Cisco – Configuring Dynamic Multipoint Virtual Private Networks DMVPN

KB ID 0000954 Dtd 09/06/14

Problem

A while back I uploaded a run through on how to deploy GRE tunnels and protect those tunnels with IPsec. That point-to-point GRE tunnel is a good solution, but if you have a lot of sites it's not a solution that scales very well. Yes you can have 2147483647 tunnel interfaces, but good luck manually configuring all those tunnels and even if you did, if you want each of your remote sites to talk to each other you have all those tunnels to configure as well.

To address that we have DMVPN, rather than a point-to-point GRE tunnel it uses multipoint GRE that can have more than two endpoints. In fact it acts like a 'network overlay' that joins all the sites together. It is typically deployed in a hub-and-spoke configuration where one router (the hub) sits at the center and each remote site (spoke) joins the multipoint GRE, tunnels back to the hub. The magic part is, if one spoke needs to speak to another spoke, this is done by building a dynamic tunnel between them 'on the fly'.

Below is the the network topology I'm going to use;

DMVPN Topology

Note: If you use GNS3 heres the topology and configs for all the routers.

I have a main site that will be the 'Hub Site' and at that site MainSiteRTR will be the NHS router. The remaining three hub sites will act as 'spokes', so my 'network overlay' will be 192.168.1.0/24.

DMVPN Configuration

Solution

Configure DMVPN Hub (NHS) Router

Before I start, all the sites are pre-configured as per figure1 above, all the routers can see each other via EIGRP.

1. To begin the setup looks very much like a standard GRE tunnel, but we define a source but no destination (we don't need to, because we specify tunnel mode gre multipoint). There are two commands for the NHRP setup, 'ip nhrp map multicast dynamic' lets EIGRP information propagate to the 'spokes'. And 'ip nhrp network-id 1' creates a group ID for the DMVPN group. All the spokes will need to share this ID to form tunnels with this NHS Router.<.p>

MainSiteRTR#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
MainSiteRTR(config)#interface Tunnel0
MainSiteRTR(config-if)# ip address 192.168.0.1 255.255.255.0
MainSiteRTR(config-if)# ip nhrp map multicast dynamic
MainSiteRTR(config-if)# ip nhrp network-id 1
MainSiteRTR(config-if)# tunnel source 1.1.1.1
MainSiteRTR(config-if)# tunnel mode gre multipoint
*Mar 1 00:02:31.651: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel0, changed state to down
MainSiteRTR(config-if)#exit
*Mar 1 00:02:41.635: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel0, changed state to up
MainSiteRTR(config)#

Configure the Branch 'Spoke' Routers

2. There are a few differences here, first 'ip nhrp map 192.168.0.1 1.1.1.1' tells the branch router to map the 192.168.0.2 private IP permanently to the 1.1.1.1 public address. 'ip nhrp map multicast 1.1.1.1' tells the router to relay ALL its multicast traffic back to the hub router. 'ip nhrp nhs 192.168.0.1' tells the spoke router where the hub router is.

Branch1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Branch1(config)#interface Tunnel0
Branch1(config-if)# ip address 192.168.0.2 255.255.255.0
Branch1(config-if)# ip nhrp map 192.168.0.1 1.1.1.1
Branch1(config-if)# ip nhrp map multicast 1.1.1.1
Branch1(config-if)# ip nhrp network-id 1
Branch1(config-if)# ip nhrp nhs 192.168.0.1
Branch1(config-if)# tunnel source 2.2.2.1
Branch1(config-if)# tunnel mode gre multipoint
*Mar 1 00:07:28.403: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel0, changed state to down
Branch1(config-if)#exit
*Mar 1 00:07:38.387: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel0, changed state to up
Branch1(config)#

3. Then configure each branch router the same, (apart from the tunnel source and the endpoint IP).

Branch 2 Router Config

Branch2#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Branch2(config)#interface Tunnel0
Branch2(config-if)# ip address 192.168.0.3 255.255.255.0
Branch2(config-if)# ip nhrp map 192.168.0.1 1.1.1.1
Branch2(config-if)# ip nhrp map multicast 1.1.1.1
Branch2(config-if)# ip nhrp network-id 1
Branch2(config-if)# ip nhrp nhs 192.168.0.1
Branch2(config-if)# tunnel source 3.3.3.1
Branch2(config-if)# tunnel mode gre multipoint
Branch2(config-if)# exit
*Mar 1 00:09:32.631: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel0, changed state to up
Branch2(config)#

