I am trying to wrap my head around this (very) simple issue of IP failover from one physical location to another. I have a few Virtual machines running at Site A where I need to rebuild the NAS. In order to do so I want to vMotion/move these VM(s) to site B but want to retain the IP addresses of the VMs/Network configuration. SiteA and SiteB's routers have a GRE tunnel between them and I defined static route for the next-hop of the IP being migrated.

In order for this to work, I have created a virtual interface on Site B's routerB and use the same gateway IP address which the VM is expecting. It works fine but when I try to access any machine from the same subnet from migrated VM, I can't do so because my requested subnet is physically not present on router B. It falls under /24 broadcast domain of the subnet on router B.

NSX solved this problem with VxLAN but What is the alternative to achieve IP failover across multiple datacenters? I run BGP on both locations and running over Brocade NetIron routers. I was thinking to create a MPLS cloud over my exiting IP network and have a virtual router interface defined accessible from both routers A and B? Or some sort of extended VLAN?

Or I would say I have 2 sites, 2 routers, I need to have a extended LAN between both sites so I can use 1 IP address from any of the locations at the same time.



1 Answer 1


One way of doing this kind of thing with pure routing is to have the "service" IP addresses be secondaries on all the servers, and have routes as appropriate.

Consider this straightforward network where the clients AC and BC go through a router to get to the servers A1, A2, B1, B2. Everything has default gateway aimed at their network's .1. RA and RB also have the obvious inter-site routes: RA sends and to RB; and RB sends and to RA.

       servers                 servers
       A1  A2                  B1  B2
       |.4 |.5                 |.4 |.5
===+===+===+===+===     ===+===+===+===+====    |.1         |.1
               RA---------RB    |.1         |.1
===+===========+===     ===+===========+====
   |.64                                |.64
   AC                                  BC
  clients                             clients

If the servers are also given addresses in as aliases on loopback interfaces (such as lo:1) A1=, B1 also, A2=, B2 also

Then you control which server gets the work by the routing on RA and RB.

If the whole network moves from site A to B, you do routes on; if it's one host at a time you do host routes eg

This structure is useful in two cases:

  • Where you want to use a local server in a content distribution method (like Google's DNS on you can have different routes on RA and RB. (> on RA,> on RB).
  • Where you want a given server in only one place at a time, you have> on RA,>RA on RB.

The scheme lends itself naturally to some virtualisation schemes, where the service networks are inside the virtual host.

Does that help at all?

Other methods include

  • MPLS, as you've said
  • VPN tunnels

EDIT: there's certainly some horrible way to do this with NAT, and there's probably a horrible way to do it with proxy ARP. The pure routing method has the advantage that you can manage the routes by whatever method you have available, such as the BGP you mentioned, but any routing update method will work. Additionally, the routing method is (I think) the only one which supports "send to my most local" CDP-type functionality, if that's something which might be helpful.

  • As I understand, If we alias different subnet to the server, the new subnet will work fine on failover but the old will stop working. This is not desired unless I am understanding it wrongly.
    – NBhatti
    Commented Nov 3, 2018 at 19:53
  • The mechanism is that the aliased addresses (eg in my example) are the addresses that the clients know. The "real" addresses (eg in my example) are just for transport. The servers always have two addresses, and the clients always refer to them by the "service" address ( in my example). Does that explain it better?
    – jonathanjo
    Commented Nov 3, 2018 at 20:05
  • I understand now. But this would require some network configuration changes in the virtual machine. Not difficult but doable. Servers will have 2 default gateways with equal distance. Only one will be reachable while the "service" IP will be routed to the active/reachable IP address. Thanks for your quick answer, let me also look into MPLS if that's an easier option which may not require much reconfiguration on the VMs.
    – NBhatti
    Commented Nov 3, 2018 at 20:23
  • Not sure what you mean by servers having "two default gateways with equal distance". In the test I just did, server A1 has eth0 as, lo:1 as, and default route to RA has route>; RB has>RA. All hosts except B1 (if it's up) will send to A1.
    – jonathanjo
    Commented Nov 4, 2018 at 1:55

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