What is HSRP and why to use it?
Hot Standby Router Protocol (HSRP) is a Cisco proprietary redundancy protocol for establishing a fault-tolerant default gateway. The protocol establishes a framework between network routers in order to achieve default gateway failover if the primary gateway becomes inaccessible, in close association with a rapid-converging routing protocol like EIGRP or OSPF. HSRP routers send multicast Hello messages to other routers to notify them of their priorities (which router is preferred) and current status (Active or Standby).
In regards to using HSRP with dynamic routing protocols, routers do not use the virtual IP addresses of HSRP to form adjacencies. HSRP is designed to provide end-users devices and servers default gateway/router redundancy. For instance, EIGRP neighbor relationship are not formed with the HSRP Virtual IP Address but with the physical interface IP address. As such if you are using HSRP along with a dynamic routing protocol you should ensure your routes prefer the active router unless offline to avoid issues as only the active HSRP router for a group will forward packets.
In the network below there are a total of two routers, two end-user computers and one server. In the example below we will configure three HSRP groups, two for the LAN and one for the WAN network. R1 will be the primary gateway for PC1, and R2 the primary gateway for PC2. This will make use of both routers to prevent network capacity from being underutilized. R1 will be the active router for the 1.1.1.0/29 network and failover and failback must occur automatically. HSRP groups will also make use of authentication to prevent any rogue routers from interfering with HSRP operations.
Verify the configuration
Now that the configuration is finished lets verify our HSRP deployment. Using the ping, traceroute and show ip route on PC1 we can verify connectivity, the path used to reach the destination and default gateway. You will find ping is successful from PC1 to Server and the path is PC1 to R1 to Server. On PC2 you will find ping is also successful but the path PC2 takes to reach Server is PC2 to R2 to Server.
On R1 issue the show standby command to verify the active HSRP routers. This will confirm R1 is the active router for groups 1 and 2, and R2 is the active router for group 3.
Now let's shutdown an interface on R1 and perform the same steps from PC1. As you can see everything functions the same with the only exception your default gateway, 10.1.0.3, is routed through the backup router R2 which is now the active HSRP router for HSRP groups 1, 2, and 3.
For the last verification step turn R1 back on and repeat the steps again. Your default gateway, 10.1.0.3, will again be routed through tR1 and is once again the active router for groups 1 and 2.
