HSRP is a Cisco proprietary protocol that provides redundancy within a subnet. A virtual router is created which acts as the default gateway and a priority number is assigned to the physical routers to determine which is active and which is a standby router.
Some key concepts with HSRP;
Priority
By default the priority number is 100. The highest priority number configured is assigned the active router if that fails then the next highest takes over which is the standby router. For example a priority number of 150 would be the active and 100 would be the standby router.
Preempt
Is a state which activates the active router automatically
Hello messages
Are exchanged periodically to activate the standby router if it detects a failure on the network.
Virtual IP address
A logical IP address that each host uses as the default gateway within a local subnet.
Practical activity
In the below video, there is a full run through of HSRP in action. There is also the side effect concept of OSPF which is a routing protocol that dynamically routes traffic based on administrative distance.
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