
Control Word Processing
The control word contains forward explicit congestion notification (FECN), backward explicit congestion
notification (BECN) and DE bits in case of frame relay connection.
Control word is mandatory for:
• Frame Relay
• ATM AAL5
• Frame Relay to Ethernet bridged interworking
• cHDLC/PPP IP interworking
• CEM (Circuit Emulation)
The system does not map bits from one transport end point to another across an AToM IP Interworking
connection.
Whenever supported, control word is also recommended for pseudowires, as it enables proper load balancing
without packet desequencing independent of L2VPN packet content. Without control word the heuristics used
to perform load balancing cannot achieve optimal results in all cases.
High-level Data Link Control over MPLS
The attachment circuit (AC) is a main interface configured with HDLC encapsulation. Packets to or from the
AC are transported using an AToM pseudowire (PW) of VC type 0x6 to or from the other provider edge (PE)
router over the MPLS core network.
With HDLC over MPLS, the entire HDLC packet is transported. The ingress PE router removes only the
HDLC flags and FCS bits.
PPP over MPLS
The attachment circuit (AC) is a main interface configured with PPP encapsulation. Packets to or from the
AC are transported through an AToM PW of VC type 0x7 to or from the other provider edge (PE) routers
over the MPLS core network.
With PPP over MPLS, the ingress PE router removes the flags, address, control field, and the FCS bits.
Frame Relay over MPLS
Frame Relay over MPLS (FRoMPLS) provides leased line type of connectivity between two Frame Relay
islands. Frame Relay traffic is transported over the MPLS network.
The Data Link Connection Identifier (DLCI) DCLI-DLCI mode is supported. A control word (required for
DLCI-DLCI mode) is used to carry additional control information.
Note
L2VPN and Ethernet Services Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 6.3.x
87
Implementing Point to Point Layer 2 Services
Control Word Processing
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