FlexVPN Hub and Spoke

The FlexVPN hub and spoke topology can be useful when you have a central site and multiple remote sites. After configuring the hub and your first spoke router, adding extra spoke routers is easy. This is a scalable solution and because we use IKEv2, everything is encrypted with IPSec. FlexVPN is an improvement over DMVPN and is sometimes (unofficially) referred to as DMVPN phase 4.

FlexVPN uses virtual tunnel interfaces (VTI), an alternative to the older crypto-maps. There are two VTI types:

  • Dynamic VTI (DVTI)
  • Static VTI (VTI)

In the first FlexVPN IKEv2 smart defaults lesson, we used static VTIs. This is a “regular” tunnel interface.

With dynamic VTI, we use a single virtual template on our hub router. Whenever a new IPSec session is needed, the router automatically creates a virtual access interface that is cloned from the virtual template.

With two remote spoke routers, you will have one virtual-template and two virtual-access interfaces.

The virtual template can include most of what you would use on a regular interface. You can add access-lists, policy-maps for QoS, etc. These are all copied to the virtual access interfaces. The DVTI makes it easy to create lots of IPSec sessions with remote peers.

With the hub and spoke topology, we use a dynamic VTI on the hub and static VTIs on the spoke routers.

Configuration

Let’s see how we can configure FlexVPN hub and spoke. This is the topology I am going to use:

Flexvpn Hub Spoke Topology

The three routers are connected on the 192.168.1.0/24 network using their GigabitEthernet 0/0 interfaces.

  • The hub router has a loopback 1 interface with an IP address we will use for the dynamic VTI interface.
  • Each spoke router has a loopback 0 interface which simulates a network behind the spoke router.

I used IOSv Software (VIOS-ADVENTERPRISEK9-M), Version 15.9(3)M2 for this example.

Configurations

Want to take a look for yourself? Here you will find the startup configuration of each device.

Hub1

hostname Hub1
!
ip cef
!
interface Loopback1
 ip address 172.16.1.254 255.255.255.255
!
interface GigabitEthernet0/0
 ip address 192.168.1.254 255.255.255.0
!
end

Spoke1

hostname Spoke1
!
ip cef
!
interface Loopback0
 ip address 1.1.1.1 255.255.255.255
!
interface GigabitEthernet0/0
 ip address 192.168.1.1 255.255.255.0
!
end

Spoke2

hostname Spoke2
!
ip cef
!
interface Loopback0
 ip address 2.2.2.2 255.255.255.255
!
interface GigabitEthernet0/0
 ip address 192.168.1.2 255.255.255.0
!
end

Hub1

We’ll start with the hub router.

IKEv2 Keyring

For this example, we’ll use pre-shared keys (PSK). You could configure a unique PSK for each spoke but to keep it simple, I’ll use a single PSK for all spoke routers. On a real network, PKI authentication would be a better option. Here is the keyring:

Hub1(config)#crypto ikev2 keyring IKEV2_KEYRING
Hub1(config-ikev2-keyring)#peer SPOKE_ROUTERS
Hub1(config-ikev2-keyring-peer)#address 0.0.0.0 0.0.0.0
Hub1(config-ikev2-keyring-peer)#pre-shared key local CISCO
Hub1(config-ikev2-keyring-peer)#pre-shared key remote CISCO

IKEv2 Authorization Policy

I’m going to use IKEv2 routing so that the hub advertises a default route to the spoke routers:

Hub1(config)#aaa new-model

Hub1(config)#aaa authorization network FLEXVPN_LOCAL local

Hub1(config)#crypto ikev2 authorization policy IKEV2_AUTHORIZATION
Hub1(config-ikev2-author-policy)#route set interface
Hub1(config-ikev2-author-policy)#route set access-list FLEXVPN_ROUTES

Hub1(config)#ip access-list standard FLEXVPN_ROUTES
Hub1(config-std-nacl)#permit any

IKEv2 Profile

We need an IKEv2 profile that contains the local and remote identity. We could create an entry for each spoke router but instead, I’ll configure the hub router so that it only checks the domain “FLEXVPN.LAB”. This makes it easier to connect spoke routers in the future.

