Cellular WAN: from legacy 3G to 4G and 5G backup links

From 3G to 4G and 5G

Routers have been able to use mobile networks for many years. The first widespread option was 3G wireless WAN: a modem card in the router, a SIM, and a configuration borrowed from dial-up modems, with a "chat script" that sent AT commands to make a data call. Speeds were a few megabits at best, so 3G was mostly a backup or a stopgap at a new site waiting for its fixed line.

3G is legacy. Many operators have already switched off their 3G networks, and others are doing so, to reuse the radio spectrum for 4G and 5G. Its replacement is 4G LTE and 5G cellular modules, which are far faster and attach to the network by themselves once given an APN. If you find a 3G router still in service, plan its replacement.

The building blocks

• SIM and data plan: a business plan, ideally with a fixed allowance and alerts. A private APN can give the router a fixed private address inside the operator's network.
• APN (access point name): tells the operator which data network to attach to. The operator gives you the name and, sometimes, a username and password.
• Antennas: placement matters more than anything. A router in a basement may need external antennas near a window or on the roof.
• Address: usually assigned by the network, often behind the operator's carrier-grade NAT, so inbound connections may not work without a VPN.

A modern Cisco configuration

On a Cisco ISR with an LTE module, you create a data profile with the APN (an exec-mode command), then configure the cellular interface. The exact slot numbers and profile commands vary by module and release, so check the module guide.

cellular 0/2/0 lte profile create 1 business.apn.example
interface Cellular0/2/0
 ip address negotiated
 ip nat outside
 dialer in-band
 dialer-group 1
dialer-list 1 protocol ip permit

Check signal strength and attach status with show cellular 0/2/0 radio and show cellular 0/2/0 network. Aim for good signal quality before blaming the configuration.

Making it a dependable backup

A backup link is only useful if traffic moves to it when needed, and back again afterwards. A floating static route (a default route with a higher administrative distance) stays unused while the main route exists. But the main route does not disappear if the fibre is up and the provider's network is broken. IP SLA tracking fixes that: the router pings a reliable address through the main link and withdraws the main route when the pings fail.

ip sla 10
 icmp-echo 198.51.100.1 source-interface GigabitEthernet0/0/0
 frequency 10
ip sla schedule 10 life forever start-time now
track 10 ip sla 10 reachability
ip route 0.0.0.0 0.0.0.0 203.0.113.1 track 10
ip route 0.0.0.0 0.0.0.0 Cellular0/2/0 250

Remember that VPN tunnels and NAT rules must also work over the cellular interface. In SD-WAN designs, the cellular link is simply another transport (often marked as the last-resort colour) and the overlay handles failover. See SD-WAN path selection.

Real-world example: a pharmacy keeps taking payments

A pharmacy on a retail park had a single fibre line. When roadworks cut it for a day, the shop could not take card payments or check prescriptions. The owner's IT support fitted an LTE module to the existing router with a business SIM on a capped data plan, mounted the antenna by the front window and set up IP SLA tracking with a floating default route. Six months later the fibre failed again; the router moved to 4G within about 30 seconds, the till and prescription system kept working, and staff only noticed when the support company phoned to tell them.