HSRP Default Value

[amarshailender]

Hi all,

IN HSRP Priority, the range is from 1 to 255, where 1 denotes the lowest priority and 255 denotes the highest priority. The default priority value is 100. The router in the HSRP group with the highest priority value becomes the active router. My query is why the default priority is set to 100 if the range starts from 1 to 255. Logically, either this range should start from 101 or the default must be set at 1. Since under such default seetings, the range of 1 to 99 does not make any sense.

I am sure there must be some intentional reason behind it. Please share.

Thanks in advance

Shailender Amar

[Abzal]

The logic could be that if a priority is set 1 then you can't lower it by track command for example by monitoring uplink interface. By setting it to 100 you more choices to decrease if you have for example 100 routers doing HSRP for one segment(highly unlikely scenario though).

Maybe other members know more detail and history it but that's my view.

[Giuseppe Larosa]

Hello @amarshailender ,

as noted by @Abzal  the main reason to have a default value of 100 instead of 1 is to leave to the HSRP routers the capability to use the tracking feature by reducing the HSRP priority if one of the WAN links fail for example by default the HSRP priority is reduced to 90 giving to another router configured for pre-emption a chance to take over and become the new active HSRP router for the group.

I have had bad experience  with setting priority to max value 255 because if the device for any reasons becomes a black hole for traffic to the internet there is no easy way to override it.

What I usually do is to have a difference of 5 in priority between the primary router and secondary router so that in normal conditions or with the same number of faults the primary is still the active router and the secondary take over when it has no or less faults then the other router.

Again this only one of the possible design choices. Other people prefer to have greter differences like 50 and then changes the amount of priority decrement for each tracked object.

Hope to help

Giuseppe

[Joseph W. Doherty]

As the other posters have noted, a mid-range value, as a default, allows you to go above or below that value, dynamically, whereas if the default had been the minimum or maximum value, you wouldn't have that as an option.

As to why 100 was used, my guess would be someone got it as a nice round decimal value that was close to the actual binary half way value of 127/128. (Since the range is 1..255 the value is likely a binary byte.)

[Cristian Matei]

Hi,

    The reason for not having a default value of the highest or lowest, and having more of a middle value like 100, is, most probably, so that you have enough room to go up/down from the default value (assuming you have multiple routers in a HSRP group, cause there can be more than 2 routers; you have routers, for example, with priority of 100, 90,80, 70,60).

   As for why the exact number of 100 and not 105, there could be some sort of logic behind the number, but for sure only people involved in the development would know. Within the same lines, why do we have an AD of 170 for EIGRP External, and not 180? You see where i'm going, sometimes numbers are just numbers.

Regards,

Cristian Matei.

Regards,

Cristian Matei.