Hub1(config)#crypto ikev2 profile IKEV2_PROFILE
Hub1(config-ikev2-profile)#match identity remote fqdn domain FLEXVPN.LAB
Hub1(config-ikev2-profile)#identity local fqdn HUB.FLEXVPN.LAB
Hub1(config-ikev2-profile)#authentication remote pre-share 
Hub1(config-ikev2-profile)#authentication local pre-share 
Hub1(config-ikev2-profile)#keyring local IKEV2_KEYRING
Hub1(config-ikev2-profile)#aaa authorization group psk list FLEXVPN_LOCAL IKEV2_AUTHORIZATION
Hub1(config-ikev2-profile)#virtual-template 1

IPSec Profile

We need an IPSec profile where we specify the IKEv2 profile:

Hub1(config)#crypto ipsec profile IPSEC_PROFILE
Hub1(ipsec-profile)#set ikev2-profile IKEV2_PROFILE

Dynamic VTI

Because the hub connects to multiple spoke routers, we need a dynamic VTI. I want to use an IP address in the same subnet as the tunnel interfaces on the spoke routers. However, you can’t configure the IP address directly on the dynamic VTI interface. If you are curious why, what happens is that the router refuses to copy the IP address to virtual-access interfaces it creates. We have to use the ip unnumbered command to “copy” the IP address to the virtual-access interfaces. I assigned the IP address I want to use for the tunnel to the loopback 1 interface. We also attach the IPSec profile to the dynamic VTI interface. Here is the configuration:

Hub1(config)#interface Virtual-Template 1 type tunnel 
Hub1(config-if)#ip unnumbered loopback 1
Hub1(config-if)#tunnel protection ipsec profile IPSEC_PROFILE

This completes the configuration of the hub router.

Spoke1

Now it’s time for our first spoke router.

IKEv2 Keyring

Let’s start with that keyring. This is where I specify the hub and the PSKs:

Spoke1(config)#crypto ikev2 keyring IKEV2_KEYRING
Spoke1(config-ikev2-keyring)#peer HUB1
Spoke1(config-ikev2-keyring-peer)#address 192.168.1.254
Spoke1(config-ikev2-keyring-peer)#pre-shared-key local CISCO
Spoke1(config-ikev2-keyring-peer)#pre-shared-key remote CISCO

IKEv2 Authorization Policy

We need an IKEv2 authorization policy because I want to advertise the network on the loopback0 interface to the hub router. Here’s what it looks like:

Spoke1(config)#aaa new-model
Spoke1(config)#aaa authorization network FLEXVPN_LOCAL local

Spoke1(config)#crypto ikev2 authorization policy IKEV2_AUTHORIZATION
Spoke1(config-ikev2-author-policy)#route set interface
Spoke1(config-ikev2-author-policy)#route set access-list FLEXVPN_ROUTES

Spoke1(config)#ip access-list standard FLEXVPN_ROUTES
Spoke1(config-std-nacl)#permit host 1.1.1.1

IKEv2 Profile

In the IKEv2 profile, we configure the identity of the remote hub router and local identity (SPOKE1.FLEXVPN.LAB). We configured the hub router so it only checks the domain part (FLEXVPN.LAB).

Spoke1(config)#crypto ikev2 profile IKEV2_PROFILE
Spoke1(config-ikev2-profile)#match identity remote fqdn HUB.FLEXVPN.LAB
Spoke1(config-ikev2-profile)#identity local fqdn SPOKE1.FLEXVPN.LAB
Spoke1(config-ikev2-profile)#authentication local pre-share 
Spoke1(config-ikev2-profile)#authentication remote pre-share 
Spoke1(config-ikev2-profile)#keyring local IKEV2_KEYRING

IPSec Profile

Let’s create an IPSec profile and specify the IKEv2 profile we want to use:

Spoke1(config)#crypto ipsec profile IPSEC_PROFILE
Spoke1(ipsec-profile)#set ikev2-profile IKEV2_PROFILE

Static VTI

On the spoke routers, we use a “regular” static VTI (tunnel interface):

Spoke1(config)#interface Tunnel 0
Spoke1(config-if)#ip address 172.16.1.1 255.255.255.0
Spoke1(config-if)#tunnel source GigabitEthernet 0/0
Spoke1(config-if)#tunnel destination 192.168.1.254
Spoke1(config-if)#tunnel protection ipsec profile IPSEC_PROFILE

This completes the configuration of the first spoke router.

