Asynchronous Router Commands
Command Function
line N Cisco routers refer to asynchronous ports as lines. N is equal to the number
of the port and is used before the rest of the commands in this table
to get into line configuration mode.
login The login command is required to force authentication of a connection.
password This command establishes the password to be used on the line.
flowcontrol The flowcontrol command can be followed with hardware or software
settings. Typically, hardware flow control provides greater control over
the data flow and allows for higher communication speeds. Software
flow control is not recommended.
speed The speed command establishes the maximum speed to be used between
the modem and access server or router. It defines the speed of both transmit
and receive, and it is noted in bits per second (bps). Note that the
modem and access server can negotiate a slower speed or data rate.
transport input The transport input command defines the protocol to use in reverse Telnet
connections. This may be LAT, MOP, NASI, PAD, RLOGIN, Telnet, or
V120; however, administrators typically use the all keyword to allow all
connection types. This is potentially less secure because a hacker could
use one of these protocols to gain access or deny service to the router. For
example, if there is no business need to use RLOGIN, why leave the
access available to allow repeated access attempts from an outsider?
stopbits Stop bits are used in asynchronous communications to define the end of
each byte. Typically, the stopbits value is set to 1 because there is little
reason to send additional bits; however, values of 1.5 and 2 are also valid.
modem The modem command is used to define the type of calls allowed. By
default, the router allows dial-in, or incoming, calls only. However, to
allow reverse Telnet or dial-out connections, in addition to dial-in, the
administrator would use the inout keyword.
It is important to note that each line, or logical interface, (specified with the line command in
Table 23.4) has an associated physical interface. This is defined by the router. There is also a line
associated with the aux interface. The asynchronous, or async, interface is the physical representation
of the interface, and configurations on async interfaces define the protocol characteristics
of the connection. This would be used to define a protocol such as Point-to-Point Protocol (PPP)
or the addressing mechanism to be used. An async interface can be a capable serial interface configured
for asynchronous services with the physical-layer async command, or the aux (auxiliary)
port on the router.
Configuration begins with the line command and the number of the interface. This is
followed with the specific information that is needed—for example, the login capabilities
and DTE-to-DCE speed. A typical configuration might appear as follows:
line 3
modem inout
stopbits 1
databits 8
parity none
transport output all
transport input all
speed 56000
flowcontrol hardware
login
password tplekprp
This configuration would allow calls in or out, with all protocols supported and login
required. We’ve also configured the data rate to 56Kbps and added a common set of modem
parameters (N, 8, 1) to the configuration. These last parameters define those communications
characteristics and must match on both sides of the connection. Hardware flow control would
be used. Flow control is used to prevent buffer overruns and maintain an efficient flow of data
by signaling the sender that it should slow down or speed up.
If there is a problem with manual configuration, it will be first noted when the administrator
attempts to use the modem. Reverse Telnet and use of the diagnostic commands associated with
that modem are most likely the best tools available for troubleshooting.
Asynchronous, or analog, connections are widely available and extremely popular methods for
providing remote access in today’s networks, but they are not without their disadvantages.
However, no other technology has come along to replace them entirely.
Modern modems provide a wide range of modulation types, which provides the user with
many connection speed options. Connecting modems to an access server is straightforward due
to the use of the universal EIA/TIA-232 (RS-232-C) serial connector.
Asynchronous modems provide a challenge for remote access connections, and configuration
options include manual, automatic (autoconfigure), or autodiscovery. Each method has its
own characteristics. For instance, manual configuration requires the knowledge of which AT
commands to use to configure the modem for dial-in access. If using automatic configuration,
the only knowledge needed is the type of modem used on the access server, and the router will
try to apply the best match. When using autodiscovery, the administrator does not need to
know what type of modem is connected to the access server; the router will query the modem
to determine its type and then apply the best matching parameters.
Know how to identify the different connection types in analog communications. The DTE-to-
DCE connection occurs between the router (DTE) and the modem (DCE). The DCE-to-DCE connection
occurs between the two modems over the phone network.
Understand the different modulation types and their speeds. You should know the 14 modulation
types and the data rates they provide as listed in Table 23.2. You should also understand
that asynchronous connections are limited to 56Kbps, or 53Kbps by FCC order.
Understand the different signals carried in the communications cable. Make sure you know
which signal does what when two devices are communicating over an EIA/TIA-232 (RS-232-C)
connection:
TXD is used for transmission of data.
RXD is for reception of data.
CD is used to signal that a connection exists between local and remote DCE.
GND is used as a reference signal.
RTS and CTS are used in hardware flow control.
Know the compression and correction standards. V.42bis is the most widely used data compression
standard. V.44 is the more recent kid on the block and is significantly more efficient
than V.42bis.
Understand the different IOS modem configuration modes. Cisco IOS has the capability
to automatically configure a modem by using the information in the modemcap database.
The router can also automatically discover the modem type and configure that modem for
asynchronous communications. You can also manually supply the router with modem configuration
commands.
Know the major attention (AT) commands used for manual modem configuration. You
should understand what the AT&F command will do to most modems. Make sure you understand
the other AT commands described in Table 23.3.
Know how to create a new modemcap database entry by using an existing database entry as a
template. To create a new modemcap database entry, use the modemcap edit new-profilename
template existing-profilename command with template being the key parameter.
Know which commands to use in line configuration mode and which to use in the logical interface
configuration mode. The commands described in Table 23.4 are used in line configuration mode
to specify the physical characteristics of the asynchronous connection. The commands used in interface
configuration mode configure parameters such as encapsulation and addresses used on the
asynchronous connection.
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