2.10 A Few Words About Router Virtuality
So far we have been discussing VRRP without talking about the adjective "virtual" in the name of protocol. Yes, we have defined the specific use of the term "virtual router" as it is intended in VRRP terminology. Now that we have an overall understanding of the protocol, we may say a few words about router virtuality in VRRP and other uses.
Encarta World English Dictionary1 describes the first usage of virtual as "being something in effect even if not in reality." The origin of the term goes back to Latin virtualis of the 14th century, derived from virtus (virtue). From "having the power" comes "so in effect." Encarta also defines virtual to mean "generated by computer." We may say that in computer and networking terminology the two usages merge as "creating something so in effect using computing or networking technology." Nowadays we use "virtual" both as a technical term and an ordinary idiom. Examples abound: virtual memory, virtual community, virtual corporation, virtual reality, java virtual machinejust to mention a few.
But specifically the terms virtual router, virtual address, virtual IP address, virtual MAC address are an integral part of our discussion. Here we'll focus on router virtuality.
Assuming reality is physical reality, "virtual" may be expressed as the opposite of physical. But independent of its metaphysical implications, this expression may be misleading, since the term "physical" immediately creates the association with the first layer of the OSI reference model: physical layer and things related to it. The contrast between physical and virtual router may be just fine; contrasting a virtual address with physical address or even a virtual with a physical interface may be very misleading. For that reason, it is safer to use "real" as a contrasting term in our context: real address, real interface, real router.
Actually, the definition "so in effect" works quite well for our purpose: a virtual router is an entity, that is, a router of a certain kind, in effect without being so in reality.
VRRP virtual routers consist of real routers behaving in such a way that in effect they act as if they were one single router, one single default first hop router on a LAN segment. We may also look at a VRRP virtual router as a form of interface virtualization. Figure 2-12 can be used to clarify what we mean by interface virtualization.
FIGURE 2-12. Single VRRP virtual router consisting of two real routers
In Figure 2-12, we have two real routers, R1 and R2. R1 has three interfaces, RR1.1, RR1.2, and RR1.3, whereas R2 has two interfaces, RR2.1 and R2.2. Here we use the notation RR.X.Y to articulate that the interface we are referring to is a real interface of a real router. R1 and R2 are the members of a VRRP virtual router to protect the interface R1.1. In this sense, we can say that R1.1 is the interface of the virtual router, VR1. The IP address(es) of the R1.1 is/are also the IP address(es) of the VR1. For that reason, we use the term virtual IP address to refer to IP(VR1) or IP(RR1.1), since it is mapped to the interface of a virtual router. In our configuration note that R2 participates in VR1 only on its interface R2.1, not on the other interface R2.2, and the same exclusion applies to R1.2 and R1.3. With all these interface virtualizations or mappings, VRRP multiple routers impersonate one single router.
There is another sense of the term "virtual" in "virtual router," the discussion of which can help us to relate better to the concept of VRRP virtual router. The term we have in mind is "multiple virtual routers" or MVRs. MVR refers to the multiple instances of the routing function within a single device. If the operating system is a multitasking OS, different instances may be implementedfor example, as different tasks. Figure 2-13 represents a "virtual router" in this second sense.
FIGURE 2-13. Multiple virtual routers within one single real router
Figure 2-13 shows one real single router hosting two virtual routers. In this example, we see a mapping of the real interfaces to the virtual interfaces. We allocate real interfacesRR1, RR2, and RR3to the virtual router on the left. We assign RR4 and RR5 to the virtual router on the right. These virtual routers operate as if they were distinct routers. They forward packets among their interfaces and collaborate with each other as if they were different devices. It's likely that they maintain different route tables. They may even be running different routing protocols. Another variation on this term is related to routing domains. In some devices handling different routing domains, the term "virtual router" is used as if a distinct router were dedicated to each routing domain.
Since routing touches both forwarding and control aspects of networking, we call attention to the fact that "virtual routing" in the "multiple virtual router" sense may also be touching both aspects. It is conceivable that MVRs constitute different control planes but share a common, albeit distributed, forwarding plane. It is not out of the question for MVRs also to have their dedicated forwarding engines.
To summarize: MVRs are different instances of the routing function within one single network element. Their role is to impersonate the control and in some cases the forwarding aspects of a distinct router. In the case of VRRP virtual router, we deal with multiple real routers collectively impersonating one single router, which they do in terms of acting as the default first hop router in a LAN segment.