What is a Stateful High Availability?
04/07/2020 209 17519
The original version of SonicOS Enhanced provided a basic High Availability feature where a Backup firewall assumes the interface IP addresses of the configured interfaces when the Primary unit fails. Upon failover, layer 2 broadcasts are issued (ARP) to inform the network that the IP addresses are now owned by the Backup unit. All pre-existing network connections must be rebuilt. For example, Telnet and FTP sessions must be re-established and VPN tunnels must be renegotiated.
Stateful High Availability (SHA) provides dramatically improved failover performance. The Primary and Backup appliances are continuously synchronized so that the Backup can seamlessly assume all network responsibilities if the Primary appliance fails, with no interruptions to existing network connections.
Stateful High Availability provides the following benefits:
- Improved reliability - By synchronizing most critical network connection information, Stateful High Availability prevents down time and dropped connections in case of appliance failure.
- Faster failover performance - By maintaining continuous synchronization between the Primary and Backup appliances, Stateful High Availability enables the Backup appliance to take over in case of a failure with virtually no down time or loss of network connections.
- Minimal impact on CPU performance - Typically less than 1% usage.
- Minimal impact on bandwidth - Transmission of synchronization data is throttled so as not interfere with other data.
How Does Stateful High Availability Work?
Stateful High Availability is not load-balancing. It is an active-idle configuration where the Primary appliance handles all traffic. When Stateful High Availability is enabled, the Primary appliance actively communicates with the Backup to update most network connection information. As the Primary appliance creates and updates network connection information (VPN tunnels, active users, connection cache entries, etc.), it immediately informs the Backup appliance. This ensures that the Backup appliance is always ready to transition to the Active state without dropping any connections.
The synchronization traffic is throttled to ensure that it does not interfere with regular network traffic. All configuration changes are performed on the Primary appliance and automatically propagated to the Backup appliance. The High Availability pair uses the same LAN and WAN IP addresses—regardless of which appliance is currently Active.
When using SonicWall Global Management System (GMS) to manage the appliances, GMS logs into the shared WAN IP address. In case of a failover, GMS administration continues seamlessly, and GMS administrators currently logged into the appliance will not be logged out, however Get and Post commands may result in a timeout with no reply returned.
The following table lists the information that is synchronized and information that is not currently synchronized by Stateful High Availability.
Information that is Synchronized
Information that is not Synchronized
VPN information (IPSec, Global VPN Client)
Dynamic WAN clients (L2TP, PPPoE, PPTP and SSL VPN Client)
Basic connection cache
Deep Packet Inspection (GAV, IPS, and Anti Spyware)
IPHelper bindings (such as NetBIOS and DHCP)
SYNFlood protection information
Dynamic ARP entries and ARP cache timeouts
Dynamic Address Objects
Active wireless client information
DHCP server information
wireless client packet statistics
Multicast and IGMP
Rogue AP list
Wireless guest status
RIP and OSPF information
Weighted Load Balancing information
Security Services and Stateful High Availability
High Availability pairs share a single set of security services licenses and a single Stateful HA license. These licenses are synchronized between the Active and Idle appliances in the same way that all other information is synchronized between the two appliances. For information on license synchronization, see “ High Availability License Synchronization Overview ” and “ Applying Licenses to SonicWall Security Appliances ” .
Stateful High Availability Example
The following figure shows a sample Stateful High Availability network. In case of a failover, the following sequence of events occurs:
- A PC user connects to the network, and the Primary SonicWall security appliance creates a session for the user.
- The Primary appliance synchronizes with the Backup appliance. The Backup now has all of the user’s session information.
- The power is unplugged from the Primary appliance and it goes down.
- The Backup unit does not receive heartbeat messages from the Primary appliance and switches from Idle to Active mode.
- The Backup appliance begins to send gratuitous ARP messages to the LAN and WAN switches using the same Virtual MAC address and IP address as the Primary appliance. No routing updates are necessary for downstream or upstream network devices.
- When the PC user attempts to access a Web page, the Backup appliance has all of the user’s session information and is able to continue the user’s session without interruption