TAILIEUCHUNG - Giáo Trình CIs+ part 90

Tham khảo tài liệu 'giáo trình cis+ part 90', công nghệ thông tin, kỹ thuật lập trình phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | From Question 56 Subject What does Frame-relay LMI and Encapsulation really do mean I think there is some confusion here about frame relay encapsulation and frame relay lmi heartbeat keepalives . Frame relay encapsulation is indeed significant end-to-end through the cloud between communicating DTE router equipment. Cisco encapsulation inserts an ethernet type field immediately after the 2 byte frame header which contains the DLCI FECN BECN and DE fields. IETF RFC 1490 encapsulation does not use ethernet type fields to identify the payload of the frame. Instead IETF calls for the use of NLPID codes Network Layer Protocol Identifiers which are common in the OSI environment. NLPIDs are to be used when the payload has an NLPID assigned to it. like IP The NLPID CC in the case of IP will follow an Unnumbered Information UI control field 03. If the payload does not have an NLPID assigned to it like IPX then IETF suggests that an OUI field organizationally unique identifier followed by an ethernet type code 8137 for example if IPX will be used. Much like an frame with SNAP the type code of 8137 will be offset further into the frame and not found immediately after the 2 byte frame header. This encapsulation must be understood by the communicating routers at either edge of the cloud. The cloud itself does not care what type of encapsulation is being used. It is strictly a DTE-DTE issue. LMI is a link intergrity and PVC status verification protocol that IS locally significant between the router and the network interface. This protocol comes in 3 flavors the original Stratacom aka cisco version ANSI s Annex D and CCITT ITU Annex A. These protocols are often collectively referred to as LMI. It is possible to run one version of LMI on the East User-Network Interface UNI and another version on the West UNI as these protocols simply identify the status of the UNI link and the PVCs found on that link. Encapsulation however must match between the DTEs. It is .

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