HSRP (Hot Standby Router Protocol) is the Cisco proprietary protocol for providing redundancy in router networks. The standard router redundancy protocol which is used by other vendors is VRRP (Virtual Router Redundancy Protocol), however Cisco has created its own proprietary protocol (HSRP) which works very well on Cisco routers.

In a Local Area Network (LAN), all hosts (PC, Servers etc) have a single default gateway address configured which is used to route packets outside the LAN. If that single default gateway fails, then communication outside the LAN is not possible. With HSRP we can have two gateway routers, one active and one standby, which will provide resiliency regarding the default gateway address. Using HSRP, the two routers will have a physical IP address configured on their LAN-facing interface, but they will have also a Virtual (HSRP address) which will be used as the default gateway address for hosts on the LAN. No matter which router gateway is up and running (either the primary or the secondary), the virtual HSRP address will stay the same.

Let’s see a diagram below to explain this functionality.

First of all, HSRP must be configured between interfaces that have Layer2 connectivity between them. From the diagram above, HSRP will be running between interfaces FE0/1 on the two LAN routers. Interface FE0/1 on RTR-A will have a physical IP address 10.10.10.1 and interface FE0/1 on RTR-B will have a physical IP address 10.10.10.2. An HSRP address 10.10.10.3 will be also configured on both routers. This address will serve as the default gateway address for all hosts on the LAN. RTR-A will be configured as the Active HSRP router by setting a higher hsrp priority.

With HSRP, we can also track a specific interface. This means that if the tracked interface of the active router fails, then HSRP will trigger a failover to the standby router.

Let’s see an actual configuration below:

Configuration

Router RTR-A
RTR-A(config)# int fa0/1
RTR-A(config-if)# ip address 10.10.10.1 255.255.255.0

! enable HSRP group 1 and set the virtual address to 10.10.10.3
RTR-A(config-if)# standby 1 ip 10.10.10.3

! preempt allows the router to become the active router when its priority is higher
RTR-A(config-if)# standby 1 preempt

! increase its priority to 110 to make it active (default priority  is 100)
RTR-A(config-if)# standby 1 priority 110

! track the WAN interface FE0/0
RTR-A(config-if)# standby 1 track fa0/0

Router RTR-B
RTR-B(config)# int fa0/1
RTR-B(config-if)# ip address 10.10.10.2 255.255.255.0

! enable HSRP group 1 and set the virtual address to 10.10.10.3
RTR-B(config-if)# standby 1 ip 10.10.10.3

! preempt allows the router to become the active router when its priority is higher
RTR-B(config-if)# standby 1 preempt

! set priority to 100 to make it the standby router (this is the default value)
RTR-B(config-if)# standby 1 priority 100

! track the WAN interface FE0/0
RTR-B(config-if)# standby 1 track fa0/0

That’s it. Now configure a default gateway address of 10.10.10.3 for your LAN hosts.

Related posts:

1. VPN with HSRP High Availability
2. Cisco Router 851 – 871 Interfaces and Basic Configuration
3. Basic Cisco Router Configuration Steps
4. ASA Firewall Active-Standby interface configuration
5. Configuring Cisco Router Interfaces
6. How to Configure DHCP on Cisco 851 or 871 Router
7. Basic Cisco 800 Router Configuration for Internet Access