HSRP (Hot Standby Router Protocol)
HSRP(Hot Standby Router Protocol) is a Cisco proprietary First Hop Redundancy protocol. HSRP first hop redundancy mechanism provide alternative path as a proactive action for any failure on a device.
By the way, HSRP Group numbers is mention at the beginning of the confiugration after “standby” keyword. There can be multiple groups of HSRP on an interface.
HSRP Router Roles
HSRP works with HSRP Groups. For each group, there are different routers that has different roles. These roles are:
• Active Router
• Standby Router
• Listenning Routers
The Active Router is the router that the traffic flow go through this router. It is the router that actively take place in the traffic flow in normal case.
The Standby Router is the backup of Active Router. When th active router fails then this router become Active and the traffic go through this router.
Lastly the Listening Router is other routers that are particippating in the HSRP group.
HSRP Active/Standby Elections
The main two roles of HSRP are Active and Standby. For these roles, routers does an election between them. In this election two parameters are important. The elections are done according to two parameters. These are:
• HSRP Priority Values
• Interface IP Addresses
First of all, HSRP Priority values are used for Active/Standby election. The highest Priority wins and the router that has highest priority becomes Active Router. If the Priority values of HSRP Routers are equal, then the Interface IP addresses are used as tie breaker.The highest IP address wins and become Active Router. The router that has the second highest IP address, becomes Standby Router.
HSRP Priority values can be manually changed. The default HSRP Priority is 100.
After the configuration of HSRP, HSRP progresses go throught some states. After these steps, Active and Standby routers are elected and the HSRP become active. So, what are these HSRP states?
These HSRP states are mentioned below respectively:
– Init: Starting state, HSRP is not running yet. Interfaces are available
– Learn: Router doesn’t know anything yet. It is wainting for hellos, to learn Virtual IP.
– Listen: Router knows Virtual IP, not elected stadby or active
– Speak: Router is actively participating in Active/Standby election bysending hellos
– Standby: Acting as backup,monitors and send hellos
– Active: Accepting and forwarding user traffic.
Only in three states, hellos are sent. Speak,Standby and Active states. As you know standby and active routers already send hellos each other. In the last one, in speak state, hellos are sent to determine the active and standby routers.
There are two HSRP versions. These HSRP versions are:
The default HSRP version is HSRPv1. It is used by IPv4 networks. In HSRPv1, HSRP Group numbers can be from 0 to 255. HSRPv1 uses 18.104.22.168 as HSRP Multicast Address to send HSRP Hellos. This is also the address that CGMP Leave process uses.The packet format of HSRPv1 is different than HSRPv2 packet format.
HSRPv2 is the enhanced version of HSRPv1. HSRPv2 supports IPv6. For Hello messaging, HSRPv2 uses a different Multicast address than HSRPv1. The new Multicast addres that HSRPv2 uses is22.214.171.124. HSRPv2 uses Group numbers from 0 to 4095.The packet format of HSRPv2 is different than HSRPv1.
By default periodic HSRP Hello packets are sent every 3 seconds. Active router is controlled by standby router with periodic hello packets by default every 3 seconds. In case of a failure of active router, this mechanism provide switching to the backup router.
The Hold Time value is 10 seconds.
Other First Hop Redundancy Protocols (FHRPs) Lessons
First Hop Redundancy Protocols (FHRPs) – Overview
First Hop Redundancy Protocols (FHRPs) – HSRP
First Hop Redundancy Protocols (FHRPs) – HSRP Configuration on Cisco
First Hop Redundancy Protocols (FHRPs) – VRRP
First Hop Redundancy Protocols (FHRPs) – VRRP Configuration on Cisco
First Hop Redundancy Protocols (FHRPs) – GLBP
First Hop Redundancy Protocols (FHRPs) – GLBP Configuration on Cisco