Wi-Fi – Circles Are Bad

Let’s start with the classic design guideline, the trope that you need overlapping circles. “Make sure they overlap 20%, that’s a proper design.” At least that’s what they say.

The queuestion is, how do you achieve that? How do you know you have the percentage overlap that you need? Ok, maybe we account for dB loss? Well then how do you properly calculate that loss? That’s easy, find a document that gives you the loss over the distance you need, correlate the two. Call it a day! Well…

Let’s take a look at the design guide by Cisco.

Source : Mobility Design Guide 8.1

Great! So I’ve got my answer! Overlapping channels, with a 19dB separation. That shouldn’t be too hard. Right? Right?!?

Well let’s see here, let me put this AP down right here in the hallway.

That’s odd. It’s not a circle. It’s like the hallway reflected the signal down the hall instead of letting it spread evenly.

This is why your wifi isn’t performing as expected. A long enough hallway will require you to place multiple APs in it to get the coverage you need. The instant you have co-channel interference you just reduced your AP count from 2 to 1.

How do we solve for this, so the circles actually behave more like circles? You place the AP in rooms.

This allows your design to reduce the chance of co-channel interference. And allows for you to account for the amount of overlap you need. But look carefully, we still don’t have circles. It’s very critical to find out the dB loss your walls will create, this circle won’t look the same in a different office.

Using multiple addresses from the same subnet

One of the first things anyone pursuing their CCNA learns is that you can’t configure multiple IP addresses in the same subnet on the same router.

The Problem 
I attempt to configure a ‘’ address on two different interfaces.

R01(config)#int gi2 
R01(config-if)#ip add
R01(config)#int gi1
R01(config-if)#ip add
% overlaps with GigabitEthernet2

Alternate Solutions
These solutions won’t be covered in detail in this blog post. They both solve and come with their own unique problems.

  1. HSRP
  2. IP Unnumbered
  3. Secondary IP Address

Alternate Vendor
Juniper allows you to configure multiple IP addresses in the same subnet.

   ge-0/0/1 {
        unit 0 {
            vlan-id 0;
        unit 1 {
            vlan-id 1;
            family inet {
    ge-0/0/2 {
        unit 0 {
            vlan-id 0;
        unit 1 {
            vlan-id 1;
            family inet {

After some quick testing it appears that Juniper originates traffic from the lowest interface and then lowest IP address.

The Rub
Why is it that Juniper allows for multiple addresses but Cisco can only in specific use cases? CEF allows for multiple destinations, even unequal cost load balancing.


  1. Historical
  2. RFC
  3. Other?

Looking through the mists of time I found this book – “Inside Cisco IOS Software Architecture.”

Unfortunately I don’t have an AGS+ and can only infer from the text it’s possible functions. The Cisco AGS+ used autonomous switching for the line cards. It was very costly in bandwidth and cpu to send a packet to the route processor. From what I can tell, the individual line cards didn’t retain a full copy of the routes. Any packet that arrived that had an unknown destination in the line card had to be passed up to the route processor. After that, the destination could be cached on the line card. The book also mentions that the AGS+ was the basis of the 7000 router and IOS.

Could this have been an early form of control plane protection? Or was it used to prevent from transferring unnecessarily across the low bandwidth bus?

My original theory was that it was based off of the RFC for requirements for an internet gateway. Here is the text in question –

“A different subnet address mask must be configurable for each interface of a given gateway. This will allow a subnetted gateway to connect to two different subnetted networks, or to connect two subnets of the same network with different masks.”

Unless I’m misreading it, it seems a pretty clear definition of what we are running into.

Is it a combination of the two above or something completely different. I would love to know. Drop me a line! admin at solutions-haven.com