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This chapter provided an overview of the VRRP and explained its basic concepts by discussing some selected configurations. We learned how to achieve load sharing or load balancing using VRRP; we also learned the technique for not compromising service quality in case of VRRP switchovers. The multiple backups gave us an opportunity to understand better the mechanism of VRRP. Our discussion also touched on different redundancy setups with different cardinalities. We covered how to configure VRRP routers for creating 1-to-1, 1-to-many, and many-to-1 redundancies.

We used stylized configurations to introduce the basic VRRP notions. Through the discussions of these configurations, we have clarified the meaning of terms such as VRRP router, virtual router, master, backup, associated IP addresses, primary IP address, and owner. They are presented here again in summary:

Default gateway or default router: A router, the IP address of which (one of its interfaces) is entered in the routing table of the selected hosts on a LAN segment. Thus, these hosts forward all packets addressed to destinations outside the local network to this router. We also use the term default first hop in this sense.

VRRP router: a router running VRRP

A virtual router: a group of VRRP routers collaborating with each other to establish a failover mechanism to protect the default gateway function. A set of IP addresses is associated with a virtual router, representing the default router for some hosts on a LAN segment.

Virtual Router ID (VRID): a label (an integer) used to name a virtual router.

Master: a VRRP router in a given virtual router that acts as the default router for the hosts on an associated LAN. Master router has the responsibility of responding to the ARP requests related to the IP addresses of the virtual router. This characteristic also entails the responsibility to forward the packets forwarded to the virtual MAC address of the virtual router.

Backup: VRRP routers in a given virtual router that are configured to take over the master role in case of the failure of the current master. This takeover is regulated by the election mechanism specified by the protocol, VRRP.

Load sharing or load balancing: Technique of distributing the traffic to different network devices so that we don't overburden one or a few of them, so that we take advantage of all resources.

Priority: An integer representing the order in which backup routers may assume mastership responsibility. The value zero indicates that the current master ceased to participitate in VRRP. The values between 1 and 255 are used in ranking the backups, 100 being the default. The value 255 indicates the ownership. See owner.

Primary IP address: An address selected from the IP addresses of a VRRP router. The comparison of IP addresses breaks the tie between backups with the same priority in the election of the master.

Owner: The VRRP router that has the virtual router's IP address(es) as real interfaces address(es). The owner, when functional, responds to the packets addressed to these IP addresses through ICMP pings, TCP connections, and so on.

Preemption mode: A flag regulating whether a new operational, nonowner can displace the current master based on its priority. When the flag is set to false, a nonowner router that becomes master cannot replace the current master even if its priority were to be higher. This replacement is permitted when this flag is set to true.

After considering different failure points in the context of our discussion, we differentiated between the ones covered by VRRP and the ones for which VRRP was of no help. A short excursion on the semantics of "virtuality" to us in the realm of multiple virtual routers gives us another occasion to relate better to the protocol under study.

Finally, equipped with basic concepts and enough familiarity with VRRP's behavior under different configurations, we reviewed some deployment use cases to get closer to the real-world use of VRRP.

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