DMVPN Phase 1 RIP Routing

In the first DMVPN lesson I explained some of its basics, in the second lesson I explained how to create a basic DMVPN phase 1 configuration. In this lesson, we’ll take a look how RIP behaves on DMVPN phase 1.

Here’s the topology we will use:

Above we have two spoke routers and one hub, our underlay network uses 192.168.123.0/24 and the overlay network uses 172.16.123.0/24. Each router has a loopback interface with a network that we can advertise in RIP.

Configuration

Tunnel Interfaces

Here is the configuration of the tunnel interfaces on the hub and spoke routers. I explained these commands in the DMVPN phase 1 basic configuration lesson so I’m not going to discuss them again:

Hub(config)#interface Tunnel0
Hub(config-if)#ip address 172.16.123.1 255.255.255.0
Hub(config-if)#ip nhrp authentication DMVPN
Hub(config-if)#ip nhrp map multicast dynamic
Hub(config-if)#ip nhrp network-id 1
Hub(config-if)#tunnel source GigabitEthernet0/1
Hub(config-if)#tunnel mode gre multipoint

Spoke1(config)#interface Tunnel0
Spoke1(config-if)#ip address 172.16.123.2 255.255.255.0
Spoke1(config-if)#ip nhrp authentication DMVPN
Spoke1(config-if)#ip nhrp map 172.16.123.1 192.168.123.1
Spoke1(config-if)#ip nhrp map multicast 192.168.123.1
Spoke1(config-if)#ip nhrp network-id 1
Spoke1(config-if)#ip nhrp nhs 172.16.123.1
Spoke1(config-if)#tunnel source GigabitEthernet0/1
Spoke1(config-if)#tunnel destination 192.168.123.1

Spoke2(config)#interface Tunnel0
Spoke2(config-if)#ip address 172.16.123.3 255.255.255.0
Spoke2(config-if)#ip nhrp authentication DMVPN
Spoke2(config-if)#ip nhrp map 172.16.123.1 192.168.123.1
Spoke2(config-if)#ip nhrp map multicast 192.168.123.1
Spoke2(config-if)#ip nhrp network-id 1
Spoke2(config-if)#ip nhrp nhs 172.16.123.1
Spoke2(config-if)#tunnel source GigabitEthernet0/1
Spoke2(config-if)#tunnel destination 192.168.123.1

Let’s do two quick checks, we want to make sure that both spokes are registered to the hub:

Hub#show dmvpn | begin 192.168.123.
     1 192.168.123.2      172.16.123.2    UP 00:22:37     D
     1 192.168.123.3      172.16.123.3    UP 00:00:32     D

And we want to make sure that we can ping all tunnel IP addresses:

Hub#ping 172.16.123.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.123.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 6/6/8 ms

Hub#ping 172.16.123.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.123.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 5/6/9 ms

Now we can focus on the RIP configuration…

RIP

Let’s enable RIP on all routers:

Hub(config)#router rip
Hub(config-router)#version 2
Hub(config-router)#network 1.0.0.0
Hub(config-router)#network 172.16.0.0
Hub(config-router)#no auto-summary

Spoke1(config)#router rip
Spoke1(config-router)#version 2
Spoke1(config-router)#network 2.0.0.0 
Spoke1(config-router)#network 172.16.0.0
Spoke1(config-router)#no auto-summary

Spoke2(config)#router rip
Spoke2(config-router)#version 2
Spoke2(config-router)#network 3.0.0.0
Spoke2(config-router)#network 172.16.0.0 
Spoke2(config-router)#no auto-summary 

Now let’s see what we have…

Hub#show ip route rip 

      2.0.0.0/32 is subnetted, 1 subnets
R        2.2.2.2 [120/1] via 172.16.123.2, 00:00:07, Tunnel0
      3.0.0.0/32 is subnetted, 1 subnets
R        3.3.3.3 [120/1] via 172.16.123.3, 00:00:06, Tunnel0

Our hub router has learned both networks on the loopback interfaces. Take a good look at the next hop IP addresses that we have here, these are the tunnel addresses. When the hub wants to reach these networks, it will have to check NHRP to find the NBMA addresses:

Hub#show dmvpn | begin Hub
Type:Hub, NHRP Peers:2, 

 # Ent  Peer NBMA Addr Peer Tunnel Add State  UpDn Tm Attrb
 ----- --------------- --------------- ----- -------- -----
     1 192.168.123.2      172.16.123.2    UP 00:29:29     D
     1 192.168.123.3      172.16.123.3    UP 00:07:24     D

When we want to reach 2.2.2.2/2 we will use 172.16.123.2 as the next hop. In our NHRP cache we see that we need to use NBMA address 192.168.123.2 to get there.

What about the spoke routers, did they learn anything?

Spoke1#show ip route rip 

      1.0.0.0/32 is subnetted, 1 subnets
R        1.1.1.1 [120/1] via 172.16.123.1, 00:00:17, Tunnel0

Spoke2#show ip route rip 

      1.0.0.0/32 is subnetted, 1 subnets
R        1.1.1.1 [120/1] via 172.16.123.1, 00:00:08, Tunnel0

Our spoke routers only have one entry, the network on the loopback interface of the hub router. What went wrong here?

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