Cisco IOS Wide-Area Networking Configuration Guide - Free Books

Configuring X.25 and LAPBConfiguring an X.25 Datagram Transport• Mapping Datagram Addresses to X.25 Hosts• Configuring PAD AccessUnderstanding Protocol Encapsulation for Single-Protocol and Multiprotocol VCsCisco has long supported encapsulation of a number of datagram protocols across X.25, using a standardmethod when available or a proprietary method when necessary. These traditional methods assign aprotocol to each VC. If more than one protocol is carried between the router and a given host, each activeprotocol will have at least one VC dedicated to carrying its datagrams.Cisco also supports a newer standard, RFC 1356, Multiprotocol Interconnect on X.25 and ISDN in thePacket Mode,which standardizes a method for encapsulating most datagram protocols over X.25. It alsospecifies how one VC can carry datagrams from more than one protocol.The Cisco IOS software can be configured to use any of the available encapsulation methods with aparticular host.After you establish an encapsulation VC using any method, the Cisco IOS software sends and receivesa datagram by simply fragmenting it into and reassembling it from an X.25 complete packet sequence.An X.25 complete packet sequence is one or more X.25 data packets that have the M-bit set in all butthe last packet. A VC that can carry multiple protocols includes protocol identification data as well asthe protocol data at the start of each complete packet sequence.Understanding Protocol IdentificationThis section contains background material only.The various methods and protocols used in X.25 SVC encapsulation are identified in a specific field ofthe call packet; this field is defined by X.25 to carry CUD. Only PVCs do not use CUD to identify theirencapsulation (because PVCs do not use the X.25 call setup procedures).The primary difference between the available Cisco and IETF encapsulation methods is the specificvalue used to identify a protocol. When any of the methods establishes a VC for carrying a singleprotocol, the protocol is identified in the call packet by the CUD.Table 10 summarizes the values used in the CUD field to identify protocols.Table 10Protocol Identification in the CUD FieldProtocol Cisco Protocol Identifier IETF RFC 1356 Protocol IdentifierApollo Domain 0xD4 0x80 (5-byte SNAP encoding) 1AppleTalk 0xD2 0x80 (5-byte SNAP encoding)Banyan VINES 0xC0 00 80 C4 2 0x80 (5-byte SNAP encoding)Bridging 0xD5 Not implementedISO CLNS 0x81 0x81 3Compressed TCP 0xD8 0x00 (multiprotocol) 4DECnet 0xD0 0x80 (5-byte SNAP encoding)IP 0xCC 0xCC 5or0x80 (5-byte SNAP encoding)Novell IPX 0xD3 0x80 (5-byte SNAP encoding)21