Using an Async Modem on the AUX Port

2017-11-03 09:05:05

Problem

You want to connect a standard asynchronous modem to the router's AUX port and use it for dial backup.

Solution

Many Cisco routers include an AUX port that is a low-speed asynchronous serial interface that can connect to a standard modem and support PPP:

Router1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router2(config)#interface Async65 Router2(config-if)#encapsulation ppp Router2(config-if)#dialer in-band Router2(config-if)#dialer pool-member 1 Router2(config-if)#ppp authentication chap Router2(config-if)#async default routing Router2(config-if)#exit Router2(config)#interface Dialer1 Router2(config-if)#ip address 10.1.99.56 255.255.255.0 Router2(config-if)#encapsulation ppp Router2(config-if)#dialer remote-name dialhost Router2(config-if)#dialer pool 1 Router2(config-if)#dialer idle-timeout 300 Router2(config-if)#dialer string 95551212 Router2(config-if)#dialer-group 1 Router2(config-if)#ppp authentication chap Router2(config-if)#exit Router2(config)#line aux 0 Router2(config-line)#modem inout Router2(config-line)#transport input all Router2(config-line)#no exec Router2(config-line)#speed 115200 Router2(config-line)#exit Router2(config)#username dialhost password dialpassword Router2(config)#ip route 0.0.0.0 0.0.0.0 10.1.99.1 180 Router2(config)#dialer-list 1 protocol ip list 101 Router2(config)#access-list 101 deny eigrp any any Router2(config)#access-list 101 permit ip any any Router2(config)#router eigrp 55 Router2(config-router)#network 10.0.0.0 Router2(config-router)#exit Router2(config)#end Router2#

Discussion

Much of this configuration is similar to the ISDN configuration shown in Recipe 13.2. It uses a dialer interface in exactly the same way. But here, because there is only one async modem in this example, we can't benefit from PPP multilink.

The first part of this configuration example sets up the AUX port to run PPP and associates it with a dialer pool:

Router2(config)#interface Async65 Router2(config-if)#encapsulation ppp Router2(config-if)#dialer in-band Router2(config-if)#dialer pool-member 1 Router2(config-if)#ppp authentication chap Router2(config-if)#async default routing

The only thing here that hasn't appeared in a previous example is the async default routing command. This command allows the async interface to support a routing protocol such as EIGRP. By default, routing protocols are disabled on async interfaces, so you need to enable it.

The number of this particular interface, Async65, wasn't selected at random. The router automatically assigns a line number to every interface that can be used for terminal access (including VTY lines, AUX lines, and Console lines), and it varies from router to router, depending on the hardware configuration. So we used the show line command to see which line number corresponded to the AUX port on this router:

Router1#show line Tty Typ Tx/Rx A Modem Roty AccO AccI Uses Noise Overruns Int 0 CTY - - - - - 0 0 0/0 - 65 AUX 9600/9600 - - - - - 0 0 0/0 - * 66 VTY - - - - - 10 0 0/0 - * 67 VTY - - - - - 19 0 0/0 - 68 VTY - - - - - 3 0 0/0 - 69 VTY - - - - - 0 0 0/0 - 70 VTY - - - - - 0 0 0/0 - 71 VTY - - - - - 0 0 0/0 - 72 VTY - - - - - 0 0 0/0 - 73 VTY - - - - - 0 0 0/0 - 74 VTY - - - - - 0 0 0/0 - 75 VTY - - - - - 0 0 0/0 - Line(s) not in async mode -or- with no hardware support: 1-64 Router1#

As you can see, the AUX port is on line 65 on this router. It's important to do this before you attempt any of the rest of the configuration, so you know what to configure.

When you use the AUX port for dial backup, you also need to configure the terminal line information for this physical port:

Router2(config)#line aux 0 Router2(config-line)#modem inout Router2(config-line)#transport input none Router2(config-line)#no exec Router2(config-line)#speed 115200

The first command here is modem inout, which configures the router to allow access to the modem, as well as allowing the modem access to the router. Then we added the command transport input none. By default, the router will act as a terminal server and allow you to connect through protocols like telnet to the AUX port. In this case, though, we want the router to reserve this port for routed traffic, so we disable all remote terminal access to the interface.

The no exec command is extremely important when using async dial, and almost universally ignored in Cisco references. By default, the router will start an EXEC session on your AUX port. So if you plug a terminal into this port, you will get a login prompt. Unfortunately, your modem doesn't know what to do with a login prompt. At best, it will just ignore it, so disabling the EXEC session is simply good form. But, at worst, we have seen problems where the modem attempts to respond to the login prompt, the EXEC session interprets this as a bad login attempt, and puts up a new prompt, to which the modem again attempts to respond. The result can be high CPU utilization and, more importantly, this activity will prevent the router from dialing. We strongly recommend disabling the EXEC session on any async dial ports, as we have done here.

And the last command in this section sets the line speed. It's important to remember that this is the speed between the router and the modem. The actual dial session will have a much lower net speed, likely less than 56 Kbps. However, it's a good idea to make the line speed as fast as the modem can support. This will ensure that you get the best possible speed. Note that the default speed here is only 9.6 Kbps. So, if you don't increase this value, you will not be able to get the full advantage of the compression capabilities of modern modems.

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