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Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) · Cisco
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OCG Pag 773 "The Phase 3 DMVPN configuration for the hub router adds the interface parameter command IP NHRP REDIRECT on the hub router. This command checks the flow of packets on the tunnel interface and sends a redirect message to the source spoke router when it detects packets hairpinning out of the DMVPN cloud. Hairpinning means that traffic is received and sent out an interface in the same cloud (identified by the NHRP network ID). For instance, hairpinning occurs when packets come in and go out the same tunnel interface. The Phase 3 DMVPN configuration for spoke routers uses the mGRE tunnel interface and uses the command IP NHRP SHORTCUT on the tunnel interface.
Hub(config)#interface tunnel 1 Hub(config-if)#ip nhrp redirect -> Hub notifies spoke routers of suboptimal traffic paths Spoke1(config)#interface tunnel 1 Spoke1(config-if)#ip nhrp shortcut -> Spokes send a resolution request for a shortcut path after receiving an NHRP redirect traffic indication message
Shortcut is needed on hub and spokes to initiate spoke-to-spoke tunnels. Redirect is an optimization that is needed for spoke-to-spoke communication only if you have summarized routes, if the spoke has the complete routing-table it is not needed. https://www.cisco.com/c/en/us/support/docs/security/dynamic-multipoint-vpn-dmvpn/211292-Configure-Phase-3-Hierarchical-DMVPN-wit.html#anc9 https://www.ciscozine.com/dmvpn-phase-3-guide/