methods for exchanging frames, monitoring frame sequence, missing frames, and
executing frame acknowledgements and retransmission when necessary.
connection. These datagrams can be encapsulated in a proprietary protocol and transported
over the LAPB connection. LAPB is a derivative of HDLC and follows HDLC frame
formats. An LAPB connection can carry a single protocol or multiple protocols, depending
on the configuration. Mapping X.25 addresses to protocol addresses enables a Layer 3
protocol to be routed through an X.25 network.
Standard Service is a required protocol used in conjunction with DDN Packet Switched
Nodes (PSNs). The Defense Communications Agency (DCA) has certified Cisco
Systems' DDN X.25 Standard Service implementation for attachment to the DDN.
Cisco's DDN implementation also includes Blacker Front-End Encryption and Blacker
Emergency Mode operation. These options are beyond the scope of our discussions. For
further information, check out www.cisco.com.
and RFC 1382. Collection of information regarding LAPB XID Table, X.25 Cleared
Circuit Table, and X.25 Call Parameter Table is not implemented. All SNMP community
values are read-only. Refer to the NMS documentation of your specific platform for
further information on supported values.
have evolved to include the widespread use of WAN technologies. With the bandwidth cost
reductions over the past few years, WAN connections are no longer cost prohibitive. These
newer WAN installations can extend the functionality of X.25. For example, X.25 can be
implemented on an ISDN D channel. In addition, a BRI 0 B+D implementation, rather than the
traditional 2 B+D implementation, can be ordered specifically for this type of X.25 deployment.
plan. There are specific pieces that have specific meaning. For example, 410-555-1212 has
specific a specific format. For instance, 410 is the area code and is shared by all phones in a
particular geographic area. The 555 is a prefix. It too is a shared number; however, the number
of devices sharing it are less than the number of devices sharing 410. The last number, 1212, is
a user-specific identifier. Although any one of the three divisions of the previous phone number
is reusable, the entire ten-digit string is globally unique.
hierarchical structure is discussed later in this chapter. The overall similarities between this and
other technologies are also touched on because the configuration of X.25 is very much the same
as the configuration of ISDN and/or Frame Relay. The obvious differences lie in the
technology-specific parameters necessary for proper configuration of the router.