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300-410 · Question #21

Refer to the exhibit. Which interface configuration must be configured on the spoke A router to enable a dynamic DMVPN tunnel with the spoke B router?

The correct answer is B. interface Tunnel0 ip address 10.0.0.11 255.255.255.0 ip nhrp network-id 1 tunnel source FastEthernet 0/0 tunnel mode gre multipoint ip nhrp nhs 10.0.0.1 ip nhrp map 10.0.0.1 172.17.0.1. For a DMVPN spoke router to establish a dynamic tunnel, it requires a multipoint GRE tunnel, NHRP configuration specifying its network ID, and crucially, the Next Hop Server (NHS) IP address and its corresponding public IP.

VPN Technologies

Question

Refer to the exhibit. Which interface configuration must be configured on the spoke A router to enable a dynamic DMVPN tunnel with the spoke B router?

Options

  • Ainterface Tunnel0 description mGRE - DMVPN Tunnel ip address 10.0.0.11 255.255.255.0 ip nhrp map multicast dynamic ip nhrp network-id 1 tunnel source 10.0.0.1 tunnel destination FastEthernet 0/0 tunnel mode gre multipoint
  • Binterface Tunnel0 ip address 10.0.0.11 255.255.255.0 ip nhrp network-id 1 tunnel source FastEthernet 0/0 tunnel mode gre multipoint ip nhrp nhs 10.0.0.1 ip nhrp map 10.0.0.1 172.17.0.1
  • Cinterface Tunnel0 ip address 10.0.0.11 255.255.255.0 ip nhrp network-id 1 tunnel source 1.1.1.10 ip nhrp map 10.0.0.11 172.17.0.2 tunnel mode gre
  • Dinterface Tunnel0 ip address 10.0.0.11 255.255.255.0 ip nhrp map multicast static ip nhrp network-id 1 tunnel source 10.0.0.1 tunnel mode gre multipoint

How the community answered

(41 responses)
  • A
    10% (4)
  • B
    83% (34)
  • C
    2% (1)
  • D
    5% (2)

Why each option

For a DMVPN spoke router to establish a dynamic tunnel, it requires a multipoint GRE tunnel, NHRP configuration specifying its network ID, and crucially, the Next Hop Server (NHS) IP address and its corresponding public IP.

Ainterface Tunnel0 description mGRE - DMVPN Tunnel ip address 10.0.0.11 255.255.255.0 ip nhrp map multicast dynamic ip nhrp network-id 1 tunnel source 10.0.0.1 tunnel destination FastEthernet 0/0 tunnel mode gre multipoint

`tunnel destination` should not be configured on a dynamic multipoint GRE tunnel; it's for static point-to-point tunnels, and `ip nhrp map multicast dynamic` is typically for the hub.

Binterface Tunnel0 ip address 10.0.0.11 255.255.255.0 ip nhrp network-id 1 tunnel source FastEthernet 0/0 tunnel mode gre multipoint ip nhrp nhs 10.0.0.1 ip nhrp map 10.0.0.1 172.17.0.1Correct

A DMVPN spoke configuration requires `tunnel mode gre multipoint` for dynamic spoke-to-spoke tunnels and to register with the NHRP server. `ip nhrp network-id 1` uniquely identifies the DMVPN domain, `tunnel source FastEthernet 0/0` specifies the public interface, and `ip nhrp nhs 10.0.0.1` along with `ip nhrp map 10.0.0.1 172.17.0.1` defines the hub's tunnel and public IP, enabling the spoke to find and register with the hub.

Cinterface Tunnel0 ip address 10.0.0.11 255.255.255.0 ip nhrp network-id 1 tunnel source 1.1.1.10 ip nhrp map 10.0.0.11 172.17.0.2 tunnel mode gre

`tunnel mode gre` is for point-to-point tunnels, not mGRE required for DMVPN spokes. The `ip nhrp map` statement is incorrectly mapping its own tunnel IP to a public IP that isn't the hub's, and the `ip nhrp nhs` command is missing.

Dinterface Tunnel0 ip address 10.0.0.11 255.255.255.0 ip nhrp map multicast static ip nhrp network-id 1 tunnel source 10.0.0.1 tunnel mode gre multipoint

`ip nhrp map multicast static` is generally not used for dynamic spokes which learn multicast mappings; `tunnel source 10.0.0.1` is unlikely to be the spoke's public interface, and `tunnel destination` is inappropriate for mGRE.

Concept tested: DMVPN spoke configuration for mGRE and NHRP

Source: https://www.cisco.com/c/en/us/support/docs/security/dynamic-multipoint-vpn-dmvpn/119510-configure-dmvpn-00.html

Topics

#DMVPN#NHRP#GRE multipoint#Spoke configuration

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