RSS Subscription 168 Posts and 2,769 Comments

DHCP Scope vs Superscope

I wanted to provide a short description of when you would use a superscope vs scope(s) in a real-world scenario. One of my coworkers had an issue described below:

The real-world scenario that I’m running into is that a client configured a single Superscope for all 175+ address ranges they have (scattered all over the country) and there are only two DHCP servers. They are using the 50/50 scenario and configuring half of the addresses on one server and half on the other. Laptops are not able to obtain a new IP Address when going from one building to another. It keeps trying to grab an address from its original location. I’m assuming that the reason is because they have a Superscope configured and not multiple scopes, but I have yet to come across Superscopes in the field.

My explanation of superscopes vs scope(s):

In an environment where you have multiple segments segmented by a Layer 3 device, each of those segments will have a different IP Range of course. That Layer 3 device will be configured with a DHCP IP (DHCP Helper IP) that is located in a different segment. Because of this, you only need to use regular scopes, as the DHCP server will see that it was requested from a different segment.

When you have different IP ranges on the same segment, the DHCP server will return a NACK because that DHCP server’s NIC is not on that segment. One way to get around this, is by adding another NIC that contains that same IP range. So if you have 3 different IP ranges on the same physical segment, you’ll need 3 different NICs. There is a way to get around this, and that is by using a superscope.

Using a superscope, you can have multiple logical IP ranges within the same physical segment and be able to hand out IP addresses even if the DHCP’s NIC does not belong to that same IP address range. Because of this, a superscope will help transition to a new scope using a different IP range for the same single physical segment.

So for the scenario stated above, all they would need to do is have 2 physical segments (one for each building), 2 different IP ranges (1 per segment), 2 scopes on the DHCP server, and have the layer 3 device configured with a DHCP Helper IP Address. So when a client moves, boots up and requests a new DHCP IP, it will broadcast, hit the Layer 3 device, the layer 3 device would see that the DHCP is on the different segment and the DHCP would see the request is from a different IP Segment and would provide them with a new IP from the appropriate scope.

Share

3 Responses to “DHCP Scope vs Superscope”

  1. on 08 Apr 2009 at 12:21 pmDan

    I’m curious if you can help me. I’m supporting users that travel from site to site – each site has a different subnet. Occaisonally when they log into a respective site, the IP address they obtain is still pointing to the last site they were at. Our network admin is a peach to work with and I’m not sure how things are set up. I know we use DHCP and the quick fix is just deleting/releasing that ‘stuck’ IP address. Is this a symptom of something that isn’t configured properly in DHCP? We get multiple calls weekly to correct this.

    Thanks,
    Dan

  2. on 07 Oct 2009 at 9:44 pmTim O

    Dan,

    I am assuming your users are laptop users. When traveling from site to site do you know if they are booting up their computers or do they just suspend them? Depending on your lease duration on your DHCP server the the computer may not be looking for a new IP if it was just suspended. Computers will not check the validity of their IP until 50% at wich poing they will aske the DHCP server if it is still valid. So if you have an 8 day lease duration the computer will not check it's IP until day 4. I would start there.

  3. on 14 Oct 2012 at 2:58 amahmdkhan ahmadzai

    how can we active two scope in same time

Trackback this post | Feed on Comments to this post

Leave a Reply