Multihoming is the ability of a spoke site to connect to two different hub devices in a hub and spoke topology, thereby providing redundancy. The hub devices function as primary and the secondary hub devices. If there are multiple spokes in the system, the same hub device may act as primary hub device for one spoke and secondary hub device for another spoke. That is, the selection of the primary and the secondary hub devices is only in the context of a spoke site. The spoke is connected to both the hub devices through an underlay network.
The hub devices can be MX series routers with an MS-MIC or SRX4000 series routers. For a specific spoke site, both the hub devices must be either MX series routers or SRX series routers. You cannot have one hub as an MX series router and another hub as an SRX series router. To enable multihoming for a site, you must select the hub and spoke topology when you create the tenant. If you enable multihoming for a site, you must specify a primary and back up site when you configure the site.
Traffic is switched from the primary hub to the secondary hub in the following scenarios:
The primary hub is down
The primary hub is up, but all the overlay tunnels between the spoke and the primary hub are down
The tunnels are up, but the iBGP session between the primary hub and vRR is down. In this case, the failover occurs only after the BGP hold-time expires and the default route is withdrawn.
In addition to hub-level redundancy, you can provide VRR-level redundancy by creating two VRRs—primary and secondary—in two different redundancy groups.