Cisco IOS Wide-Area Networking Configuration Guide - Free Books

Configuring an X.25 Datagram TransportConfiguring X.25 and LAPBTable 10Protocol Identification in the CUD Field (continued)Protocol Cisco Protocol Identifier IETF RFC 1356 Protocol IdentifierPAD 0x01 00 00 00 6 0x01 00 00 00 6QLLC 0xC3 Not availableXNS 0xD1 0x80 (5-byte SNAP encoding)Multiprotocol Not available 0x001. SNAP encoding is defined according to the Assigned Numbers RFC; the Cisco implementation recognizes only the IETF organizational unique identifier(OUI) 0x00 00 00 followed by a 2-byte Ethernet protocol type.2. The use of 0xC0 00 80 C4 for Banyan VINES is defined by Banyan.3. The use of 0x81 for CLNS is compatible with ISO/IEC 8473-3:1994.4. Compressed TCP traffic has two types of datagrams, so IETF encapsulation requires a multiprotocol VC.5. The use of 0xCC for IP is backward-compatible with RFC 877.6. The use of 0x01 00 00 00 for PAD is defined by ITU-T Recommendation X.29.Once a multiprotocol VC has been established, datagrams on the VC have protocol identification databefore the actual protocol data; the protocol identification values are the same as those used byRFC 1356 in the CUD field for an individual protocol.Note IP datagrams can be identified with a 1-byte identification (0xCC) or a 6-byte identification (0x80followed by the 5-byte SNAP encoding). The 1-byte encoding is used by default, although the SNAPencoding can be configured.Mapping Datagram Addresses to X.25 HostsEncapsulation is a cooperative process between the router and another X.25 host. Because X.25 hostsare reached with an X.121 address (an X.121 address has 0 to 15 decimal digits), the router must have ameans to map protocols and addresses of the host to its X.121 address.Each encapsulating X.25 interface must be configured with the relevant datagram parameters. Forexample, an interface that encapsulates IP typically will have an IP address.A router set up for DDN or BFE service uses a dynamic mapping technique to convert between IP andX.121 addresses. These techniques have been designed specifically for attachment to the DDN networkand to Blacker encryption equipment. Their design, restrictions, and operation make them work well forthese specific applications, but not for other networks.You must also establish the X.121 address of an encapsulating X.25 interface using the x25 addressinterface configuration command. This X.121 address is the address to which encapsulation calls aredirected, and is also the source X.121 address used for originating an encapsulation call. It is used by thedestination host to map the source host and protocol to the protocol address. An encapsulation VC mustbe a mapped at both the source and destination host interfaces. A DDN or BFE interface will have anX.121 address generated from the interface IP address, which, for proper operation, should not bemodified.For each X.25 interface, you must explicitly map the protocols and addresses for each destination hostto its X.121 address. If needed and the destination host has the capability, one host map can beconfigured to support several protocols; alternatively, you can define one map for each supportedprotocol.To establish an X.25 map, use the following command in interface configuration mode:22