Spoke2

We’ll configure the exact same thing (except for the IP addresses and local identity) on the second spoke router:

Spoke2(config-ikev2-author-policy)#crypto ikev2 keyring IKEV2_KEYRING
Spoke2(config-ikev2-keyring)#peer HUB1
Spoke2(config-ikev2-keyring-peer)#address 192.168.1.254
Spoke2(config-ikev2-keyring-peer)#pre-shared-key local CISCO
Spoke2(config-ikev2-keyring-peer)#pre-shared-key remote CISCO

Spoke2(config)#aaa new-model

Spoke2(config)#aaa authorization network FLEXVPN_LOCAL local 

Spoke2(config)#crypto ikev2 authorization policy IKEV2_AUTHORIZATION 
Spoke2(config-ikev2-author-policy)# route set interface
Spoke2(config-ikev2-author-policy)# route set access-list FLEXVPN_ROUTES

Spoke2(config-if)#ip access-list standard FLEXVPN_ROUTES
Spoke2(config-std-nacl)# permit 2.2.2.2

Spoke2(config-ikev2-keyring-peer)#crypto ikev2 profile IKEV2_PROFILE
Spoke2(config-ikev2-profile)#match identity remote fqdn HUB.FLEXVPN.LAB
Spoke2(config-ikev2-profile)#identity local fqdn SPOKE1.FLEXVPN.LAB
Spoke2(config-ikev2-profile)#authentication remote pre-share
Spoke2(config-ikev2-profile)#authentication local pre-share
Spoke2(config-ikev2-profile)#keyring local IKEV2_KEYRING
Spoke2(config-ikev2-profile)#aaa authorization group psk list FLEXVPN_LOCAL default   

Spoke2(config-ikev2-profile)#crypto ipsec profile IPSEC_PROFILE
Spoke2(ipsec-profile)#set ikev2-profile IKEV2_PROFILE

Spoke2(ipsec-profile)#interface Tunnel0
Spoke2(config-if)#ip address 172.16.1.2 255.255.255.0
Spoke2(config-if)#tunnel source GigabitEthernet0/0
Spoke2(config-if)#tunnel destination 192.168.1.254
Spoke2(config-if)#tunnel protection ipsec profile IPSEC_PROFILE

This completes our configuration of the hub and spoke routers.

Verification

Let’s verify our work. First, let’s see if there is an IKEv2 security association (SA):

Hub1#show crypto ikev2 sa
 IPv4 Crypto IKEv2  SA 

Tunnel-id Local                 Remote                fvrf/ivrf            Status 
1         192.168.1.254/500     192.168.1.1/500       none/none            READY  
      Encr: AES-CBC, keysize: 256, PRF: SHA512, Hash: SHA512, DH Grp:5, Auth sign: PSK, Auth verify: PSK
      Life/Active Time: 86400/463 sec

Tunnel-id Local                 Remote                fvrf/ivrf            Status 
2         192.168.1.254/500     192.168.1.2/500       none/none            READY  
      Encr: AES-CBC, keysize: 256, PRF: SHA512, Hash: SHA512, DH Grp:5, Auth sign: PSK, Auth verify: PSK
      Life/Active Time: 86400/461 sec

 IPv6 Crypto IKEv2  SA 

Our hub router has two SAs, one for each spoke router. Let’s check the IPSec SA:

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Forum Replies

  1. Hi Rene and Team,

    How about if I use 1 Router is same ISP, spoke routers will have different IP public, so we need one default route to ISP. So my question is: in the case. can we do Hub advertise 0.0.0.0/0 to spoke routers ? and when I did lab in this case, My lab can not advertise subnet to peer :frowning: , can you show idea to troubleshoot ?
    Thank a lot !

  2. Hello Tung

    You must remember that in such a topology, you have an overlay network and an underlay network. The underlay network in the lesson is the 192.168.1.0/24 network. This represents the Internet or any other network over which you will create your tunnels. This underlay network can be anything, using the same ISPs or different ISPs, just as long as the routers have connectivity between them. The routing between routers here is the responsibility of the ISPs. In fact, a static IP is needed only for the hub. The IP addresses of the spokes can actuall

    ... Continue reading in our forum

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