Hot Standby Routing Protocol or HSRP, is a Cisco proprietary protocol that allows two or more routers to work together to represent a single IP address for a particular network. HSRP, as well as Virtual Route Redundancy Protocol (VRRP) are considered high-availability network services that allow for almost immediate fail over to a secondary interface when the primary interface becomes unavailable.
HSRP is a fairly simple concept that works by having one router within an HSRP group be selected as the primary or active router. That primary will handle all routing requests while the other routers within the HSRP group simply wait in a standby state. These standby routers remain ready to take on the entire traffic load if the primary router becomes unavailable. In this scenario, HSRP provides high network availability since it routes IP traffic without depending on a single router.
To really dig into the Nitty Gritty of HSRP, check out RFC 2281 for full details on the inner workings of this widely used protocol.
The hosts that use the HSRP address as a gateway never know the actual physical IP or MAC address of the routers in the group. Only the virtual IP address that was created within the HSRP configuration along with a virtual MAC address is known to other hosts on the network.
Basic HSRP Configuration
Before we discuss more advanced HSRP concepts, lets create a basic HSRP configuration to get an idea of how this all works. For this scenario we will use a topology consisting of just two routers. Keep in mind that one or both of these routers could be multilayer switches such as a 6509 or 3750 as well. But for this discussion let’s just refer them as routers.
R1 and R2 will both be configured to be in standby group 1. The HSRP address will be given an IP address of 192.168.1.1/24. All hosts on the segment and in the VLAN will use this address as their default gateway.
R1(config)#interface ethernet0
R1(config-if)#ip address 192.168.1.2
R1(config-if)#standby 1 ip 192.168.1.1R2(config)#interface ethernet0
R2(config-if)#ip address 192.168.1.3
R2(config-if)#standby 1 ip 192.168.1.1
To see the status of HSRP use the commandshow standby
This is the first command you should run to ensure that HSRP is running and configured properly.
R1#show standby
Ethernet0 – Group 1
Local state is Standby, priority 100
Hellotime 3 sec, holdtime 10 sec
Next hello sent in 0.776
Virtual IP address is 192.168.1.1 configured
Active router is 192.168.1.3, priority 100 expires in 9.568
Standby router is local
1 state changes, last state change 00:00:22
R2#show standby
Ethernet0 – Group 1
Local state is Active, priority 100
Hellotime 3 sec, holdtime 10 sec
Next hello sent in 2.592
Virtual IP address is 192.168.1.1 configured
Active router is local
Standby router is 192.168.1.2 expires in 8.020
Virtual mac address is 0000.0c07.ac05
2 state changes, last state change 00:02:08
We can see that R2 has been selected as the Active router (“Local state is Active”), the virtual router’s IP is 192.168.1.1, and R1 is the standby router.
Controlling the Active HSRP Router
There are more HSRP values that you’ll need to change from time to time to ensure complete control over your network traffic. For example, what if we wanted R1 to be the Active router instead of R2? To force a particular router to be the active router in an HSRP group you will need to use the priority command.
The default priority is 100. The higher priority will determine which router is active. If both routers are set to the same priority, the first router to come up will be the active router.
Using our example above, this is how the commands would look.
R1(config)#interface ethernet0
R1(config-if)#ip address 192.168.1.2
R1(config-if)#standby 1 ip 192.168.1.1
R1(config-if)#standby 1 priority 200<– Add this to force R1 to be active
R2(config)#interface ethernet0
R2(config-if)#ip address 192.168.1.3
R2(config-if)#standby 1 ip 192.168.1.1
Keeping the Active Router Active
In our scenario above, if R1 fails, R2 will become active. This is perfect! But, if R1 comes back up and returns to service, R2 will continue to stay active. This may not be a preferred behavior. There are times when you may always want R1 to be in an active state in the HSRP group. Cisco provides a way for use to control this by using the Preempt command. Preempt forces a router to be active after recovering from a failure.
Here again is our two router topology, with the preempt command added.
R1(config)#interface ethernet0
R1(config-if)#ip address 192.168.1.2
R1(config-if)#standby 1 ip 192.168.1.1
R1(config-if)#standby 1 priority 200
R1(config-if)#standby 1 preempt<– Add this to force R1 to return to active state after failure
R2(config)#interface ethernet0
R2(config-if)#ip address 192.168.1.3
R2(config-if)#standby 1 ip 192.168.1.1
Advanced HSRP Configuration – Load Balancing
So now you can see how great HSRP is and how it allows us to have high availability between multiple routers for a single network. But our standby routers aren’t doing anything and are just sitting there! Depending on the model router you are using, this can be a lot money just sitting idle.
To solve this problem, we can configure HSRP to be load balanced between routers. This doesn’t help us with a single HSRP group, but for multiple HSRP groups we can spread the load and have each HSRP group be active on different routers.
By configuring multiple HSRP groups on a single interface, HSRP load balancing can be achieved.
Here is how we accomplish this.
R1(config)#interface ethernet0
R1(config-if)#ip address 192.168.1.2
R1(config-if)#standby 1 ip 192.168.1.1
R1(config-if)#standby 1 priority 200
R1(config-if)#standby 1 preempt
R1(config-if)#standby 1 name nework-one
!
R1(config)#interface ethernet1
R1(config-if)#ip address 10.1.1.2
R1(config-if)#standby 2 ip 10.1.1.1
R1(config-if)#standby 2 name nework-twoR2(config)#interface ethernet0
R2(config-if)#ip address 192.168.1.3
R2(config-if)#standby 1 ip 192.168.1.1
R2(config-if)#standby 1 name nework-one
!
R2(config)#interface ethernet1
R2(config-if)#ip address 10.1.1.3
R2(config-if)#standby 2 ip 10.1.1.1
R2(config-if)#standby 2 priority 200
R2(config-if)#standby 2 preempt
R2(config-if)#standby 2 name nework-two
In this example above, ethernet0on Router 1 is active for standby group 1 and Router 2 is standby. Forethernet1, HSRP group 2, Router 2 is active and Router 1 is standby. This allows us to have each router working for us and forwarding packets to best utilize our investment in our networking equipment. We’ve also added the HSRP group name command to help better describe each HSRP group. This can be a life saver when you have several HSRP groups that you need to track.
One last note on HSRP standby groups. You can have multiple interfaces and networks configured using the same standby group number if the fail over behavior needed is the same.
However if you need to have a different behavior, meaning different priority, preempt, etc., (as seen in our load balanced scenario above) then a separate group is needed.
There is much more to HSRP then this article covers. For now, we wanted to give you a good foundation for the configuration of HSRP on a Cisco router.
Do you still have questions about this config or another question about HSRP? Leave a comment below and let’s work on it!
Reference from https://www.routerfreak.com/
Hot Standby Router Protocol (HSRP)
Hot Standby Router Protocol (HSRP): Frequently Asked Questions
Understanding and Troubleshooting HSRP Problems in Catalyst Switch Networks
More Related Readings:
Cisco First Hop Redundancy Protocols: HSRP, VRRP, GLBP
