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DMVPN Phase 3 RIP Routing

In this lesson we’ll take a look how to configure RIP on a DMVPN phase 3 network. If you are unsure what the differences between the different DMVPN phases are then you might want to start with my introduction to DMVPN first.



We will use the DMVPN phase 3 basic configuration for this example. Here’s the topology we will use:

DMVPN Example Topology with hub, two spokes and loopback interfaces.

We have a hub router and two spoke routers. Here’s the DMVPN phase 3 configuration:

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
Hub(config-if)#ip nhrp redirect

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 mode gre multipoint
Spoke2(config-if)#ip nhrp shortcut
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 mode gre multipoint
Spoke2(config-if)#ip nhrp shortcut

Let’s make sure the hub has two NHRP registrations:

Hub#show dmvpn 
Legend: Attrb --> S - Static, D - Dynamic, I - Incomplete
        N - NATed, L - Local, X - No Socket
        T1 - Route Installed, T2 - Nexthop-override
        C - CTS Capable
        # Ent --> Number of NHRP entries with same NBMA peer
        NHS Status: E --> Expecting Replies, R --> Responding, W --> Waiting
        UpDn Time --> Up or Down Time for a Tunnel
==========================================================================

Interface: Tunnel0, IPv4 NHRP Details 
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 00:00:04     D
     1 192.168.123.3      172.16.123.3    UP 00:00:03     D

So far so good, can we ping the spoke routers?

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 = 7/8/10 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 = 7/7/8 ms

Our pings are working, time to configure RIP…

RIP

Unlike DMVPN phase 2, our spoke routers don’t need specific entries in their routing tables. The only thing they’ll need is a summary that covers the networks behind all spoke routers or a default route. I’ll configure the hub router so that it will only advertise a default route to the spoke routers:

Hub(config)#ip prefix-list DEFAULT_ROUTE permit 0.0.0.0/0

Hub(config)#router rip
Hub(config-router)#version 2
Hub(config-router)#no auto-summary 
Hub(config-router)#network 172.16.0.0
Hub(config-router)#default-information originate 
Hub(config-router)#distribute-list prefix DEFAULT_ROUTE out Tunnel 0

We will configure the spoke routers so that they advertise everything to the hub:

Spoke1(config)#router rip
Spoke1(config-router)#version 2
Spoke1(config-router)#no auto-summary 
Spoke1(config-router)#network 172.16.0.0
Spoke1(config-router)#network 2.0.0.0
Spoke2(config)#router rip
Spoke2(config-router)#version 2
Spoke2(config-router)#no auto-summary 
Spoke2(config-router)#network 172.16.0.0
Spoke2(config-router)#network 3.0.0.0

That should do it, let’s check the routing tables:

Hub#show ip route 

      2.0.0.0/32 is subnetted, 1 subnets
R        2.2.2.2 [120/1] via 172.16.123.2, 00:00:16, Tunnel0
      3.0.0.0/32 is subnetted, 1 subnets
R        3.3.3.3 [120/1] via 172.16.123.3, 00:00:24, Tunnel0
Spoke1#show ip route rip 

R*    0.0.0.0/0 [120/1] via 172.16.123.1, 00:00:28, Tunnel0
Spoke2#show ip route rip 

R*    0.0.0.0/0 [120/1] via 172.16.123.1, 00:00:02, Tunnel0

Our hub has learned the two networks on the loopback interfaces of our spoke routers. The spokes only have a default route.

In my example I am able to advertise a default route since the underlay network is directly connected. In a real world scenario, your spoke routers probably already use a default route towards the ISP. In that case, you should advertise a summary instead of a default route.

Before we test spoke-to-spoke connectivity, let me show you the NHRP cache on the two spoke routers:

Spoke1#show dmvpn | begin Peer
Type:Spoke, NHRP Peers:1, 

 # Ent  Peer NBMA Addr Peer Tunnel Add State  UpDn Tm Attrb
 ----- --------------- --------------- ----- -------- -----
     1 192.168.123.1      172.16.123.1    UP 00:22:10     S
Spoke2#show dmvpn | begin Peer
Type:Spoke, NHRP Peers:1, 

 # Ent  Peer NBMA Addr Peer Tunnel Add State  UpDn Tm Attrb
 ----- --------------- --------------- ----- -------- -----
     1 192.168.123.1      172.16.123.1    UP 00:22:32     S

Right now the two spoke routers only have an entry for the hub router, this will change in a minute. Before I test connectivity between spoke1 and spoke2, let’s enable a debug:

Hub, Spoke1 & Spoke2#debug nhrp 
NHRP protocol debugging is on

Now let’s send a ping:

Spoke1#ping 3.3.3.3 source loopback0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 3.3.3.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 7/8/10 ms

When both spoke routers don’t know about each others NBMA addresses then this will produce a lot of debug information. In the debug below I filtered some of the lines so that you only see how spoke1 learns the NBMA address of spoke2:

Hub#
NHRP: NHRP successfully resolved 172.16.123.2 to NBMA 192.168.123.2
NHRP: Attempting to Redirect, remote_nbma: 192.168.123.2
NHRP: inserting (192.168.123.2/3.3.3.3) in redirect table
NHRP: Pre-setting NBMA address in NHRP Traffic Indication to: 192.168.123.2
NHRP: Attempting to send packet via DEST 2.2.2.2
NHRP: Switching NHRP Packet using pre-set NBMA: 192.168.123.2
NHRP: Encapsulation succeeded.  Tunnel IP addr 192.168.123.2
NHRP: Send Traffic Indication via Tunnel0 vrf 0, packet size: 97
      src: 172.16.123.1, dst: 2.2.2.2
NHRP: 121 bytes out Tunnel0 
NHRP: Receive Resolution Request via Tunnel0 vrf 0, packet size: 85
NHRP: netid_in = 1, to_us = 0
NHRP: nhrp_rtlookup yielded interface Tunnel0, prefixlen 32
NHRP: netid_out 1, netid_in 1
NHRP: nhrp_cache_lookup_comp returned 0x0
NHRP: Attempting to forward to destination: 3.3.3.3
NHRP: Forwarding: NHRP SAS picked source: 172.16.123.1 for destination: 3.3.3.3
NHRP: Attempting to send packet via DEST 3.3.3.3
NHRP: NHRP successfully resolved 172.16.123.3 to NBMA 192.168.123.3
NHRP: Encapsulation succeeded.  Tunnel IP addr 192.168.123.3
NHRP: Forwarding Resolution Request via Tunnel0 vrf 0, packet size: 105
      src: 172.16.123.1, dst: 3.3.3.3
NHRP: 129 bytes out Tunnel0

Above you can see that the receives the packet with destination 2.2.2.2 from spoke1. The hub will then send a NHRP traffic indication to the spoke1 router. This will trigger a NHRP resolution request on spoke1 which is received by the hub.

The resolution request is then forwarded by the hub to spoke2.

Here’s what it looks like on spoke1:

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Forum Replies

  1. It would have been nice if you had used loopback address of 1.1.1.1 for spoke 1 and loop address of 2.2.2.2 for spoke 2 and then loop address of 3.3.3.3 for hub in your examples. Would have been a lot easier to understand the debugs and illustrations that way.

  2. Can I ask why did you use default information originate and the distribute list with a default route in? What was the purpose of the DL?

  3. Hello Chris,

    With DMVPN phase 3, the spoke routers don’t need a specific route…only a default route.

    I used the distribute-list to ensure the spoke routers only receive that default route, nothing else.

    Rene

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