Network Switch vendors categorize the operating system software (in switches) into two types – Monolithic Operating Systems and Modular Operating Systems.
According to them, a monolithic switch operating system is built using one large set of code and does not provide isolation between various processes it is running. A process or module could be any individual switch function like 802.1x, SSH, sFlow, etc. So, if there is a problem with one process, then the whole switch is affected and in some cases the switch may have to be rebooted causing interruption to all users connected to it.
Also, in order to apply software updates to the switch operating system, switches need to be temporarily taken down and this causes usage disruption. For this reason, maintenance activities are performed in the evenings/weekends.
It seems, modular switches do not have these limitations. The OS is developed in such a way that each module (or process) is isolated and independent of each other. So, faults in one module are not supposed to affect the working of other modules or the working of the switch. Instead, the faulty individual module can be identified, isolated and restarted without any disruption to other processes running in the switch.
If a new module (like telnet, for example) needs to be added to the existing functions of the switch, it can be done by giving a few commands on the CLI. So, the module can be downloaded and installed anytime without requiring a switch reboot and without disrupting the other processes running in the switch. Similarly, a module can be terminated at any point of time without affecting the other switch activities, using a modular switch operating system.
Even from the manufacturer’s side, since each module is independent of others, they can be developed and maintained by different teams and are mostly independent from the main OS code.
Since modular switch operating systems are more reliable than monolithic operating systems, they are used in mission critical applications like connecting servers in data centers, connecting virtual servers, etc. Some vendors have deployed the same modular operating system across their entire line of switching platform – from edge switches to core switches. This makes upgrading/managing multiple switches easier.
Reference: “Innovation with Extreme XOS”, a white paper from Extreme Networks; “Continuous systems, non-stop operations with Junos operating system”, a white paper from Juniper Networks.
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