07-30-2012 04:03 AM
These allow you to treat you physical box as if it were multiple boxes that each have their own routing tables. Interfaces on the device can be assigned to different routing instances and they will then each use the routing tables and dynamic routing parameters of that virtual device.
This is convenient in situations where you have multiple outbound paths or multiple customers that you need to control the interfaction between them. Each routing instance is independent and are treated as separate routers. You then can create either an limited routing interaction between them or none at all.
The terminology is messy. Virtual router was used by ScreenOS for this function. Junos generally calls the same function routing instances. But you see in QFX documentation the use of Virtual Router Routing Instances. But whatever the name used the concept is the same.
The other caveat is that there is a limited number depending on the specific device.
07-30-2012 07:56 PM
Check KB16081 for a nice summary of various routing-instance types, including virtual-router.