ccna 1 price

ccna 1 price.Frame Relay Encapsulation and HeaderA router connected to Frame Relay encapsulates each Layer 3 packet in a Frame Relay header and tail before sending the packet out over the access link. This header and tail are defined by Frame Relay (or the Data Link Layer Access Procedure Frame (LAPF, Link Access Procedure Frame) specification ITU Q. 922-A). This sparse LAPF frame structure uses the FCS at the end of the message and the DLCI field in the header (described later) to provide error detection. Figure 13-5 shows a schematic of the frame structure.LAPF Header Information LAPF HeaderElFCS- FECN,BECN,DE (1 bit each)DLCI (usually 10 bits) Figure 13-5 LAPF HeaderHowever, the header used by the router is longer than the standard LAPF header because the standard header does not provide all of the fields normally required by a router. In particular, the Protocol Type field is not shown in Figure 13-5. Each data link header requires a field that defines the type of packet and is placed after the data link header. If Frame Relay uses only the LAPF header, the DTE (including the router) cannot support multiprotocol communication because there is no way to identify the protocol type in the information field.Two solutions are listed below that compensate for the lack of a protocol type field in the standard Frame Relay header. Cisco and three others created an additional header that is placed between the LAPF header and the Layer 3 packet, as shown in Figure 13-5. It includes a 2-byte protocol type field with a value that matches the same field that Cisco uses for HDLC.RFC 1490 (later replaced by RFC 2427), Multiprotocol Interconnect Frame Relay defines a second solution. RFC 1490 was developed to ensure interoperability between multiproducers over Frame Relay DTEs. This RFC defines a protocol type field similar to the header that is also placed between the LAPF header and the Layer 3 packet, and includes many other options.