Should this device fail, then all of the users who are on a specific segment using it as their default gateway will be unable to reach any other subnet including the Internet. There are a number of different solutions to this problem, and most of these are all grouped together and referred to as First Hop Redundancy Protocols FHRP. This article specifically looks at HSRP and how it operates along with the main concepts that should be known before attempting to configure it. The general idea behind HSRP is rather simple: Configure multiple devices that all exist on the same subnet and are able to act as gateways to the hosts on the subnet. Basic redundancy can be configured by configuring some of the hosts to use one gateway and the other hosts to use another.
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Most of the company in the world has a connection to the Internet. This can be done manually or automatically via DHCP. After some time, your boss wants to implement some redundant methods so that even the Router fails, all PCs can still access the Internet without any manual configuration at that time.
So we need one more router to connect to the Internet as the topology below: But now we have a problem: There is only one default gateway on each host, so if Router1 is down and we want to access the Internet via Router2, we have to change the default gateway to Also, when Router1 comes back we have to manually change back to the IP address on Router1. And no one can access to the Internet in the time of changing the default gateway.
HSRP can solve all these problems! For example, the virtual IP address is All the hosts will point their default gateway to this IP address. One router, through the election process, is designated as active router while the other router is designated as standby router. Both active and standby router listen but only the active router proceeds and forwards packets.
Standby router is backup when active router fails by monitoring periodic hellos sent by the active router multicast to When a failure on the active router detected, the standby router assumes the role of the forwarding router.
Because the new forwarding router uses the same virtual IP and MAC addresses, the hosts see no disruption in communication. A new standby router is also elected at that time in the case of there are more than two routers in a HSRP group. By default, the hello timer is set to 3 seconds and the dead timer is set to 10 seconds. It means that a hello packet is sent between the HSRP standby group devices every 3 seconds, and the standby device becomes active when a hello packet has not been received for 10 seconds Note: The virtual MAC address of HSRP version 1 is But please notice that the virtual MAC address can be configured manually.
HSRP version 1 hello packets are sent to multicast address It indicates HSRP is not running. It happens when the configuration changes or the interface is first turned on Learn The router has not determined the virtual IP address and has not yet seen an authenticated hello message from the active router.
In this state, the router still waits to hear from the active router. For example, if there are 3 routers in HSRP group, the router which is not in active or standby state will remain in listen state. Speak The router sends periodic HSRP hellos and participates in the election of the active or standby router. Standby In this state, the router monitors hellos from the active router and it will take the active state when the current active router fails no packets heard from active router Active The router forwards packets that are sent to the HSRP group.
The router also sends periodic hello messages Please notice that not all routers in a HSRP group go through all states above. In a HSRP group, only one router reaches active state and one router reaches standby state. Other routers will stop at listen state. Suppose there are 2 routers A and B in the network; router A is turned on first. It enters the initial state. Then it moves to listen state in which it tries to hear if there are already active or standby routers for this group.
After learning no one take the active or standby state, it determines to take part in the election by moving to speak state. Now it starts sending hello messages containing its priority. These messages are sent to the multicast address When it does not hear a hello message with a higher priority it assumes the role of active router and moves to active state. In this state, it continues sending out periodic hello messages. Now router B is turned on. It also goes through initial and listen state.
Suppose router A is in active state while router B is in standby state. If router B does not hear hello messages from router A within the holdtime 10 seconds by default , router B goes into speak state to announce its priority to all HSRP members and compete for the active state. But if at some time it receives a message from the active router that has a lower priority than its priority because the administrator change the priority in either router , it can take over the active role by sending out a hello packet with parameters indicating it wants to take over the active router.
This is called a coup hello message. A hello packet is sent every 3 seconds by default. The hold time dead interval is 10 seconds. F0xx , where xx is the hexadecimal number of HSRP group. HSRP version 2 supports group numbers.
How to configure HSRP on Cisco – Basic to Advanced
From the group of routers configured in an HSRP group, there is one router elected as the active router and another as a standby router. The active router assumes the role of forwarding packets sent to the virtual IP address. If the active router fails, the standby router takes over as the new active router. This address is also referred to as the standby IP address. Each router in this group must define with the same virtual IP address using the standby [group-number] ip [Virtual ip-address] command.
CCNA 5.5: Configure, verify, and troubleshoot basic HSRP
This command was introduced. Usage Guidelines To use this command, you must be in a user group associated with a task group that includes the proper task IDs. If you suspect user group assignment is preventing you from using a command, contact your AAA administrator for assistance. The hsrp ipv4 command activates HSRP on the configured interface.
Introduction to The Hot Standby Router Protocol (HSRP)