According to the Juniper documentation, the best VPN type for connecting two remote sites to the corporate headquarters site while ensuring that all traffic is secured and sent directly between sites is IPsec ADVPN. ADVPN stands for Auto Discovery VPN, which is a feature that allows the SRX Series devices to dynamically establish IPsec tunnels between remote sites without requiring a full mesh configuration1. IPsec ADVPN uses NHRP (Next Hop Resolution Protocol) to discover the optimal path between two remote sites and create a shortcut tunnel that bypasses the hub device2. This reduces the latency and bandwidth consumption of the traffic and improves the performance and scalability of the VPN.
To configure IPsec ADVPN on the SRX Series devices, the following steps are required:
Configure the hub device as an NHRP server and assign it a unique NHRP network ID and a public IP address3.
Configure the spoke devices as NHRP clients and register them with the hub device using the same NHRP network ID and the hub’s public IP address3.
Configure the IPsec VPN parameters on the hub and spoke devices, such as the IKE and IPsec proposals, policies, and gateways4.
Configure the routing protocols on the hub and spoke devices, such as OSPF or BGP, to advertise the routes between the sites.
Once the IPsec ADVPN is configured, the hub and spoke devices will establish IPsec tunnels with each other and exchange NHRP information. When a spoke device needs to send traffic to another spoke device, it will send an NHRP resolution request to the hub device, which will reply with the public IP address of the destination spoke device. The source spoke device will then initiate a shortcut IPsec tunnel with the destination spoke device and send the traffic directly to it2.
The following VPN types are not suitable for this scenario:
Hub-and-spoke IPsec VPN: This type of VPN requires that all traffic between the remote sites go through the hub device, which adds latency and consumes bandwidth. It also does not scale well as the number of remote sites increases.
Layer 2 VPN: This type of VPN allows the remote sites to extend their Layer 2 networks over a Layer 3 network, such as the internet. It is typically used for data center interconnection or service provider networks. However, it does not provide any security or encryption for the traffic, and it may not be compatible with the existing network infrastructure.
Full mesh Layer 3 VPN with EBGP: This type of VPN allows the remote sites to exchange Layer 3 routing information over a Layer 3 network, such as the internet, using EBGP (External Border Gateway Protocol). It is typically used for enterprise networks or service provider networks. However, it requires that each remote site has a unique AS (Autonomous System) number and a public IP address, and that each remote site establishes a BGP session with every other remote site. This can be complex and cumbersome to configure and maintain, and it may not provide any security or encryption for the traffic.
References: 1: Auto Discovery VPN Overview 2: Understanding Auto Discovery VPN 3: Configuring NHRP on the Hub and Spoke Devices 4: Configuring IPsec VPN on the Hub and Spoke Devices : [Configuring Routing Protocols on the Hub and Spoke Devices] : [Hub-and-Spoke VPNs Overview] : [Layer 2 VPNs Feature Guide for Security Devices] : [Layer 3 VPNs Feature Guide for Security Devices]
