The Access Server

You use the access server as the primary configuration device. From this device, you configure the other routers and switches through a reverse Telnet session. The Cisco routers that accomplish this function the most efficiently are the Cisco routers with the SCSI-II 68-pin async port and the eight-to-one octopus cable. The most common routers that provide this port are the following:

• Cisco 2509/2510

• Cisco 2511/2512

• Cisco 2600/3600 with SCSI-II 68-pin 16/32-port async port

Any Cisco router might be configured for asynchronous communications on its serial ports. A Cisco router with multiple serial ports, such as a Cisco 2522, can also be used as an access server. Regardless of whether you use a router like the 2509 with its SCSI-II 68-pin port or a Cisco 2522 with its 10 serial ports, you need require special cables and a unique configuration.

The most common routers that serve the function of an access server are the Cisco 2509/2510 and the Cisco 2511/2512 routers. The Cisco 2509 has one Ethernet port and eight asynchronous ports, whereas the Cisco 2510 has one Token Ring port and eight asynchronous ports. The eight asynchronous ports utilize a Cisco eight-to-one cable, referred to as the octal cable. The part number is CAB-OCTAL-KIT, which also includes modem head- shells for any asynchronous devices, such as modems. The RJ-45 end, of one of the eight cables, plugs into the console port of each router in the lab. You use what Cisco calls reverse Telnet to configure the routers through this cable. This should be called internal Telnet ”there is nothing reverse about the way Telnet is used to configure the routers. Actually, Telnet is used as the protocol to communicate with a device through a specific internal port or line number.

NOTE

For a complete list of the pinouts of these cables, head-shells, and any other cables mentioned in this book, see Appendix D, "Common Cable Types and Pinouts."

The 2511 access server uses a 68-pin connector and breakout cable, which provides 8 RJ-45 ports on each cable. These ports can also use RJ-45 “to “DB-25 adapters to connect to asynchronous devices. The Cisco 2511/2512 is modeled in the same format but supplies an additional eight asynchronous ports.

Figure 1-1 illustrates the back of a Cisco 2511 router and the eight-to one cable.

Figure 1-1. Cisco 2511 Router and the Eight-to-One Octopus Cable

The other form of access server is using serial ports on a router such as a Cisco 3600 series with an eight-port serial module or the Cisco 2522 series with ten serial ports. You can to connect serial cables to various head shells to run asynchronous communications in numerous ways. Essentially, the best combinations require a RS-232 serial cable, either DTE or DCE, an RJ-45 “to “DB-25 adapter, and a rolled or straight-through cable. For the precise pinouts of the RS-232 serial cables and the rolled cable, see Appendix D. The RJ-45 “to “DB-25 adapter has three flavors: a DTE M/F, DCE M/F, and MMOD. The female DTE head-shell from Cisco has the label Terminal on one side. The MMOD head-shell (the most common) has the label MODEM on it. This is the same head-shell used on Cisco 4000 and 7000 series routers for the console port, and it is the same head-shell used to connect the terminal server to a modem.

Two common and inexpensive cabling options involve using RS-232 cables as a terminal server cables:

• Cable method 1 ” Use a Cisco female RS-232 “to “DB-60 serial cable, the RJ-45 “to “DB-25 head-shell labeled MODEM, and a Cisco rolled cable. Connect the cables in the logical fashion; then, from the configuration mode, under the interface mode, enter the physical-layer async command. This forces asynchronous communications out the serial port. In the next section, you learn how to completely use this command along with the transport input command to configure reverse Telnet sessions to your router's console ports.

• Cable method 2 ” This method is similar to cable method 1, but you use different cables and head-shells. This method requires a Cisco male RS-232 “to “DB-60 serial cable, the RJ-45 “to “DB-25 head-shell labeled TERMINAL, and a Cisco rolled cable. Again connect the cables in the logical fashion, and add the physical-layer async command under the serial interface.

NOTE

The serial port must be a synchronous/asynchronous port to use these cable methods. For example, the ports on a Cisco 2501 are synchronous only, so these cable methods will not work.

Table 1-1 shows all the pinouts for the RJ-45 “to “DB-25 adapters, while Table 1-2 shows asynchronous device cabling options.

Table 1-1. Pinouts for the RJ-45 “to “DB-25 Adapters

Adapter RJ-45 Pins	DTE M/F Pins TERMINAL	DCE M/F Pins	MMOD Pins MODEM
1	4	5	5
2	20	6	8
3	2	3	3
4	7	7	7
5	7	7	7
6	2	2	2
7	20	20	20
8	4	4	4

Table 1-2. Asynchronous Device Cabling Options

Access Server Port	RJ-45 Console Cable Type	Head-Shell Adapter	End Device
Console or aux	Rolled	DTE pinout	Serial cable
Console or aux	Straight	DCE pinout	Serial cable
Console or aux	Rolled	MMOD/MODEM	Modem

The access server can perform multiple roles. Not only can you use this device for easy configuration of the other routers in your stack, but the other serial and LAN ports can be used independently of the terminal server functions. For example, the terminal server could be configured as a route generator or a backbone router. The access server can also be used for remote configuration of the lab through analog dialup. Both of these topics are discussed in detail later in this chapter. The access server is discussed in the section "Configuring Analog Remote Access."