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Strange behavior with DHCP

GlennWelker
Occasional Advisor

Strange behavior with DHCP

Looking for some help with our HP gear and Meraki access points.

So what is happening, is that our wireless clients seem to be addresses from what appears to be random buildings. DHCP is simple ip helper addresses set on our vlans.

Our network varies from each building

10.73.3.x - High school APs (vlan 3, ip address 10.73.50.1)
10.73.50.x - Wireless clients (vlan 50, ip address 10.73.50.1)

10.74.3.x - Middle school APs (vlan 3, ip address 10.74.3.1)
10.74.50.x - Wireless clients (vlan 50, ip address 10.74.50.1)

Meraki access point ports are all tagged in the wireless client vlan. The buildings are also trunked back to the main switch at the High School. Strangely even though our scopes are set to the individual buildings, our wireless clients may be issued an IP in a different building even though they will be put in the right vlan 50.

Any ideas on why this might be happening?

7 REPLIES
Y0DA
Occasional Visitor

Re: Strange behavior with DHCP

Can you paste the config of the switch port the AP is connected to?

parnassus
Honored Contributor

Re: Strange behavior with DHCP

Many if...but, at a first read:

Supposing you Subnets are /24...if both buildings have Switches in which the VLANs you reported are respectively defined (VLAN 3 and VLAN 50 on Switch of Building "A" and VLAN 3 and VLAN 50 on Switch of Building "B") AND if both locations are then trunked together to permit those VLANs to flow inbetween AND if the VLAN Ids are the same on both Switches (VLAN Id 3 and VLAN Id 50) then how is it possible that VLAN 3 and 50 are concurrently associated respectively - each one VLAN - to two differents subnets (VLAN 3: 10.73.3.0/24 on "A" versus 10.74.3.0/24 on "B" and VLAN 50: 10.73.50.0/24 on "A" versus 10.74.50.0/24 on "B")?

Pardon me if I misunderstood your whole network topology and its VLANs logic...but if the above picture is somewhat correct, shouldn't both VLAN Ids 3 and 50 use globally uniques Subnets across the whole campus site ("A"+"B")?

I hope to have not been hit by any big solar flare today...

GlennWelker
Occasional Advisor

Re: Strange behavior with DHCP

I think your assumptions are correct. Two different subnets, same vlan.

We have used this in other areas, such as all workstation computers fall into vlan 18.

So the high school's ips are different than the middle school, etc.

 

GlennWelker
Occasional Advisor

Re: Strange behavior with DHCP

I can't post all of it but I can try to clarify details.

Essentially what is happening is that the DHCP server seems to be offering multiple addresses from each of our buildings but our wireless client isn't accepting any of them.

This doesn't happen with physical interfaces. It only seems to be happening in this case due to the vlan tagging.

 

parnassus
Honored Contributor

Re: Strange behavior with DHCP


GlennWelker wrote:

I think your assumptions are correct. Two different subnets, same vlan.

That's worrying. I'm quite sure it's not a best practice.

16again
Respected Contributor

Re: Strange behavior with DHCP

Two different buildings can re-use the same VLAN structure, and all having different IP subnets.

This works out fine if there's no L2 link in between them.  Inter-connecting must be done in L3 (routed) mode,

parnassus
Honored Contributor

Re: Strange behavior with DHCP

Yes, that's true ...and I hope it is so! ...but that's also the reason because I asked the OP on my first post above (and the reason because I started with a lot of "if" trying to figure out his network topology...encouraging the OP to provide few more details).