How to ensure the high reliability of network data? Let's take a look at the dual-machine hot standby solution!

1. Introduction to dual-machine hot standby

definition

Hot-Standby Backup means that when two devices are identified as the active (master) device and the standby (Backup) device, the active device forwards services, and the standby device is in the monitoring state, and the active device regularly sends status information and information to the standby device to be backed up, and when the active device fails, the standby device takes over the service operation of the active device in a timely manner.

objective

As users have higher and higher requirements for network reliability, how to ensure uninterrupted network transmission has become a problem that must be solved. In particular, it is necessary to ensure the uninterrupted operation of the network at the entrance or access point of some important services, such as the Internet access point of the enterprise and the database server of the bank. If only one device is used at these service points, no matter how reliable it is, the network will inevitably bear the risk of service interruption due to a single point of failure.

In order to solve the above problems, a dual-machine hot standby is introduced. Dual-node hot standby implements the backup function of dual-node services, and implements batch backup and real-time backup of service information through the backup link, ensuring that services can be switched to the backup device smoothly without interruption when the primary device fails, thereby reducing the risk of single point of failure and improving network reliability.

2. Dual-machine hot standby mode

The dual-node hot standby solution has two basic modes based on the networking situation: primary/standby mode and load balancing mode.

Active/standby mode

As shown in the show, the active device RouterA processes all services and transmits the generated session information to the backup device RouterB through the active and standby channels for backup. RouterB does not handle business and is only used for backup.

Figure 1 Networking diagram of the active/standby hot standby mode (normal operation)

When the primary RouterA fails, RouterB takes over from the active RouterA, as shown in the following figure. Since the session information has been backed up on the standby device, it can ensure that the newly initiated session can be established normally, and the current ongoing session will not be interrupted, improving the reliability of the network.

Figure 2 Networking diagram of active/standby hot standby mode (faulty)

After the original active device recovers, you can configure whether to switch back service traffic to the original active device as needed.

Load balancing method

As shown in the show, in a wireless service network, AC1 is the primary device and AC2 is the backup device for service traffic on AP1. AC1 handles all services and transmits the generated session information to the backup device AC2 through the active/standby channel for backup. AC2 does not handle services and is only used for backup. For service traffic on AP2, AC2 is the primary device and AC1 is the backup device. AC2 processes all services and transmits the generated session information to the backup device AC1 through the active/standby channel for backup. AC1 does not handle services and only uses it for backup.

In this way, AP1's service traffic is forwarded through AC1, and AP2's service traffic is forwarded through AC2, implementing traffic load balancing.

Figure 3 Networking diagram of hot standby load balancing (normal operation)

If AC1 fails, as shown in Figure 4, service traffic on AP1 is automatically forwarded to the backup device AC2 to ensure network reliability. However, for service traffic on AP2, the active device AC2 works normally and the traffic forwarding path remains unchanged.

Figure 4 Networking diagram of the hot-standby load balancing mode (faulty)

After the original active device recovers, you can configure whether to switch back service traffic to the original active device as needed.

3. Dual-machine hot standby implementation mechanism

The implementation of dual-node hot standby is mainly divided into two parts:

1. Data synchronization under normal circumstances, which ensures that the information of the active and standby devices is consistent;
2. Traffic switchover during fault and fault recovery ensures that services can run without interruption after a fault.

Data synchronization

If the active device fails and traffic switches to the backup device, the session entries between the active and backup devices must be the same, otherwise the session may be interrupted. Therefore, a mechanism is needed to synchronize and save relevant information to the backup device when a session is established or an entry changes on the active device. The active/standby service processing module of the HSB provides data backup by establishing an active/standby channel between two devices that back up each other, maintaining the link status of the active/standby channel, and providing packet sending and receiving services.

There are two ways to synchronize data: batch backup and real-time backup.

• Batch backups
• After the active device has been working for a period of time, there may already be a large number of session entries, at this time, add the backup device, and configure the dual-node hot backup function on the two devices, the active device that runs first will synchronize the existing session entries to the newly added backup device at one time, this process is called batch backup.
• Real-time backups
• During the operation of the active device, new session entries may be generated. To ensure that the entries on the active and standby devices are completely consistent, the active device backs up new entries or changes to the backup device in a timely manner, a process called real-time backup.

Traffic switching

Dual-node hot standby can be switched over through VRRP or dual-link. VRRP is only applicable to active/standby backup mode. Dual-link can be used for active/standby backup and load balancing.

• Use VRRP to switch traffic
• The active/standby service backup group of the HSB is responsible for the linkage with VRRP, binds the HSB backup group to VRRP, negotiates the active and standby status of the service based on the status of VRRP, and the active/standby status of the HSB active and standby backup group is the same as that of VRRP, monitors the changes in the status of the bound active and standby channels and VRRP, and notifies each service module to switch traffic.
• As shown in the figure, the VRRP function is configured on RouterA and RouterB, where RouterA is configured as the master device of the VRRP backup group and RouterB is configured as the backup device of the VRRP backup group. Based on the active/standby status of VRRP, the HSB active and standby service backs up information about RouterA as the active device and RouterB as the backup device for the hot standby (that is, the selection of the active and standby devices for the dual-node hot standby is the same as that of the active and standby devices in the VRRP group), and the HSB active/standby service backs up the information on the active and standby devices on the active and standby devices of the VRRP group.
• Figure 5 Networking diagram of using VRRP to implement traffic switchover between two nodes (before switchover)

• If RouterA fails, the VRRP backup group selects RouterB as the new master device of the VRRP backup group based on the VRRP priority, forwards service traffic, and switches traffic.

Figure 6 Networking diagram of using VRRP to switch traffic between two hot standers (after switchover)

• Traffic switching is achieved through dual links
• As shown in the figure, a dual link is established between the AC and the AP. For the service traffic on the AP, the selection of the primary link and the standby link is as shown in the following figure: AC1 acts as the primary device and the service traffic is forwarded through AC1. AC1 is the primary device and AC2 is the backup device based on the active/standby status of the dual-node hot standby device (that is, the selection of the active/standby hotstandby device is the same as that of the active/standby device of the dual-link node). The active/standby HSB service backs up information from AC1 to AC2.
• Figure 7 Traffic switchover through dual-link hot standby (before switchover)

• If AC1 fails, the AP senses that AC1 is faulty and automatically switches the original standby link to the primary link, and the service traffic on the AP is forwarded through AC2. Figure 8 Switching traffic over a dual-link hot standby (after switchover)