What Does "Multiple WAN" Mean and How Do You Combine Multiple WAN Links?
"Multiple WAN" means a single site uses two or more separate WAN or internet connections instead of one. You combine them in one of three ways: failover (one link backs up another), load balancing (traffic is spread across links), or aggregation/bonding (links are treated as one higher-capacity path). The right choice depends on whether you need resilience, more total throughput, or more throughput for a single session — and on whether your ISP and hardware support it.
The three ways to combine WAN links
| Method | What it does | Effect on a single session | Typical use |
|---|---|---|---|
| Failover | Uses one link; switches to another if the first fails | Session may drop during switchover | Keeping a site online when a link dies |
| Load balancing | Distributes flows across links | Usually stays on one link (per-flow) | Adding capacity across many users/sessions |
| Aggregation / bonding | Treats multiple links as one logical path | Can exceed one link's speed (per-packet) | Point-to-point transport needing more than one link |
These are not mutually exclusive. A site can load-balance across two links and fail over to a third.
Failover vs. load balancing vs. aggregation
Failover is the simplest. You keep a primary WAN and a standby. When the primary goes down, traffic moves to the backup. The benefit is availability, not speed. A single long-lived session (a call, a download) often breaks when the switch happens, because the public IP or path changes.
Load balancing spreads traffic across links so the total capacity is higher than any one link. The key distinction is per-flow versus per-packet:
- Per-flow load balancing assigns each connection (flow) to one link. A single download still runs at one link's speed, but many users or many sessions together use all links. This is the common case for general internet access, and it preserves session continuity because each flow stays on one path.
- Per-packet load balancing splits even a single flow across links, so one session can exceed one link's speed. This requires both ends to cooperate and reassemble packets in order; otherwise packets arrive out of order and performance suffers. It is used in controlled point-to-point setups, not general ISP connections.
Aggregation / bonding combines links into one logical channel. Done per-packet with matching equipment at both ends, it gives a single path faster than any individual link. Done without cooperation at the far end, it usually degrades to load balancing.
How multiple T3/DS3/E3 links fit in
T3, DS3, and E3 are TDM (time-division multiplexing) WAN links. When you have several of them between two sites, you can bond them to carry Ethernet traffic as one higher-bandwidth path. E3Switch describes exactly this in its Multi-TDM Gigabit LAN Extender application: high-bandwidth Ethernet transport over multiple DS3/E3/T3 TDM links, with fine-grained per-packet load balancing over the bonded links. It is a point-to-point design — one unit at each end of the DS3/E3 transport — typically connecting two distant customer locations that each have an Ethernet network. The same source notes it is not for a single-ended connection to an ISP.
That point-to-point requirement is the practical limit of aggregation: both ends must run compatible equipment. If you only control your own site, you generally cannot bond links to an arbitrary ISP.
Practical limits to check before combining links
- ISP support. Bonding or per-packet aggregation usually needs cooperation at the far end. Failover and per-flow load balancing generally do not.
- Session continuity. Failover and per-packet schemes can break or reorder sessions; per-flow load balancing usually keeps each session intact.
- Cost. More WAN links mean more recurring circuit cost, plus hardware that can handle multiple WAN interfaces.
- Hardware. You need a router, load balancer, or converter that supports the number and type of links you have. For TDM links, that means T3/E3/DS3-capable equipment.
- Addressing. Multiple links may mean multiple public IPs, which affects inbound services and some applications.
A simple decision guide
- Need the site to stay up when a link fails? → Failover.
- Need more total capacity for many users or sessions, and can accept that one session uses one link? → Per-flow load balancing.
- Need one session faster than any single link, and control both ends? → Aggregation / bonding (per-packet).
- Have multiple T3/DS3/E3 links between two sites and need higher point-to-point Ethernet throughput? → Bonded TDM transport, with matching units at both ends.
If you are unsure, start with failover plus per-flow load balancing: it covers most availability and capacity needs without requiring ISP cooperation or risking session breakage.