Reliability Design

Two or more fixed switches can be virtualized into one logical switch using stacking technology, whereas two modular switches can be virtualized into one logical switch using clustering technology. When the master member switch in a stack or Cluster Switch System (CSS) fails, the backup member switch takes over all the services without interrupting services. On a large or midsize campus network, deploying switches using the stacking or clustering technology is recommended for higher reliability. The stacking or clustering technology has the following advantages:

• Improving reliability

  Member switches in a stack or CSS work in redundancy mode. In Figure 2-61, two modular core switches Switch A and Switch B set up a CSS and back up each other. In the event of a failure on Switch A, Switch B takes over services from Switch A to ensure service continuity.

  Figure 2-61 Improving reliability
  
• Increasing the number of ports

  As demonstrated in Figure 2-62, if the port density of the original switch cannot meet the access requirements of users, you can set up a stack by attaching new switches to the original switch to increase the number of ports.

  Figure 2-62 Increasing the number of ports
  
• Simplifying the network topology

  In Figure 2-63, two switches at each network layer set up a stack or CSS, which is similar to a single logical device. Eth-Trunks are used between stacks and between a stack and a CSS. Such deployment approach increases link bandwidth and reliability, and avoids using loop prevention protocols such as MSTP. On a Layer 3 network, members in a stack or CSS share one routing table. This shortens the route convergence time upon a network fault and makes it easy to manage, maintain, and expand a network.

  Figure 2-63 Simplifying the network topology
  

If switches with the native WAC function serve as WACs, the clustering or stacking technology is used to ensure the WAC reliability. If standalone WACs are used, you are advised to deploy them in HSB mode to improve the WAC reliability. In HSB mode, there are two devices, one acting as the active and the other the standby. The active device forwards services and the standby device monitors the forwarding. In addition, the active device sends the standby device the status information and information that needs to be backed up in real time. In the case that the active device becomes faulty, the standby device takes over services. As shown in Figure 2-64, two standalone WACs working in HSB mode are connected to the core switches that set up a CSS in off-path mode. The Eth-Trunks between the WACs and core switches work in active/standby mode. When the active WAC fails, the standby WAC takes over services to forward WLAN packets.

Figure 2-64 Deploying WACs in HSB mode

When firewalls function as egress devices, you are advised to deploy HSB to improve firewall reliability. As illustrated in Figure 2-65, the firewalls act as egress devices of the campus network and are directly connected to the stacked core switch. The two firewalls are configured to work in HSB mode, and the member links in their interconnected Eth-Trunk are in active/standby mode. When the active firewall is faulty, the standby firewall takes over services and forwards service packets.

Figure 2-65 Deploying firewalls in HSB mode