DMVPN with NHRP

Basic DMVPN overview

In a dual cloud topology, two DMVPN networks are used to exchange traffic between devices. Two mGRE or two P2P-GRE interfaces are configured at each site not each device. Two tunnels are configured on a single CPE site and two tunnels are configured on a dual CPE site (one tunnel per CPE device). Dual Cloud Topologies can support the Hub and Spoke Deployment Model and the Spoke-to-Spoke Deployment Model. Multiple NHRP servers are required at the hub site (one for each cloud). Also, this helps to protect the customer’s hub from a single point failure.

Basic DMVPN Configuration

====================
! IKE Policy Configuration
!===================
! Creates the ISAKMP policy named “1″.
crypto isakmp policy 1
! Sets the encryption algorithm for protection suite.
encr aes 256
! Sets the hash algorithm for protection suite.
hash sha
! Sets the authentication method for protection suite.
authentication pre-share
! Set the Diffie-Hellman group.
group 5
! Set lifetime for ISAKMP security association
lifetime 86400
! Sets a wildcard pre-shared key for remote peers.
crypto isakmp key [PRESHARED_KEY] address 0.0.0.0
! Sets the keepalive interval for use with all peers. The Number of seconds
! between keep alives is set to 10. The number of seconds between retries
! is set to 4. The on-demand mode only sends DPD messages when needed.
crypto isakmp keepalive 10 4 on-demand
!===================
! IKE Policy Configuration
!===================

crypto isakmp policy 1
encr aes 256
hash sha
authentication pre-share
group 5
lifetime 86400
crypto isakmp key cisco address 0.0.0.0
crypto isakmp keepalive 10 4 on-demand

!=====================
! IPSEC Policy Configuration
!=====================
! Creates an IPSEC transform named “T1″ and defines the associated settings.
crypto ipsec transform-set T1 esp-aes 256 esp-sha-hmac
! Sets the IPSEC mode of opration to “transport” mode.
mode transport
! Creates an IPSEC policy profile named “P1″.
crypto ipsec profile P1
! Specifies the transform set to be used.
set transform-set T1
! Specifies the PFS Diffie-Hellman group.
set pfs group5

!====================
! IPSEC Policy Configuration
!====================

crypto ipsec transform-set T1 esp-aes 256 esp-sha-hmac
mode transport
crypto ipsec profile P1
set transform-set T1
set pfs group5

!================
! Hub - DMVPN Tunnel
!================

! Creates a tunnel interface.
interface Tunnel 0
! Defines the interface description.
description DMVPN Hub Primary Cloud
! Sets the bandwidth informational parameter.
bandwidth 10000
! Sets the IP address of the tunnel interface.
ip address [TUNNEL_IP] [TUNNEL_MASK]
! Sets the IP Maximum Transmission Unit.
ip mtu 1400
! Sets the NHRP authentication string.
ip nhrp authentication [NHRP_PRESHARED_KEY]
! Dynamically learn NBMA mapping for broadcasts/multicasts
ip nhrp map multicast dynamic
! Sets the NHRP NBMA network identifier.
ip nhrp network-id [NHRP_NETWORK_ID]
! Sets the NHRP advertised holdtime.
ip nhrp holdtime 600
! Specifies the interface throughput delay.
delay [TUNNEL_DELAY]
! Sets the source IP address of the tunnel packets.
tunnel source [PUBLIC_INTERFACE_INSTANCE]
! Enables Multipoint GRE mode.
tunnel mode gre multipoint
! Sets a tunnel security or selector key.
tunnel key [TUNNEL_KEY]
! Enables IPSEC tunnel protection.
tunnel protection ipsec profile P1

!=================
! Hub - DMVPN Tunnel
!=================

interface Tunnel 0
description DMVPN Hub Primary Cloud
bandwidth 10000
ip address 172.17.0.1 255.255.255.0
ip mtu 1400
ip nhrp authentication cisco
ip nhrp map multicast dynamic
ip nhrp network-id 1
ip nhrp holdtime 600
delay 100
tunnel source GigabitEthernet0/1
tunnel mode gre multipoint
tunnel key 1
tunnel protection ipsec profile P1

