Exam 300-730 topic 1 question 99 discussion

To resolve the issue with the DMVPN tunnel not coming up, the solution would be to ensure that devices between the hub and spoke are not blocking the GRE (Generic Routing Encapsulation) traffic. DMVPN uses GRE encapsulation to create the overlay network, and ESP (Encapsulating Security Payload) is used to secure the GRE-encapsulated packets. However, in this scenario, the packets are leaving the spoke router, but not reaching the hub router, indicating a possible blockage of GRE traffic. By ensuring that devices between the hub and spoke are not blocking GRE traffic, you allow the encapsulated packets to traverse the network and reach the intended destination. This can involve checking the configuration of any firewalls, routers, or other network devices along the path to ensure they are configured to allow GRE traffic. While it is also important to ensure that ESP traffic is not blocked to maintain secure communication, in this specific case, the issue lies with the GRE encapsulation itself. Once the GRE traffic is allowed, the DMVPN tunnel should be able to establish successfully.

For me... the first packet that the router should send is the IKE negotiation and then IPSEC negotiation. So I would go with B.

Answer is C When DMVPN does not work, before you troubleshoot with IPsec, verify that the GRE tunnels work fine without IPsec encryption. https://www.cisco.com/c/en/us/support/docs/security/dynamic-multipoint-vpn-dmvpn/111976-dmvpn-troubleshoot-00.html#toc-hId--121156399

B, some devices between the hub and spoke might be blocking the ESP traffic

the traffic is encrypted, C

B. ESP since the traffic is encrypted. I have tested in the LAB with an ACL blocking ESP and GRE in my lab: show ip access-list Extended IP access list BlockGRE 5 deny esp any any (44 matches) 10 deny gre any any log 20 permit ip any any (1645 matches)