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===== DMVPN w/ GRE =====
GRE tunnels, but dynamic, yanno?

Going to skip the explanation until I hit this part in the books, but until then - a refresher on quick Phase 2 configuration from a Hub/spoke topology with one HUB. And excuse the terrible diagram, I'll build a better one one I'm not pressed for time!

REFER TO THE ABOVE DIAGRAM AND NOTE THE CONFIGS BELOW!

==== CONFIGS ====
=== R1 the HUB ===
<code>
interface Tunnel10
 ip address 192.168.1.1 255.255.255.0  <-- Tun Addr
 no ip redirects
 ip nhrp network-id 1         <-- Activates NHRP
 tunnel source Loopback0
 tunnel mode gre multipoint   <-- Enable DMVPN
end

</code>
=== R3 Spoke ===
<code>
interface Tunnel10
 ip address 192.168.1.3 255.255.255.0    <-- Set IP for Tun INT
 no ip redirects
 ip nhrp map 192.168.1.1 1.1.1.1         <-- Map Phys/NBMA address for NHS aka How do I get to 192.168.1.1?
 ip nhrp network-id 3                    <-- Useful for multi-hub environments. Details later...
 ip nhrp nhs 192.168.1.1                 <-- Specify NHS for tunnel traffic to traverse.
 tunnel source Loopback0                
 tunnel mode gre multipoint              <-- Enable Dynamic Multipoint VPN
end

</code>
=== R5 Spoke ===
<code>
interface Tunnel10
 ip address 192.168.1.5 255.255.255.0
 no ip redirects
 ip nhrp map 192.168.1.1 1.1.1.1
 ip nhrp network-id 5
 ip nhrp nhs 192.168.1.1
 tunnel source Loopback0
 tunnel mode gre multipoint
end

</code>

==== Verification and Flow ====
<code>
R3#traceroute 5.5.5.5                           <-- Loop to Loop
Type escape sequence to abort.
Tracing the route to 5.5.5.5
VRF info: (vrf in name/id, vrf out name/id)
  1 10.2.3.2 36 msec 52 msec 48 msec
  2 10.1.2.1 52 msec 56 msec 48 msec
  3 10.1.4.4 76 msec 72 msec 68 msec
  4 10.4.5.5 72 msec 56 msec 76 msec

Tracing the route to 192.168.1.5                 <-- Tun to Tun
VRF info: (vrf in name/id, vrf out name/id)
  1 192.168.1.5 88 msec 76 msec 76 msec
  
R3#sh dmvpn | b Type     
Type:Spoke, NHRP Peers:2,

# Ent  Peer NBMA Addr Peer Tunnel Add State  UpDn Tm Attrb
 ----- --------------- --------------- ----- -------- -----
     1 1.1.1.1             192.168.1.1    UP 00:01:24     S
     1 5.5.5.5             192.168.1.5    UP 00:01:23     D
     
R3#sh ip nhrp 
192.168.1.1/32 via 192.168.1.1
   Tunnel10 created 00:05:46, never expire 
   Type: static, Flags: used 
   NBMA address: 1.1.1.1 
192.168.1.5/32 via 192.168.1.5
   Tunnel10 created 00:04:05, expire 01:56:06
   Type: dynamic, Flags: router used 
   NBMA address: 5.5.5.5

</code>

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