!===================
! Spoke - DMVPN Tunnel
!===================

! Creates a tunnel interface.
interface Tunnel [TUNNEL_INSTANCE]
! Defines the interface description.
description [TUNNEL_DESCRIPTION]
! Sets the bandwidth informational parameter.
bandwidth 10000
! Sets the IP address of the tunnel interface.
ip address [TUNNEL_IP] [TUNNEL_MASK]
! Sets the IP Maximum Transmission Unit.
ip mtu 1400
! Sets the NHRP authentication string.
ip nhrp authentication [NHRP_PRESHARED_KEY]
! Creates a static NHRP mapping for the hub router.
ip nhrp map [HUB_TUNNEL_IP] [HUB_PUBLIC_IP]
! NBMA mapping for broadcasts/multicasts. Used to dynamically learn
! destinations from client registrations on hub.
ip nhrp map multicast [HUB_PUBLIC_IP]
! Sets the NHRP NBMA network identifier.
ip nhrp network-id [NHRP_NETWORK_ID]
! Sets the NHRP advertised holdtime.
ip nhrp holdtime 600
! Sets the IP address of the NHRP server located at the hub.
ip nhrp nhs [HUB_TUNNEL_IP]
! Specifies the interface throughput delay.
delay [TUNNEL_DELAY]
! Sets the source IP address of the tunnel packets.
tunnel source [PUBLIC_INTERFACE_INSTANCE]
! Sets the destination IP address of the tunnel packets.
tunnel destination [HUB_PUBLIC_IP]
! Sets a tunnel security or selector key.
tunnel key [TUNNEL_KEY]
! Enables IPSEC tunnel protection.
tunnel protection ipsec profile P1

!===================
! Spoke - DMVPN Tunnel
!===================

interface Tunnel 0
description DMVPN Spoke Primary Cloud
bandwidth 10000
ip address 172.17.0.2 255.255.255.0
ip mtu 1400
ip nhrp authentication cisco
ip nhrp map 172.17.0.1 10.0.0.1
ip nhrp map multicast 10.0.0.1
ip nhrp network-id 1
ip nhrp holdtime 600
ip nhrp nhs 172.17.0.1
delay 100
tunnel source FastEthernet0
tunnel destination 10.0.0.1
tunnel key 1
tunnel protection ipsec profile P1


Next-Hop Resolution Protocol (NHRP)

When traffic is exchanged between two devices in a DMVPN network, the devices must learn where the GRE interface for the other device publicly resides in the NBMA network. This information is learnt through the Next-Hop Resolution Protocol (NHRP). When a device on the Spoke’s private network initiates a session to a device on the Hub’s private network, a routing table lookup is executed and determines the destination network is reachable through IP address 10.0.0.1 via interface Tunnel 0. The session traffic is then forwarded out the Tunnel0 interface where an NHRP lookup is executed. The map entry in the Spoke NHRP Table indicates the Hub GRE (End-Point) is located at NBMA IP address 10.0.0.1. From this information, the session traffic is forwarded out a dynamic GRE/IPSEC tunnel to the hub as demonstrated in the diagram. The Hub receives the session traffic and forwards it to the destination device.

NHRP Registration Order of Operation

A static NHRP mapping is configured for the NHRP server configured at the Hub which forces the spoke to initiate an NHRP Registration Request to the Hub. The NHRP Registration Request causes an IKE session to be initiated with the Hub. Once the IKE/IPSEC sessions are established, the NHRP Registration request is forward over the IPSEC/GRE tunnel.

The Hub processes the NHRP Registration Request received from Spoke1, creates an entry in the NHRP table and sends an NHRP Registration Reply to Spoke1 indicating the spoke was successfully registered.

Data exchanged between the hub and spokes are sent over the IPSEC/GRE tunnel.