DMVPN Phase 3 Single Hub – OSPF – Hub example

Traffic Flow:

• Packet is sent from Spoke’s 1 network to Spoke’s 2 network via Hub (according to routing table)
• Hub routes packet to Spoke2 but in parallel sends back the NHRP Redirect message to Spoke1 containing information about suboptimal path to Spoke2 and tunnel IP of Spoke2
• Spoke1 then issues the NHRP Resolution request of Spoke’s 2 NBMA IP address to NHS with destination IP of Spoke’s 2 tunnel, this NHRP Resolution request is sent targeted to Spoke2 via NHS (according to routing table) – it is normal hop by hop NHRP forwarding process
• Spoke2 after receiving resolution request including NBMA IP of Spoke1 sends the NHRP Resolution reply directly to Spoke1 – Reply does not traverse the Hub!
• Spoke1 after receiving correct NBMA IP of Spoke2 rewrites the CEF entry for destination prefix – this procedure is called NHRP Shortcut
• Spokes don’t trigger NHRP by glean adjacencies but NHRP replies updates the CEF

DMVPN Phase 3 and OSPF

• Configure OSPF p2m type (all spokes are aware of whole topology)
• Advertise spoke’s connected routers
• Disable split horizon on hub (Spoke to Spoke prefix advertisement)
• The one of OSPF limitation is single area routes summarization

R1 – DMVPN Phase 3 – OSPF Hub configuration example:

router ospf 111
router-id 10.1.1.1
summary-address 10.1.0.0 255.255.0.0
network 10.1.1.0 0.0.0.255 area 0
network 172.16.1.0 0.0.0.255 area 0
POD1_R1#

interface Tunnel0
ip address 172.16.1.1 255.255.255.0
no ip redirects
ip mtu 1400
ip nhrp authentication gmlabs
ip nhrp map multicast dynamic
ip nhrp network-id 111
ip nhrp shortcut
ip nhrp redirect
ip ospf network point-to-multipoint
tunnel source Loopback0
tunnel mode gre multipoint
tunnel key 123
tunnel protection ipsec profile DMVPN_PROFILE
end