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DMVPN, or Dynamic Multipoint Virtual Private Network, is an overlay hub-and-spoke technology that enables enterprises to connect their offices across a non-broadcast multi-access (NBMA) network, such as the internet. One of the most common implementations of DMVPN is as a backup Wide Area Network (WAN) connection over the internet, leveraging its advantages over traditional Layer 2 WAN circuit alternatives like Frame Relay or Asynchronous Transfer Mode (ATM).
NHRP, or Next Hop Resolution Protocol, is the core protocol that enables DMVPN's dynamic spoke-to-spoke traffic flows and dynamic IP addressing capabilities. NHRP acts as a mapping protocol, allowing the NBMA address of a spoke to be dynamically associated with its VPN address, much like how Frame Relay or ATM maps IP addresses to DLCI numbers or VPI/VCI pairs.
In a DMVPN network, at least one Next Hop Server (NHS), typically located on the hub, receives NHRP registration information from each spoke, containing their dynamically allocated NBMA address and VPN address. This information is then used to facilitate spoke-to-spoke traffic flows via NHRP resolution requests or traditional hub-and-spoke communication.
DMVPN configurations are divided into three phases, each with unique properties and routing capabilities, allowing for flexibility in meeting specific design or policy requirements:
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