Virtual Router Redundancy Protocol (VRRP)
from Wikipedia:
The Virtual Router Redundancy Protocol (VRRP) is a computer networking protocol that provides for automatic assignment of available Internet Protocol (IP) routers to participating hosts. This increases the availability and reliability of routing paths via automatic default gateway selections on an IP subnetwork.
The protocol achieves this by creation of virtual routers, which are an abstract representation of multiple routers, i.e. Primary/Active and Secondary/Standby routers, acting as a group.
The virtual router is assigned to act as a default gateway of participating hosts, instead of a physical router. If the physical router that is routing packets on behalf of the virtual router fails, another physical router is selected to automatically replace it. The physical router that is forwarding packets at any given time is called the Primary/Active router.
VRRP provides information on the state of a router, not the routes processed and exchanged by that router.
Each VRRP instance is limited, in scope, to a single subnet. It does not advertise IP routes beyond that subnet or affect the routing table in any way.
Essential Basic Points About VRRP
A virtual router has to use 00-00-5E-00-01-XX for its Media Access Control (MAC) address.
The last byte of the VRRP MAC address (XX) is the Virtual Router IDentifier (VRID). This is different for each virtual router within the network.
This address is used by only one physical router, and it responds with this MAC address whenever an ARP request is sent to the virtual router’s IP address.
VRRP packets are sent encapsulated into IP packets. These are communicated to the IPv4 multicast address assigned to VRRP.
Physical routers within the virtual router talk to each other using packets labeled with the multicast IP address 224.0.0.18 and IP protocol number 112.
Routers have a priority set from 1 to 254. The router with the highest priority becomes the Primary/Active.
The default priority is 100. For the MAC address owner the priority will always be 255.
All physical routers functioning as a virtual router have to be located on the same local area network (LAN).
A failure to receive a multicast packet from the Primary/Active router for a period longer than 3 times the advertisement timer causes the Secondary/Standby routers to regard the Primary/Active router as offline.
In this case the virtual router switches to an unsteady state and initiates an election process to select the next Primary/Active router from the Secondary/Standby routers. This is achieved by means of multicast packets.
VRRP is based on Cisco’s proprietary Hot Standby Router Protocol (HSRP) concepts. The protocols are similar but not compatible.
Routers running VRRP dynamically elect primary and backup routers. Assignment of primary and backup routers can also be forced by setting priorities from 1 through 255, whereby 255 is the highest priority.
The default primary router sends VRRP advertisements to backup routers at regular intervals.
The default interval is set to 1 second. If a backup router does not receive an advertisement within the set period, the backup router with the next highest priority takes over, becoming the primary router and begins forwarding packets.