DHCP Scope 50/50 rule vs 80/20 rules

[xzevallos]

I have two C9200 switches acting as DHCP servers.  Initially I had the network address scopes split in half between the two switches (50/50 rule).  But then I read in many web sites that an 80/20 split is more resilient.  So I put 80% of the addresses in the primary switch and 20% in the secondary switch.  But I can't wrap my head around why the 80/20 rule is better than 50/50.  Can someone please explain this to me?

[balaji.bandi]

If you like to split the DHCP - suggest to do 50/50 is best option

If you split the Load 80/20 - if the Switch has 80 DHCP address go down ? the situation is going to be disaster.

And also using DHCP on switches not suggested, Suggest to have dedicate DHCP Server is adviseble.

[Richard Burts]

The original post describes a primary switch and a secondary switch. Can you provide more information about the switches, their relationship to each other, and their relationship to the devices receiving DHCP addresses?

If you have one DHCP scope/pool that is close to the clients and another DHCP scope/pool that is further away then 80/20 might make sense. Or is one was primary and the other was some type of Disaster Recovery then 80/20 might be ok. But if both switchs are connected in the same vlan/same subnet then I agree that 50/50 is the better alternative.

[xzevallos]

Hello Richard.  I have two DHCP switches co-located.  However, any DHCP packets from the secondary switch (the one with 20%) must go through the primary switch to get to the relay router.  So the primary switch should usually respond first.  From what you are saying, 80/20 would make sense.

But I still don't understand why the 80/20 rule is considered better than 50/50 rule by many.  Companies like Cisco and Microsft recommend 80/20 over 50/50, so there has to be some benefit.  I'm just asking for someone to explain the benefit.

[Richard Burts]

One way to look at the question of what is the benefit is to think about what is happening when your network architecture has 2 servers/switches providing DHCP. Are then operating as equals to load share (in which case 50/50 would be very appropriate) or they operating as primary and backup? Clearly yours are primary and backup. Since the primary switch will respond to the majority of DHCP requests it would benefit from having a majority of the addresses and so 80/20 is beneficial.

[Richard Burts · Accepted solution]

Another way to think about this is in terms of how many client devices will be requesting DHCP and how many addresses are available in the pools. If you have twice as many addresses as clients (250 addresses for 125 clients) then a 50/50 is optimal (but very few of us have that luxury). As the ratio of clients to addresses changes then the ratio of addresses between servers changes and moves away from 50/50 toward 80/20. Many of us do not want to do case by case calculation and just adopt 80/20 as a good solution.

[Richard Burts]

I am glad that our explanations have been helpful. Thank you for marking this question as solved. This will help other participants in the community to identify discussions which have helpful information. This community is an excellent place to ask questions and to learn about networking. I hope to see you continue to be active in the community.