HSRP Fundamentals

We are going to configure HSRP together, step by step.
You need to understand every mechanism, so we will walk through them one by one.

By the end of this module, you will configure everything yourself.

The HSRP Group

HSRP starts by grouping router interfaces into an HSRP group, identified by a group number.

Figure 1 – R1 and R2 form HSRP Group 1

In the topology, R1 and R2 both join HSRP group 1 on their LAN interface.

The HSRP Virtual IP

The group then shares one Virtual IP address (VIP).
That address belongs to no physical interface: the group itself answers for it.

Figure 2 – The Virtual IP becomes the default gateway

• Here the group uses 192.168.10.3 as the VIP, while R1 keeps .1 and R2 keeps .2 on their physical interfaces.

• Every host in 192.168.10.0/24 points its default gateway at .3.

Let's configure HSRP on R1 first:

R1# conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)# int g0/0
R1(config-if)# ip address 192.168.10.1 255.255.255.0
R1(config-if)# no shut
R1(config-if)# standby 1 ip 192.168.10.3

%HSRP-6-STATECHANGE: GigabitEthernet0/0 Grp 1 state Init -> Init
%HSRP-6-STATECHANGE: GigabitEthernet0/0 Grp 1 state Speak -> Standby
%HSRP-6-STATECHANGE: GigabitEthernet0/0 Grp 1 state Standby -> Active
R1(config-if)# end

R1 walked through the states on its own and took the Active role: no other router answered.
The three commands did the work: they set the physical IP address, brought the interface up, and created HSRP group 1 with the VIP 192.168.10.3.

R1 is Active because it is alone in the group for now: with no other router to compete against, it wins the election by default.

Now do the same on R2:

R2# conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R2(config)# int g0/0
R2(config-if)# ip address 192.168.10.2 255.255.255.0
R2(config-if)# no shut
R2(config-if)# standby 1 ip 192.168.10.3

%HSRP-6-STATECHANGE: GigabitEthernet0/0 Grp 1 state Init -> Init
%HSRP-6-STATECHANGE: GigabitEthernet0/0 Grp 1 state Speak -> Standby
R2(config-if)# end

R2 settled into Standby: the Active role was already taken.

Verify it on R1:

R1# show standby
GigabitEthernet0/0 - Group 1 (HSRP-V1)
  State is Active
    2 state changes, last state change 00:01:24
  Virtual IP address is 192.168.10.3
  Active virtual MAC address is 0000.0c07.ac01
    Local virtual MAC address is 0000.0c07.ac01 (v1 default)
  Hello time 3 sec, hold time 10 sec
    Next hello sent in 1.324 secs
  Preemption disabled
  Active router is local
  Standby router is 192.168.10.2, priority 100 (expires in 8.912 sec)
  Priority 100 (default 100)
  Group name is "hsrp-Gi0/0-1" (default)

Focus on one line for now: the group answers for 192.168.10.3.

Every other line of this output (State, Priority, timers, virtual MAC) is exactly what the next lessons decode.

Run the same command on R2:

R2# show standby
GigabitEthernet0/0 - Group 1 (HSRP-V1)
  State is Standby
    2 state changes, last state change 00:01:24
  Virtual IP address is 192.168.10.3
  Active virtual MAC address is 0000.0c07.ac01
    Local virtual MAC address is 0000.0c07.ac01 (v1 default)
  Hello time 3 sec, hold time 10 sec
    Next hello sent in 1.324 secs
  Preemption disabled
  Active router is 192.168.10.1, priority 100 (expires in 8.912 sec)
  Standby router is local
  Priority 100 (default 100)
  Group name is "hsrp-Gi0/0-1" (default)

Both routers agree on the same virtual IP address, 192.168.10.3.
The group is formed.

Inside each group, one router holds the Active role.
Let's look at the Active and Standby roles, confirmed with show standby on both routers.

The Two Roles

• The Active router owns the virtual addresses.
  It answers ARP requests for the VIP and forwards every packet sent to the virtual gateway.

Figure 3 – Active and Standby roles inside the group