Configuring Frame Relay Control Bit Translation
On interfaces with Frame Relay CCC encapsulation, you can configure Frame Relay control bit translation, as defined in the IETF documents:
- Internet draft draft-martini-frame-encap-mpls-00.txt, Frame Relay Encapsulation over Pseudo-Wires (expired December 2002)
- Internet draft draft-martini-l2circuit-encap-mpls-07.txt, Encapsulation Methods for Transport of Layer 2 Frames Over IP and MPLS Networks (expired December 2004)
To support Frame Relay services over IP and MPLS backbones using Layer 2 VPNs and Layer 2 circuits, you can configure translation of the Frame Relay control bits. When you configure translation of Frame Relay control bits, the bits are mapped into the Layer 2 circuit control word and preserved across the IP or MPLS backbone.
Junos OS allows you to translate the following Frame Relay control bits:
- Discard eligibility (DE)—A header bit used to identify lower priority traffic that can be dropped during periods of congestion.
- Forward explicit congestion notification (FECN)—A header bit transmitted by the source router requesting that the destination router slow down its requests for data.
- Backward explicit congestion notification (BECN)—A header bit transmitted by the destination router requesting that the source router send data more slowly.
By default, translation of Frame Relay control bits is disabled. If you enable Frame Relay control bit translation, the bits are translated in both directions (CE to PE and PE to CE):
- From CE to PE—At ingress, the DE, FECN, and BECN header bits from the incoming Frame Relay header are mapped to the control word.
- From PE to CE—At egress, the DE, FECN, and BECN header bits from the control word are mapped to the outgoing Frame Relay header.
The Frame Relay control bits do not map to MPLS EXP labels, and do not affect class-of-service (CoS) behavior inside the provider network.
You enable or explicitly disable translation of Frame Relay control bits by including the translate-discard-eligible and translate-fecn-and-becn statements:
You can include these statements at the following hierarchy levels:
- [edit interfaces interface-name unit logical-unit-number family ccc]
- [edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number family ccc]
If you enable or disable Frame Relay control bit translation on one CE-facing interface, you must configure the same Frame Relay control bit translation settings on the other CE-facing interface.
If you change the Frame Relay control bit translation settings, the circuit goes down and comes back up, which might result in traffic loss for a few seconds.
If you enable Frame Relay control bit translation, the number of supportable Layer 2 virtual private networks (VPNs) and Layer 2 circuits is reduced to one eighth of what the router can support without Frame Relay control bit translation enabled.
You can classify and rewrite the control word DE bit based on the packet loss priority (PLP) by using the translate-plp-control-word-de statement at the [edit interfaces interface-name unit logical-unit-number family ccc] hierarchy level.
- When you configure the translate-plp-control-word-de statement on the ingress PE router, the DE bit in the control word is rewritten based on the PLP.
- When you configure the translate-plp-control-word-de statement on the egress PE router, the PLP is derived based on the control word DE bit and the DE bit in the outgoing Frame Relay header is rewritten based on the PLP.
- By default, control word classification and rewrite is disabled.
The mapping of the PLP to the DE bit in the control word and the outgoing Frame Relay packet is fixed. For rewriting, the PLP values low and medium-low are mapped to the DE bit 0 and the PLP values high and medium-high are mapped to the DE bit 1. For classifying, the DE bit 0 is mapped to the PLP value low and the DE bit 1 is mapped to the PLP value high.
 | Note: The translate-discard-eligible and translate-plp-control-word-de statements are mutually exclusive—that is, you can configure only one of these statements. |
For ATM2 IQ interfaces, the control word contains a field to carry ATM cell loss priority (CLP) information by default. For more information, see Configuring Layer 2 Circuit Transport Mode.
For more information about Layer 2 circuits, see the Junos OS VPNs Configuration Guide and the Junos OS Routing Protocols Configuration Guide. For a comprehensive example, see the Junos OS Feature Guides.
