A Virtualization Technologies Primer: Theory--Part III

Virtual and Logical Routers
A VRF is not the same thing as a completely virtualized device, even if they are sometimes confused as such (it is true that some marketing literature encourages such confusion). They simply allow routers to support multiple address spaces. This is some distance from a fully-virtualized device, where resources can be more or less arbitrarily allocated to tasks.

Virtualized devices do exist, however, and, to cut through the fog of confusion, it is helpful to have a taxonomy of terms to start with:

A logical router (LR) uses hardware partitioning to create multiple routing entities on a single device. An LR can run across different processors on different cards of a router. All the underlying hardware and software resources are dedicated to an LR. This includes network processors, interfaces, and routing and forwarding tables. LRs provide excellent fault isolation, but do require abundant hardware to implement.
• A virtual routers (VR) uses software emulation to create multiple routing entities. The underlying hardware is shared between different router processes (note that we mean an entire instance of something like the nonkernal parts of IOS, not a single router process). In a well-implemented virtual router, users can see and change only the configuration and statistics for "their" router.

Note
The previous definitions and Figure 2 were derived from RST-4314 2004 Networkers "Advances in Router Architecture: The CRS-1 and IOS-XR," by David Tsiang and David Ward.

From the preceding list and Figure 2, which gives a pictorial idea of the difference between VRs and LRs, you can see that only the LR is completely virtualized. Because of the cost involved of having all that extra hardware and device management, LRs tend to be high-end systems. A VR is a software-based virtualization solution, where all the tasks share the same hardware resources.

In both cases, the granularity of what is virtualized can differ. Some implementations allow multiple router processes (for instance, one VR per customer domain), others allow you to allocate resources to tasks (an LR can have Border Gateway Protocol [BGP] running on one hardware subsystem and Intermediate System-to-Intermediate Systems [IS-IS] on another, for example).

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