Configuring Basic 4G LTE Connectivity on the Cisco 819 Router

en March 06 , 2018
I've recently had the mixed fortune to have set up a couple of Cisco routers for 3G and 4G data services. It turns out to be surprisingly simple, although I found myself having to flit around between a handful of different documents to work out how to get it working. Luckily I was sat next to someone who previously worked in one of the NL's biggest mobile carriers while I was working on it, which saved me a bit of head scratching at times.
As a little bonus I had two different devices to work on - the first being an old Cisco 1841 router with a 3G WIC installed, the second being a Cisco 819 (4G LTE model). As it turns out the concepts are pretty similar but the syntax is moderately different between the two platforms, so in time I'll write up the process for each. This post and its accompanying video will explain the 4G version.

Building Blocks
While there are a few mobile-specific pieces of configuration, anyone who has previously worked on ISDN, async modems or ADSL on Cisco routers will probably find a lot of familiar concepts. Here are the main elements of a 3G / 4G configuration:

Cellular Profile
This is where the APN address and authentication mode are configured. These are saved to the modem's NVRAM as soon as they are applied. Here's an example of how to set a cellular profile on the two different platforms:


Note that this example is for three, a NL mobile carrier which is interesting because the APN uses no authentication. I found during my testing that an 819 router running IOS 15.2 does not have the option to use authentication type "none". Under 15.3 the option is there and works fine - luckily I had another 819 with 15.3 installed which worked and so a) I knew that's what the problem was and b) I could copy the image across!
Also note that this is applied at the exec prompt rather than in config mode.
Most carriers use CHAP authentication, these just require the authentication type and credentials added to the command, for example:


The number "1" here indicates which slot on the modem will be used to store the profile. This is significant later on because there may be multiple APNs configured and the router needs to know which to use when connecting.

Cellular Interface
The physical radio interfaces are referred to using the "Cellular" prefix, in this case Cellular0. The Cellular interface is where the dialer, authentication and IP details are normally configured - I say normally as there are many different ways to configure dialers depending on what kind of load balancing and resilience are required. For a typical 3G / 4G deployment, though, you will only have one physical interface and so the simplest way is to forget about pools and put the config straight onto that.
Here is an example configuration showing the key elements:


The key thing to notice here is that unlike the old 3G config there is no "*98*1#" type diaper string. If you want to use alternative profiles you have to mess around with config under the "controller cellular 0" context.
The rest is fairly standard dialer stuff, in this example I've made the dialer-list so that any IP traffic will cause it to connect.
Note that the encapsulation specified on this 4G interface is SLIP. When 4G is not available it will fall back to 3G (which uses PPP encapsulation) - it does this transparently and does not need the encap changed.

Sundry Config
At this point the router should be able to connect to the cellular network. For most purposes, though, you will need to either set up NAT or some sort of VPN tunnel for the connection to be of any use. These are set up the same way as for any other setup.

Testing and Diagnostics

How can you tell whether the cellular connection is coming up? The first clue is that log entries similar to the following should appear:


If the above log entries don't appear it could be because the modem is not ready yet. The modems in the 819 routers I was playing with took an incredibly long time to boot. The following logs indicate that the modem has (finally) booted up:


However at this point the modem will still need to attach to the cellular network, which can take a little time. To check whether the modem is attached to the radio network, use the following commands:


Note that the band and channel need to be populated, the network should display the expected carrier name and the packet service should show as attached. The actual band and service type will vary depending on carrier, coverage, area and equipment used.
If the radio interface is up but a data connection cannot be established then all the usual debugs may be used:

Note on Earlier Releases
Prior to IOS 15.3 you had to define your own chat string and apply it to the line - later releases do this automatically. If your "debug chat" output shows anything about "ATDT" or expecting "CONNECT" then this probably applies to you. Making and applying the chat script is pretty simple:


Basically this says to define a script called "lte" which waits for nothing, sends "AT!CALL" to the modem and expects to get "OK" in return within 20 seconds. Then that script gets attached to line 3, which in the 819 router is the 4G cellular modem.
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