Branch 3 Router Config

Branch3#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Branch3(config)#interface Tunnel0
Branch3(config-if)# ip address 192.168.0.4 255.255.255.0
Branch3(config-if)# ip nhrp map 192.168.0.1 1.1.1.1
Branch3(config-if)# ip nhrp map multicast 1.1.1.1
Branch3(config-if)# ip nhrp network-id 1
Branch3(config-if)# ip nhrp nhs 192.168.0.1
Branch3(config-if)# tunnel source 4.4.4.1
Branch3(config-if)# tunnel mode gre multipoint
Branch3(config-if)# exit
*Mar 1 00:11:05.259: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel0, changed state to down
Branch3(config)#
*Mar 1 00:11:15.247: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel0, changed state to up
Branch3(config)#

Verifying and Testing the DMVPN Setup

4. On the main site;

MainSiteRTR#show dmvpn
Legend: Attrb --> S - Static, D - Dynamic, I - Incompletea
        N - NATed, L - Local, X - No Socket
        # Ent --> Number of NHRP entries with same NBMA peer

Tunnel0, Type:Hub, NHRP Peers:3,
 # Ent  Peer NBMA Addr Peer Tunnel Add State  UpDn Tm Attrb
 ----- --------------- --------------- ----- -------- -----
     1         2.2.2.1     192.168.0.2    UP    never D
     1         3.3.3.1     192.168.0.3    UP    never D
     1         4.4.4.1     192.168.0.4    UP    never D

5. On a branch site (Note: There is only one tunnel to the Main Site);

Branch1#show dmvpn
Legend: Attrb --> S - Static, D - Dynamic, I - Incompletea
        N - NATed, L - Local, X - No Socket
        # Ent --> Number of NHRP entries with same NBMA peer

Tunnel0, Type:Spoke, NHRP Peers:1,
 # Ent  Peer NBMA Addr Peer Tunnel Add State  UpDn Tm Attrb
 ----- --------------- --------------- ----- -------- -----
     1         1.1.1.1     192.168.0.1    UP 00:08:24 S

6. But if from the same branch site you ping another branch site, it will dynamically build a tunnel to that site also. (Note: The S denotes static, and the D denotes dynamic).

Branch1#ping 192.168.0.4
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.0.4, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 32/60/108 ms
Branch1#show dmvpn
Legend: Attrb --> S - Static, D - Dynamic, I - Incompletea
        N - NATed, L - Local, X - No Socket
        # Ent --> Number of NHRP entries with same NBMA peer

Tunnel0, Type:Spoke, NHRP Peers:2,
 # Ent  Peer NBMA Addr Peer Tunnel Add State  UpDn Tm Attrb
 ----- --------------- --------------- ----- -------- -----
     1         1.1.1.1     192.168.0.1    UP 00:08:52 S
     1         4.4.4.1     192.168.0.4    UP    never D

Protect and Encrypt the Tunnel(s) with IPSEC

7. BE AWARE the traffic traveling over these tunnels is still being sent in cleartext, it's simply been 'encapsulated' with GRE. To rectify that we can protect the tunnels with IPSEC.

MainSiteRTR#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
MainSiteRTR(config)#crypto isakmp policy 10
MainSiteRTR(config-isakmp)# authentication pre-share
MainSiteRTR(config-isakmp)# encryption aes
MainSiteRTR(config-isakmp)# group 2
MainSiteRTR(config-isakmp)# hash sha
MainSiteRTR(config-isakmp)# crypto isakmp key Sh@reds3cret address 2.2.2.1
MainSiteRTR(config)# crypto isakmp key Sh@reds3cret address 3.3.3.1
MainSiteRTR(config)# crypto isakmp key Sh@reds3cret address 4.4.4.1
MainSiteRTR(config)# crypto ipsec transform-set TFS-PNL esp-aes esp-sha-hmac
MainSiteRTR(cfg-crypto-trans)# crypto ipsec profile PF-PNL
MainSiteRTR(ipsec-profile)# set transform-set TFS-PNL
MainSiteRTR(ipsec-profile)# interface Tunnel0
MainSiteRTR(config-if)# tunnel protection ipsec profile PF-PNL
*Mar 1 00:25:34.055: %CRYPTO-6-ISAKMP_ON_OFF: ISAKMP is ON
MainSiteRTR(config-if)# exit
MainSiteRTR(config)#

