what is dmvpn

MVPN Components 
Multiple GRE tunnel interfaces: a single GRE interface that can secure several IPsec tunnels, reducing the overall scope of the DMVPN configuration
IPsec tunnel endpoint discovery: meaning that static crypto maps between individual IPsec tunnel endpoints do not have to be configured
Routing Protocols: which can allow the DMVPN to find routes between different endpoints much more effectively
NHRP: which can deploy spokes with assigned IP addresses that can then be connected to from the central DMVPN hub.
DMVPN Phases
There are three distinct types, or phrases, of DMVPN design, all of which can be found on the Cisco DMVPN design guide. To summarize them briefly, however, they are as follows:
what is dmvpn

DMVPN Phase 1 uses HUB-and-spoke tunnel deployment. The tunnels through which inter-branch connections are made are only built through the central DMVPN hub and the individual spokes, working much like a traditional VPN system.
DMPVN Phase 2 uses spoke-to-spoke tunnel deployment, meaning that data doesn’t have to travel to a central hub first, so long as there are specific routes in place for the spoke subnets.
Reduces the likelihood of downtime by securing routing with IPsec technology.
Understanding DMVPN 

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