WAN technologies: leased lines, PPP, MPLS, Metro Ethernet, broadband and VPNs

What a WAN is

A wide area network connects sites in different cities or countries. Unlike a LAN, the organisation rarely owns the links: it rents them from service providers. A private WAN uses dedicated circuits or provider-managed services; a public WAN uses the internet, protected with VPNs.

Leased lines and PPP

A leased line is a dedicated point-to-point circuit between two sites: predictable and private, but expensive and inflexible. Serial links on such circuits often run PPP (Point-to-Point Protocol):

• LCP (Link Control Protocol) sets up, tests and ends the link.
• NCPs (Network Control Protocols) carry each Layer 3 protocol, such as IPCP for IPv4.
• Authentication: PAP sends the password in clear text once (two-way); CHAP uses a three-way challenge and hashed response, repeats it during the session, and never sends the password. Use CHAP.

username HQ-RTR password Shared-Chap-Secret
interface serial 0/0/0
 encapsulation ppp
 ppp authentication chap
 ip address 10.255.0.1 255.255.255.252

Provider services: Frame Relay, MPLS and Metro Ethernet

• Frame Relay (legacy): virtual circuits identified by DLCIs, with a committed information rate. Common in the 1990s and 2000s, now largely retired.
• MPLS: the provider forwards traffic using labels and keeps each customer separate in its own VPN. Offers any-to-any connectivity, quality of service and service-level agreements (SLAs). It separates customers but does not encrypt.
• Metro Ethernet: Ethernet delivered across a city or region; simple to use because sites look like they are on one big switch.

Broadband, mobile and internet VPNs

• Broadband: DSL over telephone lines, cable, and fibre to the premises. Cheap and fast, but usually best effort without an SLA.
• Mobile (4G/5G): quick to deploy; ideal for backup links, temporary sites and IoT.
• Internet VPN: any internet link plus an IPsec tunnel gives private, encrypted site-to-site connectivity at low cost. See VPNs and IPsec.

Choosing a WAN solution

Most organisations now mix two or more of these and manage them together with SD-WAN.

Real-world example: mixing WAN options in a logistics company

A logistics company runs a head office, two large distribution centres and fifteen small depots. The head office and distribution centres connect over an MPLS service from one provider, because their warehouse systems need predictable delay and the provider offers a service level agreement. Each of these sites also has a business broadband line with an IPsec VPN to head office as a backup.

The small depots have only a few PCs and handheld scanners, so MPLS would cost too much. They use broadband with an IPsec VPN as the main link and a 4G router as backup. When a road crew cut the fibre to one distribution centre, traffic moved to the broadband VPN automatically, and the warehouse kept shipping. The mix gives each site the right balance of cost and reliability.