8. Then simply repeat on the branch routers, the only difference is the peer addresses.

Branch 1 Router Config

Branch1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Branch1(config)#crypto isakmp policy 10
Branch1(config-isakmp)# authentication pre-share
Branch1(config-isakmp)# encryption aes
Branch1(config-isakmp)# group 2
Branch1(config-isakmp)# hash sha
Branch1(config-isakmp)#crypto isakmp key Sh@reds3cret address 1.1.1.1
Branch1(config)#crypto isakmp key Sh@reds3cret address 3.3.3.1
Branch1(config)#crypto isakmp key Sh@reds3cret address 4.4.4.1
Branch1(config)#crypto ipsec transform-set TFS-PNL esp-aes esp-sha-hmac
Branch1(cfg-crypto-trans)#crypto ipsec profile PF-PNL
Branch1(ipsec-profile)# set transform-set TFS-PNL
Branch1(ipsec-profile)#interface Tunnel0
Branch1(config-if)# tunnel protection ipsec profile PF-PNL
Branch1(config-if)#exit
Branch1(config)#
*Mar 1 00:36:47.179: %CRYPTO-6-ISAKMP_ON_OFF: ISAKMP is ON
Branch1(config)#

Branch 2 Router Config

Branch2#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Branch2(config)#crypto isakmp policy 10
Branch2(config-isakmp)# authentication pre-share
Branch2(config-isakmp)# encryption aes
Branch2(config-isakmp)# group 2
Branch2(config-isakmp)# hash sha
Branch2(config-isakmp)#crypto isakmp key Sh@reds3cret address 1.1.1.1
Branch2(config)#crypto isakmp key Sh@reds3cret address 2.2.2.1
Branch2(config)#crypto isakmp key Sh@reds3cret address 4.4.4.1
Branch2(config)#crypto ipsec transform-set TFS-PNL esp-aes esp-sha-hmac
Branch2(cfg-crypto-trans)#crypto ipsec profile PF-PNL
Branch2(ipsec-profile)# set transform-set TFS-PNL
Branch2(ipsec-profile)#interface Tunnel0
Branch2(config-if)# tunnel protection ipsec profile PF-PNL
Branch2(config-if)#exit
Branch2(config)#
*Mar 1 00:37:57.239: %CRYPTO-6-ISAKMP_ON_OFF: ISAKMP is ON
Branch2(config)#

Branch 3 Router Config

Branch3#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Branch3(config)#crypto isakmp policy 10
Branch3(config-isakmp)# authentication pre-share
Branch3(config-isakmp)# encryption aes
Branch3(config-isakmp)# group 2
Branch3(config-isakmp)# hash sha
Branch3(config-isakmp)#crypto isakmp key Sh@reds3cret address 1.1.1.1
Branch3(config)#crypto isakmp key Sh@reds3cret address 2.2.2.1
Branch3(config)#crypto isakmp key Sh@reds3cret address 3.3.3.1
Branch3(config)#crypto ipsec transform-set TFS-PNL esp-aes esp-sha-hmac
Branch3(cfg-crypto-trans)#crypto ipsec profile PF-PNL
Branch3(ipsec-profile)# set transform-set TFS-PNL
Branch3(ipsec-profile)#interface Tunnel0
Branch3(config-if)# tunnel protection ipsec profile PF-PNL
*Mar 1 00:39:10.515: %CRYPTO-6-ISAKMP_ON_OFF: ISAKMP is ON
Branch3(config-if)#exit

9. To test re-establish the tunnels, and this time you can see they are protected;

MainSiteRTR#show crypto isa sa
IPv4 Crypto ISAKMP SA
dst             src             state          conn-id slot status
1.1.1.1         3.3.3.1         QM_IDLE           1002    0 ACTIVE
1.1.1.1         4.4.4.1         QM_IDLE           1003    0 ACTIVE
1.1.1.1         2.2.2.1         QM_IDLE           1001    0 ACTIVE
Download the DMVPN labs and have a play

Related Articles, References, Credits, or External Links

Cisco Simple GRE Tunnels (With IPSEC)

Author: Migrated

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