Frame Relay Headers and Encapsulation
Routers create Frame Relay frames by using different consecutive headers. The first header is the
ITU Link Access Procedure for Frame-Mode Bearer Services (LAPF) header. The LAPF header
includes all the fields used by Frame Relay switches to deliver frames across the FR cloud,
including the DLCI, DE, BECN, and FECN fields.
The Frame Relay encapsulation header follows the LAPF header, holding fields that are important
only to the DTEs on the ends of a VC. For the encapsulation header, two options exist:
■ The earlier-defined Cisco-proprietary header
■ The IETF-defined RFC 2427 (formerly RFC 1490) encapsulation header
The cisco option works well with Cisco routers on each end of the VC, with the ietf option
being required for multivendor interoperability. Both headers include a Protocol Type field
to support multiple Layer 3 protocols over a VC; the most commonly used is the RFC 2427
Network Layer Protocol ID (NLPID) field. Figure 15-2 shows the general structure of the
headers and trailers.
Each VC uses the Cisco encapsulation header unless configured explicitly to use the IETF header.
Three methods can be used to configure a VC to use the IETF-style header:
■ Use the encapsulation frame-relay ietf interface subcommand, which changes that
interface’s default for each VC to IETF instead of cisco
■ Use the frame-relay interface-dlci number ietf interface subcommand, overriding the
default for this VC
■ Use the frame-relay map dlci . . . ietf command, which also over-rides the default for this VC
For example, on an interface with ten VCs, seven of which need to use IETF encapsulation, the
interface default could be changed to IETF using the encapsulation frame-relay ietf interface
subcommand. Then, the frame-relay interface-dlci number cisco command could be used for
each of the three VCs that require Cisco encapsulation.
Frame Relay Encapsulation Options
RFC
2427
Packet
LAPF
Header Cisco LAPF
Packet Trailer
LAPF
Header
LAPF
Trailer
ITU and ANSI Conform to
RFC Specifications. Header
Includes NLPID Field.