I want to understand VRRP, but most resources online use complex examples that are fairly hard to understand. I want to understand the big picture.
What problem does VRRP(Virtual Router Redundancy Protocol) solve?
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VRRP, and other FHRPs such as HSRP, allow you to have a default gateway configured on your hosts that does not really exist. One router will take over the virtual router IP and MAC addresses, and if that router fails, then another router that was in standby can take over the virtual addresses. This lets you have backup routers in case one router fails. Some vendors will let you track the router uplink interfaces and decrement the router priority when that fails, allowing a switchover to a different router in the event of a link failure.
Basically, you are fooling the hosts so that you maintain a router to reach other networks without needing to reconfigure a different router or the hosts in the event of a failure.
VRRP is used to create a virtual router that can be made redundant by using two routers in a cluster. One router is active, the other is in standby.
When the active router fails, the virtual router's IP and MAC addresses are taken over by the standby router. That way, you can create a failover scenario with next to no connectivity loss.
Using NAT with those redundant routers is much more complicated. Either you lose all NAT sessions in case of a failover or they need to be closely synchronized between the routers. There is no standard for that but proprietary solutions exist.