DMVPN- Details Analysis

DMVPN - Phase1,2, and 3

• NBMA networks and all traffic broadcast and uni-cast must be sent across the network as uni-cast packets.
• This simply means multicast traffic destined for an IGP neighbor will always be encapsulated in a uni-cast packet for delivery
• Hub router always knows the identities of all the spokes for which it is the next-hop server (NHS)
• ip nhrp map multicast dynamic command on the hub is used to dynamically create mappings in the NHRP multicast table for each spoke that registers with it
• Spoke routers create a static multicast mapping for hub 
•  ip nhrp map multicast [ nbma address of hub] 
•  ip nhrp nhs [overlay address] [underlay address] multicast

PHASE I

• Only provides hub-spoke tunnel deployment. This means GRE tunnels are only build between the hub and spokes
• Traffic destined to networks behind spokes is forced to first traverse the hub.
• Restricts it form building dynamic spoke-to-spoke tunnels. Because all spoke-to-spoke traffic in DMVPN phase 1 always traverses the hub.
• Flood a default summary route to the spokes for all traffic. This is achieved in EIGRP using the ip summary-address eigrp [asn] 0.0.0.0 0.0.0.0 command under the tunnel interface

PHASE1- Configuration
Hub 
Router(config)# interface tunnel 0
Router(config-if) # ip address 172.16.1.1 255.255.255.0
Router (config-if) # no ip redirect
Router (config-if) # ip nhrp authentication cisco
Router (config-if) # ip nhrp multicast dynamic
Router (config-if) # ip nhrp network-id 1
Router (config-if) # ip summary-address eigrp 1 0.0.0.0 0.0.0.0
Router (config-if) # tunnel source G0/1
Router (config-if) # tunnnel mode gre multipoint

Spoke
Router (config) # interface tunnel 0
Router (config-if) # ip address 172.16.1.2 255.255.255.0
Router (config-if) # no ip redirect
Router (config-if) # ip nhrp authentication cisco
Router (config-if) # ip nhrp map 172.16.1.1 192.168.1.1
Router (config-if) # ip nhrp map multicast 192.168.1.1
Router (config-if) # ip nhrp network-id 1
Router (config-if) # ip nhrp nhs 172.16.1.1
Router (config-if) # tunnel source 192.168.1.2
Router (config-if) # tunnel destination 192.168.1.1

PHASE 2

• Traffic between the spokes would always hit the hub.
• The hub burdened with encapsulate/decapsulate overhead for the spoke-to-spoke traffic
• Phase 2 improved on phase 1 by allowing spokes to build a spoke-to-spoke tunnel on demand with these restrictions
• Spoke must use multipoint GRE tunnels
• The spokes must receive specific routes for all remote spoke subnets
• The next hop of the entry in the routing table must list the remote spoke as the next hop

CONFIGURATION

• Need to configure mGRE on the spokes allows multiple GRE tunnels to be formed using a single tunnel interface ( tunnel mode gre multipoint)

•     Spokes must receive specific routes for all remote spoke subnets. For EIGRP, this is accomplished by disabling split horizon on the tunnel interface

• The hub uses its tunnel 0 interface to reach the networks behind the spokes. Split horizon will prevent the hub from advertising those networks to remote spokes.
• no ip split-horizon eigrp command

• After the NHRP resolution is complete, traffic will not pass through Hub anymore.
• Because the next hop for each prefix must be preserved. In Phase 2, it is not possible to summarize from the hub to the spokes

PHASE2- Configuration
Hub 
Router(config)# interface tunnel 0
Router(config-if) # ip address 172.16.1.1 255.255.255.0
Router (config-if) # no ip redirect
Router (config-if) # ip nhrp authentication cisco
Router (config-if) # ip nhrp multicast dynamic
Router (config-if) # ip nhrp network-id 1
Router (config-if) # no ip next-hop-self eigrp 1
Router (config-if) # no ip split-horion eigrp 1
Router (config-if) # tunnel source G0/1
Router (config-if) # tunnnel mode gre multipoint

Spoke
Router (config) # interface tunnel 0
Router (config-if) # ip address 172.16.1.2 255.255.255.0
Router (config-if) # no ip redirect
Router (config-if) # ip nhrp authentication cisco
Router (config-if) # ip nhrp map 172.16.1.1 192.168.1.1
Router (config-if) # ip nhrp map multicast 192.168.1.1
Router (config-if) # ip nhrp network-id 1
Router (config-if) # ip nhrp nhs 172.16.1.1
Router (config-if) # tunnel source 192.168.1.2
Router (config-if) # tunnel mode gre multipoint

PHASE 3

• Spokes register their Tunnel/NBMA mappings with the hub (or hubs). This allows the hub to dynamically discover all spokes and establish routing adjacency and routing information exchanged.
• In fact, the hub is not required to preserve the reachability information and also summarize information to send to all spokes

• Use NHRP traffic indication messages from the hub to signal to the spokes that a better path exists to reach the target network
• " ip nhrp redirect" on the hub and " ip nhrp shortcut"
• This command provides the hub to the send the NHRP traffic indication message while the shortcut command tells the spokes to accept the redirect and install the shortcut route

PHASE 3 CONFIGURATION
HUB 

Router(config)# interface tunnel 0
Router(config-if) # ip address 172.16.1.1 255.255.255.0
Router (config-if) # no ip redirect
Router (config-if) # ip nhrp authentication cisco
Router (config-if) # ip nhrp multicast dynamic
Router (config-if) # ip nhrp network-id 1
Router (config-if) # ip nhrp redirect
Router (config-if) # ip summary-address eigrp 1 0.0.0.0 0.0.0.0
Router (config-if) # tunnel source G0/1
Router (config-if) # tunnnel mode gre multipoint

SPOKE
Router (config) # interface tunnel 0
Router (config-if) # ip address 172.16.1.2 255.255.255.0
Router (config-if) # no ip redirect
Router (config-if) # ip nhrp authentication cisco
Router (config-if) # ip nhrp map 172.16.1.1 192.168.1.1
Router (config-if) # ip nhrp map multicast 192.168.1.1
Router (config-if) # ip nhrp network-id 1
Router (config-if) # ip nhrp shortcut
Router (config-if) # ip nhrp nhs 172.16.1.1
Router (config-if) # tunnel source 192.168.1.2
Router (config-if) # tunnel mode gre multipoint
 
REFERENCE

1. INE_BLOG_DMVPN_EXPLAIN
2. INE_DMVPN_VIDEO
3. CISCO_DMVPN_CONFIGURATION_DOC