Information Technology Reference
In-Depth Information
LAPB Frame Types
The control field in the LAPB frame indicates one of three frame types: information,
supervisory, and unnumbered. The frame types are described as follows:
Information frames carry the upper-layer information. Sequencing is part of flow control
and error recovery. I frames send and receive sequence numbers.
Supervisory frames carry control information, such as requests for transmissions,
acknowledgment, and status information. S frames only receive sequence numbers.
Unnumbered frames perform functions such as link setup or disconnection and error
reporting. U frames do not use sequence numbers.
Protocol Translation
Cisco routers can be protocol translators, which enable communication between X.25 and
IP end stations. The only TCP/IP application supported is Telnet. An IP host can Telnet to a
specific IP address, which gets translated by the protocol translator to an X.121 address and
forwarded to an X.25 end station. The global command that performs this function is translate
tcp ip-address x25 x.121 . To translate from X.25 to Telnet, the parameters are reversed:
translate x25 x.121 tcp ip-address.
Mapping
IP packets can be encapsulated in X.25 packets for transport over an X.25 network, which is
referred to by Cisco as mapping . An X.25 map statement maps the IP address to the X.121
address associated with the remote IP router. In contrast to frame relay DLCIs, X.121 addresses
are globally significant, which results in mapping to the destination X.121 address. Figure 5-4
illustrates the use of mapping.
The map command configured on Router A indicates that the next hop IP address, 172.18.1.6,
which is on the remote side of the X.25 network, is reachable through the X.121 address
34343434. The broadcast keyword informs the router to encapsulate broadcast packets that
originate on the IP network and forward them to the remote IP router.
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