Virtual Machines
To increase capacity and reduce expensive hardware costs we can run multiple virtual servers/PCs on top of a single device. Virtual machines share network, CPU, storage and RAM of the underlying hardware known as the host. Note that VMs are typically called guests, and the physical hardware they run on is called the host. In order for virtual machines to work, the host needs to be running hypervisor software (otherwise known as a VMM (Virtual Machine Monitor). There are two types of hypervisors, see below:
Type-1 (Bare-metal)
This type of hypervisor sits directly on the host's hardware, allowing virtual machines (VMs) to be run directly on top. Looking at the below image there are 3 virtual machines running on 1 physical host. The host itself does not have an operating system but uses a hypervisor for the VMs to run and for management. A popular hypervisor is ESXi from VMWare. This type of hypervisor is commonly found in datacentres.
Type-2 (Hosted Hypervisor)
A type-2 hypervisor runs on the operating system of the physical host. For example Oracle VirtualBox or VMWare Player. A type-2 hypervisor is commonly found in personal-use situations only.
Containers
Containers are essentially software packages that contain all of the application software code, required libraries and other dependencies for the code to run. Containers are lightweight and are run on a Container Engine such as Docker. Because there can be 100s or 1000s of containers, automation is required to manage the containerised estate. This can be done using a Container Orchestrator.
Containers have a number of advantages over VMs:
-
Containers boot up in milliseconds as opposed to minutes for VMs
-
VMs use more storage due to having a full OS, containers are typically in the Megabytes
-
Containers use less hardware resource as compared to VMs as they aren't each running their own OS
-
VMs are portable between the same hypervisor however containers are even more portable and can run on nearly any container service
One disadvantage to containers over VMs is that should the underlying OS which is hosting the containerised instances crash then all of the apps running within the containers are affected.
VRFs
VRF stand for Virtual Routing and Forwarding, and it is essentially a way of applying virtualisation to a router. With VRFs, routers can have a number of different routing tables for each individual instance. VRFs are most commonly found within Internet Service Providers where a number of customers will connect to a router within the ISP's infrastructure. Using VRFs allows for customer network traffic to be segregated. VRFs also allow the same router to use the same IP address within different VRFs as they are segregated, duplicate IP addressing is not a concern between VRFs.
The CCNA doesn't require a full understanding of how VRFs work, just that you need to be aware of them. Just remember that VRFs allow for multiple virtual routing tables.