What is High Availability?
In any mission critical application, where a business or organisation absolutely relies on its IT facilities and where failure would cause severe disruption, ensuring that availability becomes paramount. "Redundancy" ensures that any facility or device has backup in case of primary failure. A UPS, which switches to battery power if the mains supply fails, is an example of redundancy, as is server or disk RAID mirroring. It means that if one facility fails, something else can take over automatically.
These principles apply to your WAN (Internet) connection too. If staying online is vital to your organisation, you can provide redundancy to your WAN connectivity. The most common method is to have two or more WAN (Internet) connections so that traffic can use the secondary connection if the primary fails. This is called WAN failover (or 'backup'). Most DrayTek routers have multiple WAN interfaces to support this.
Going one step further, allowing for hardware failure, High Availability is a facility whereby two or more routers can be set up in a master/slave configuration. The primary (master) unit normally provides the Internet connectivity but, in the case of hardware or WAN connectivity failure, the secondary (backup) router automatically kicks-in and takes over Internet connectivity. Depending on the configuration, the master/slave routers could access the same Internet connections, or each have their own independent connection (see hot vs. active modes later).
When the failed connection or hardware is either fixed or replaced, the primary (master) router will resume service without any reconfiguration of the network required. In mission-critical environments, this removes the router or line as a single point of failure.
A pair of Vigor 2860 routers in master/slave configuration.
High Availability on the LAN
Your LAN has to be able to be connected to both routers. This is achieved by using a "Virtual IP" on the local network, which would be configured as the gateway on client PCs. This Virtual IP address is not linked to a physical device but is instead managed by routers configured for high availability so that usage of this IP address is not tied to a single physical device.
During normal operation the primary router takes ownership of this virtual IP. Clients on the network would use the virtual IP as an internet / VPN gateway and these connections would be handled by the primary router. Responses would return from the virtual IP address, making use of the virtual IP address transparent to devices on the network.
In the event that the secondary router has to take over, the secondary router would take ownership of this virtual IP. Clients on the network would send requests to the virtual IP as they would normally and once the failover to the secondary router has completed, traffic would go to and from the virtual IP without the client needing any reconfiguration.
The virtual IP can be set up for each subnet if making use of multiple subnets and VLANs on the DrayTek routers. It is also possible to have multiple high availability groups on the same physical network through the use of different High Availability Group ID numbers and Authentication Keys.
High Availability on the WAN
The High Availability on DrayTek routers is aware of the connectivity on each of its WAN interfaces. When using multiple WAN interfaces with high availability, the primary router will check whether its WAN interfaces have working access to the internet; if all of the WAN interfaces are unable to access the internet, the secondary router will take over in providing connectivity.
There are two types of high availability in regards to WAN connectivity that can be used in different scenarios:
This type of high availability has both routers connected to the same internet connection(s). The primary router would use the internet connections as normal while the secondary router would keep the internet connections in standby or offline unless a failover occurs.
If the primary router does go offline, the secondary router would take over the internet connection(s) and bring them online, resuming normal connectivity.
This is achieved by using a switch to share the same physical interface between the primary and secondary routers and is well suited to Ethernet WAN interfaces.
When managing multiple routers being used for high availability and using the Hot Standby redundancy method, DrayTek routers can automatically synchronise the configuration of the primary router with the secondary router(s), which simplifies the management of multiple routers.
2. Active Standby
This type of high availability uses different WAN interfaces on each router, which is used where the primary and secondary routers would have different physical connections and cannot share the physical interface, such as the VDSL2 interface of a Vigor 2860 router.
These connections could be made to different ISPs to reduce the risk of internet connectivity loss being caused by a single point of failure, in this case the ISP, while still having redundant hardware.
It is not possible for a pair of DSL routers to connect to the same DSL line directly because if one modem has 'sync' another modem cannot connect, or if they both attempt sync. at the same time, they will clash.