T1: A Survival Guide

Multiprotocol Encapsulation with RFC 1490

When frame relay was first standardized, the information field did not have any sort of protocol tag, and the only way to transmit multiple protocols across a frame relay network was to use a separate PVC for each one. Service costs, however, are typically dominated by the number of PVCs, so subscribers asked network equipment vendors to develop a multiprotocol encapsulation for frame relay. RFC 1490, which has since been updated by RFC 2427, specifies the most common multiprotocol encapsulation. Even though the current version of the specification is RFC 2427, RFC 1490 was in force for so long that many continue to refer to “RFC 1490 encapsulation”; therefore, this book uses RFC 1490 as well.

When using RFC 1490 encapsulation, the LAPF frame’s information field carries a second header that tags the higher-level protocol carried in the LAPF frame. Within the RFC 1490 header, a Network Layer Protocol Identifier (NLPID) is used to identify the higher-layer protocol carried inside the LAPF frame. NLPIDs are 1-byte values assigned in ISO/IEC TR 9577. Some examples are shown in Table 10-1.

Table 10-1. Common NLPIDs

Protocol	ISO/IEC TR 9577 NLPID (hexadecimal)
IP	CC
PPP	CF
Q.933	08
Subnetwork Access Protocol (SNAP)	80

RFC 1490 headers begin with 0x03 to indicate that unnumbered information is being sent. After the unnumbered information header, a single padding byte, set to zero, may be sent to assist in aligning other headers ...