Solving DHCP Scope exhaustion - multiple /24 subnets or a single /23 subnet?

We have a client with a remote site with a 10.0.9.0/24 network. The DHCP scope on this network is nearly exhausted, with only about 20 addresses available. We could possibly increase this to 30 by re-IPing some static devices and shrinking our exclusion range. This may not be a long-term solution.

I've been tasked with engineering a long-term solution. Caveat: It's been 14 years since I did any serious Cisco router/switch configuration.

It was recommended that I create 3 additional VLANs with 3 additional subnets (10.0.10.0, etc). This has the advantage of allowing me to leave most of my static assigned IPs alone, but requires a ton of switch and router configuration for inter-VLAN routing. With a single DHCP server, this complicates the DHCP setup as well, with multiple DHCP scopes and a superscope. Most of this configuration can be done with minimal or no downtime for clients.

The other option is to resubnet the site to 10.0.10.0/23. I would have to re-IP every static device, but changing IPs and subnet masks is a bit easier (for me) than configuring multiple VLANs and inter-VLAN routing across 4 different subnets. (Note: The 10.0.8.0/24 network is in use at another site, so we can't use 10.0.8.0/23 to avoid changing the 10.0.9.* IP addresses at this site.) This configuration will require significant downtime as network devices are re-IPd.

My understanding between the two solutions is that a single /23 subnet has a larger broadcast domain (510 clients) where as 4 /24 subnets have 4 smaller broadcast domains. With ~220 devices, I don't see broadcast traffic as a significant factor. The switches in use are Cisco 2960-s models with limited layer-3 capability. Therefore most of the inter-VLAN routing would fall back on the site's router, increasing router overhead.

Is there an industry standard preferred method for resolving DHCP scope exhaustion? Are there any pros/cons to either solution that I haven't mentioned her that may make a difference? Is there a third, better solution?

Replay

I've been tasked with engineering a long-term solution. Caveat: It's been 14 years since I did any serious Cisco router/switch configuration.

If you are tasked with creating a long-term solution, you should really use your first option:

It was recommended that I create 3 additional VLANs with 3 additional subnets (10.0.10.0, etc). This has the advantage of allowing me to leave most of my static assigned IPs alone, but requires a ton of switch and router configuration for inter-VLAN routing. With a single DHCP server, this complicates the DHCP setup as well, with multiple DHCP scopes and a superscope. Most of this configuration can be done with minimal or no downtime for clients.

I think you really overestimate the work involved. The network engineers I know really, really hate to readdress end-devices, and they would much rather make the few simple changes to the network to add a few VLANs.

Category: dhcp Time: 2016-07-29 Views: 0
Tags: dhcp

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