Exam 300-730 topic 1 question 118 discussion

To resolve the issue of VPN tunnels between a spoke and two DMVPN hubs not coming up, the network administrator should ensure bidirectional UDP 500/4500 traffic. UDP ports 500 and 4500 are used for Internet Key Exchange (IKE) and IPsec traffic, respectively. To establish and maintain VPN tunnels, it is crucial to allow bidirectional traffic on these ports between the spoke and the hubs. By default, UDP port 500 is used for IKE Phase 1 negotiations, and UDP port 4500 is used for IKE Phase 2 negotiations in the case of NAT traversal. It is important to ensure that network devices, firewalls, and any intermediate network devices allow traffic on these ports. In some scenarios, network administrators may encounter issues due to network devices blocking or restricting UDP 500 or 4500 traffic. By ensuring that bidirectional UDP 500/4500 traffic is allowed, the VPN tunnels between the spoke and the two DMVPN hubs should come up successfully.