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Occasional Contributor II

Re: DHCP scope are all oversubsribed

we had wireshark not seen anything unusual, dhcp server is common for wired and wireless, but i am sure no sharing b/w them.

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Occasional Contributor II

Re: DHCP scope are all oversubscribed

We had similar issue when we tried the even VLAN assignment. Users will connect and get a lease on VLAN1 (for example), then they will disconnect and connect again before their lease for VLAN1 expire and after the user entry in the controller times out, and get put by the controller in another VLAN2 but the even assignment where they get another lease. This can happen multiple times for the same user and he ends up with number of active leases in different VLANS. We have never had problems with hash based VLAN assignment , so this may not be exactly your case.

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All-Decade MVP 2020

Re: DHCP scope are all oversubsribed

 
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Ryan Holland, ACDX #1 ACMX #1
The Ohio State University
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All-Decade MVP 2020

Re: DHCP scope are all oversubsribed

A couple items:

1. show ap vlan-usage indicates what Mac addresses in the controller's station table have been assigned to each vlan. This is separate from L3/dhcp information. I just wanted to clear this up in case of ambiguity.

2. If I'm a client and receive IP-1, then immediately send out a DHCPDISCOVER packet, the dhcp server (per the RFC) will send a DHCPOFFER ...UNLESS your vendor (MS) has a "one lease per client" option. Can you verify there are discovers from clients within the max lease time window after they've already received an address?

3. What are your lease times configured?
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Ryan Holland, ACDX #1 ACMX #1
The Ohio State University
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Occasional Contributor II

Re: DHCP scope are all oversubscribed

Hell dpi,

 

we are very close in syptom with even algorithm, after change to even from hash i had same indications as u mentioned, how u sorted out the issue, were u had DHCP scope full issue. Hash the issue we faced is that twelve vlans in the pool, 3 r fully subscribed but controller still assign clients the vlan id of fully subscribed scopes.

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Occasional Contributor II

Re: DHCP scope are all oversubsribed

Hello Ryan,

 

1) show ap vlan-usage, and show vlan-assignment, are they give same info?, how to find L3/dhcp info.

 

 

 

I will check for ur second question.

thanks

 

 

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All-Decade MVP 2020

Re: DHCP scope are all oversubscribed

Ah! Welcome aboard the ?vlan pooling hash algorithm is uncomfortably inefficient? train!

I track this very closely in our environment and observe up to a 20% variance between how subscribed the most and least utilized network are. 20% is just awful. The problem is that while the controller does see the DHCP traffic from the client/server, it does nothing with this from an intelligence standpoint. This is something wherein I envy Cisco customers. Cisco?s ?VLAN select? has the controller aware of DHCP traffic. If it observes that clients are not getting a lease from a particular vlan, that vlan is marked dirty is essentially taken out of the pool for a period of time. In your case then, those 3 fully subscribed would not be in the pool, thus leaving 9 available for use by the hash algorithm.

I have Aruba performing a deep analysis/comparison between the efficiencies of hash vs. even, and I?m hopeful they can support their claim that even is more efficient by illustrating this with metrics. I?ll certainly share with anyone that wants it (if I ever receive it).

For now, our solution is to throw a ton of address space at the problem, but moving forward, we?re also going to place our students? mobile devices (phones/tablets) onto RFC1918 space.

- Ryan -
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Ryan Holland, ACDX #1 ACMX #1
The Ohio State University
Highlighted
Occasional Contributor II

Re: DHCP scope are all oversubscribed

We reverted back to hash and haven't used the even VLANs since. If you have hash for a VLAN assignment algorithm and still have full DHCP scopes, you may need to add more VLANs to the pool.

Our understanding for the even VLAN failure/feature is that because the controllers don't communicate with the DHCP servers they don't know if the users already have DHCP lease on one of the subnets/VLANs. The controller will keep information for the last VLAN assignment until the user entry the controller user table expires, and after that if the same user reconnects he will get put into the next in line VLAN (which most likely won't be the same as before).

There may be a way to match the lease time with the user table entry timeout, but that will lead to too short leases or too long timeout  (at least in our case).

Highlighted
All-Decade MVP 2020

Re: DHCP scope are all oversubscribed

fwiw, we are using 10 minute leases. If you have a lot of churn in your wireless network, consider shorter leases.

- Ryan -
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Ryan Holland, ACDX #1 ACMX #1
The Ohio State University
Highlighted
All-Decade MVP 2020

Re: DHCP scope are all oversubsribed

Hmm . . . oddly enough, they?re different it seems. When I run these two commands, I get the same format (vlan and client #s) but with different values. I don?t know from where each is populated, so maybe Aruba will see this thread and chime in.

Where to get L3/dhcp info? Your DHCP server. My experience is that using Windows event viewer for MS DHCP logs is hideous. Take a look at ?Splunk? as a logging repository, which could do a lot of MS DHCP analysis for you.

- Ryan -
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Ryan Holland, ACDX #1 ACMX #1
The Ohio State University
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