Connecting the Modem to a Router Console Port

Verification Kit equipment:

  • Verikit modem
  • RS-232 to RJ-45 adaptors (may be labeled MDCE or Modem)
  • DB-9 to RJ-45 FDTE terminal adapter (imprinted with "Terminal")
  • RJ-11 phone cable
  • Black RJ-45 to RJ-45 console cable
Warning Warning:   Unprotected modems should not be left connected to the console port. The console ports do not log users off when the carrier detect is lost, which can leave a security hole. To avoid this, use a secure modem, or connect via the AUX port if the modem is to be left permanently.
 

To connect the modem to the router:

Step 1  Plug one MDCE adaptor into the modem, and optionally plug another into the router console port where necessary (to routers such as 4000 and 7000 series). see Figure 1

Step 2  Plug one end of the console cable into the MDCE adaptor on the modem, and the other end into the RJ45 router console port (or optionally to a MDCE adapter if the console port was a DB25 connector on routers such as Cisco 4000 and 7000 series).

Step 3  Plug one end of the verikit RJ-11 gray phone cable into the left RJ-11 port of the modem (if USR), and plug the other end into the wall jack. Record the dial in number.

Step 4  Set the dip switches on the modem so that switch 1, 4 and 7 are in the down position, and all the other switches are in the up position.

Step 5  Plug the modem power supply into the modem and into a wall outlet.

Step 6  Switch the modem on. Check to make sure all devices are powered on and all cables are secured. You are now ready to dial in to the modem to access the Router Console.

Figure 1:  Connecting the Modem to the Router

RJ-45-to-DB-25 adapter (MDCE adapter) RJ-45-to-RJ-45 console cable
DB-25-to-RJ-45 modem cable adapter Console Cable
 
Note Note:   There are several advantages to connecting a modem to the console port of a router instead of the AUX port:

Passwords can be recovered remotely. You may still need someone on-site with the router to toggle the power, but aside from that, it's identical to being there with the router.
 
Note Note:  

There are several Disadvantages of connecting a modem on the console port instead of the AUX port:

The console port may not support RS232 modem control (data set ready/Data Carrier Detect (DSR/DCD), data terminal ready (DTR). Therefore, when the console EXEC session terminates (logout), the modem connection will not drop automatically; the user will need to manually disconnect the session.

More seriously, if the modem connection should drop, the EXEC session will not automatically reset. This can present a security hole, in that a subsequent call into that modem will be able to access the console without entering a password. The hole can be made smaller by setting a tight exec-timeout on the line. However, if security is important, it is recommended to use a modem that can provide a password prompt.

Since the console port does not support hardware (Clear to Send/Ready to Send (CTS/RTS) flow control, it is recommended to use no flow control. If data overruns are encountered, however, software (XON/XOFF) flow control may be enabled.

Some routers (for example, Cisco 1600s) do not have AUX ports..
 

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