Cisco Router and Google Cloud VPN

Posted by John Marrett on Nov 8, 2017 9:29:02 AM

Cisco Router and Google Cloud VPN.png

In a recent consulting engagement, our Professional Services team needed to help build a VPN connection between a series of Cisco Routers and a Google Cloud environment. We were surprised to discover that at the time we completed the project, Google did not provide a configuration guide for this common configuration type and wanted to share our experience building the connection. They have since added documentation for IKEv2 based ASR configurations. Our post highlights some important architectural details as well as the configuration requirements (tunnel mode, phase 2 timers) for the tunnels. It also makes use of an inner VRF, unlike Google's example.

Before we get into the details of the configuration, it's important to understand the architectural decisions we made. If you wish to make a connection from a single site to two different GCP VPN endpoints you cannot use a traditional static IPSec connection, you must use a routed VPN configuration. The GCP environment will load balance traffic between availability zones across these different paths based on the destination IP address and ignoring different source IP ranges. This means that for redundant connectivity between GCP and a site you must use a routed VPN configuration. 

The following Google documentation states very opaquely "When egressing from on-premises the routing to Cloud VPN and the choice of child SA within Cloud VPN are made based on destination IP only." What this means is that, if you create a static VPN configuration for the same subnet in two different locations you will see outgoing traffic load balanced between them and portions of it dropped if it doesn't match the VPN traffic selector. As a result, the only really viable option for redundant connectivity to a remote site is to configure a routed VPN.

Now onto some interesting elements of the VPN configuration.
Here are some base device configuration, a hostname, and a VRF:

hostname: rooooter
ip vrf gcp-test
rd 65001:1

We create a keyring for the peer and define our PSK:

crypto keyring GCP-test-keyring
  pre-shared-key address key 8OrVZR6YcVog0XEvXoHfGTz7

Define an ISAKMP policy that matches Google's requirements, note that the phase 2 uses tunnel and not transport mode. This is critical for the tunnel to be properly established:

crypto isakmp policy 10
 encr aes
 authentication pre-share
 group 2
crypto ipsec transform-set hs1-transport esp-aes esp-sha-hmac
 mode tunnel

Create an ISAKMP and IPSec profile for the tunnel:

crypto isakmp profile gcp-test-isa
   vrf gcp-test
   keyring gcp-test-keyring
   match identity address
crypto ipsec profile gcp-test-profile
 set security-association lifetime kilobytes disable
 set security-association lifetime seconds 10800
 set transform-set hs1-transport
 set pfs group2
 set isakmp-profile gcp-test-isa

Need help with Networking

Next, we need to define the tunnel that will face the Google Cloud environment. The tunnel mode ipsec ipv4 command is critical. Otherwise, your VPN will build but no traffic will be carried through it:

interface Tunnel2
 ip vrf forwarding gcp-test
 ip address
 ip mtu 1400
 tunnel source GigabitEthernet1
 tunnel mode ipsec ipv4
 tunnel destination
 tunnel protection ipsec profile gcp-test-profile

Finally, we define our BGP router configuration:
router bgp 65001
 bgp log-neighbor-changes

We have created a loopback interface for a test network to announce:

interface Loopback0
 ip vrf forwarding gcp-test
 ip address
 address-family ipv4 vrf gcp-test
  redistribute connected
  neighbor remote-as 65003
  neighbor activate

With this configuration in place, you should build the VPN connection to Google Cloud, see your BGP session established and begin bidirectional announcement of routes.

The Stack8 Professional Services team are experts who can assist you with the issues your organization faces daily. For more information, please visit. If you require a managed solution and not just assistance setting up a new connection, our Managed Services team can help build you a custom solution to address your communications needs.

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Topics: Cisco Router, Google Cloud, VPN, GCP

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