relay lmi-type interface subcommand.
that allow the switches and DTEs to successfully deliver the frame across the network.
Encapsulation refers to the process of using such headers and trailers to contain data supplied
by a higher layer. (Refer to Chapter 3 for additional concepts about encapsulation.)
Q.922-A. The sparse LAPF framing provides error detection with an FCS in the trailer, as well
as the DLCI, DE, FECN, and BECN fields. Figure 8-5 diagrams the frame.
field in the header must define the type of header that begins the Information field. If Frame
Relay is using only the LAPF header, then DTEs (including routers) cannot support multi-
protocol traffic because there is no way to identify the type of protocol in the Information field.
(For more information about the concept of a Protocol Type field in data link headers, refer to
Chapter 3.)
other companies created an additional header, which comes first in the Information field shown
in Figure 8-5. It includes a 2-byte-long Protocol Type field, with values matching the same field
used for HDLC by Cisco. The second solution was defined in RFC 1490, "Multiprotocol
Interconnect over Frame Relay," which was written to ensure multivendor interoperability
between Frame Relay DTEs. This solution includes use of a Protocol Type field and adds many
options, including support for bridged frames. ITU and ANSI later incorporated RFC 1490
headers into specs Q.933 Annex E and T.617 Annex F, respectively.