DMVPN-Part1

DMVPN solves some of the scalability issues with GRE tunnels.

Highly scalable

Easy configuration

DMVPN Phase1:

In phase1 of DMVPN, the hub is a multipoint GRE tunnel and the spokes are point-to-point GRE tunnels. It means

·         Spoke to spoke traffic must go through hub

·         Simplified routing-just a default route on spokes will do

·         Summarization and default routing can be used on the hub

·         Next hop is always changed by the hub

Here is the sample topo for discussion

The IP address used is just to illustrate that the spoke just needs an internet connection to be part of DMVPN and each of them can be in any arbitrary network as along as connectivity is available.

For our discussions and configurations,

NBMA address –169.254.100.xx

Overlay DMVPN network—14.1.1.xx

We do ping tests on loopbacks of the devices, ip address is 150.1.x.x

Configuration:

Spoke Configuration,

R3#    sh run int tun0

Building configuration...

Current configuration : 352 bytes

!

interface Tunnel0

 ip address 14.1.1.3 255.255.255.0

 ip mtu 1400

 ip nhrp authentication NHRPAUTH

 ip nhrp group INE

 ip nhrp map multicast 169.254.100.5

 ip nhrp map 14.1.1.5 169.254.100.5

 ip nhrp network-id 1

 ip nhrp nhs 14.1.1.5    ----------------àOverlay address of the HUB

 ip tcp adjust-mss 1360

 tunnel source GigabitEthernet0/0.100

 tunnel destination 169.254.100.5--- àNBMA/Public/Underlay address of the HUB

 tunnel key 2

end

HUB Configuration

R5#sh run int tun0

Building configuration...

Current configuration : 293 bytes

!

interface Tunnel0

 ip address 14.1.1.5 255.255.255.0

 no ip redirects

 no ip split-horizon eigrp 100

 ip nhrp authentication NHRPAUTH

 ip nhrp group INE

 ip nhrp map multicast dynamic

 ip nhrp network-id 1

 tunnel source GigabitEthernet0/0.100

 tunnel mode gre multipoint----àMode must be multipoint on HUB, should specify destination on spokes.

 tunnel key 2

end

Once the DMVPN network is established and a routing protocol say EIGRP is enabled, the routers will form adjacency as they are connected in a LAN.

The spoke will multicast the hello packets to HUB.

The hub will multicast the hello packets to the spokes, it’s basically replication of the packets.

The routes learned on a router

Ping from spoke R1 to spoke R3 will go as

The destination is reachable via dmvpn tunnel,so the icmp ping request will get gre encapsulated

Icmp ping request sent to hub

Hub will re encapsulate and send it to spoke3

Spoke3 will send the reply to hub

Hub will reencapsulate and send the reply to spoke1