NAT-T (NAT Traversal) In English for SD-WAN:Encapsulates IPsec traffic in UDP (usually port 4500)

NAT-T (Network Address Translation Traversal) in SD-WAN

NAT-T is a mechanism that enables IPsec-based SD-WAN tunnels to function correctly when one or both endpoints are behind a NAT device. Since traditional IPsec ESP traffic (protocol 50) does not work well with NAT—because NAT modifies IP headers and breaks integrity checks—NAT-T solves this limitation by encapsulating IPsec packets inside UDP.

When NAT is detected during IKE negotiation, the SD-WAN device automatically switches to NAT-T mode and wraps IPsec ESP packets inside UDP, typically using port 4500 (with port 500 used initially for IKE). This allows the traffic to traverse NAT routers and firewalls that are configured to permit UDP traffic, ensuring tunnel stability.

In SD-WAN deployments, NAT-T is especially important because:

Branch edges often use private IP addresses behind ISP routers

Multiple sites may share a single public IP

Firewalls may block native ESP traffic

Dynamic WAN links (broadband, LTE, 5G) frequently involve NAT

By enabling NAT-T, SD-WAN overlays can maintain secure, encrypted connectivity, support dynamic path selection, and ensure reliable tunnel establishment across heterogeneous networks without requiring special firewall configurations.

In summary, NAT-T ensures seamless IPsec tunnel establishment and reliability in SD-WAN environments where NAT is present, making it a critical feature for modern WAN architectures.