https://www.rfc-editor.org/rfc/rfc2333
RFC 2333: NHRP Protocol Applicability Statement
rfc2333nhrpprotocolapplicability
https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9500/software/release/16-10/configuration_guide/ip/b_1610_ip_9500_cg/m9-1610-ip-config-nhrp-cg.html
IP Configuration Guide, Cisco IOS XE Gibraltar 16.10.x (Catalyst 9500 Switches) - Configuring NHRP...
https://www.cisco.com/c/en/us/support/docs/security-vpn/ipsec-negotiation-ike-protocols/41940-dmvpn.html
Dynamic Multipoint IPsec VPNs (Using Multipoint GRE/NHRP to Scale IPsec VPNs) - Cisco
This document discusses Dynamic Multipoint IPsec VPNs (DMVPN) and why a company might want to design or migrate their network to make use of this new IPsec VPN...
dynamicmultipointipsecvpnsusing
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_nhrp/command/reference/irn_book.html
Cisco IOS IP Routing: NHRP Command Reference - Cisco
Cisco IOS IP Routing: NHRP Command Reference
cisco iosip routingnhrpcommandreference