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Community Member

How DHCP scope will work between two Wireless contoller

Dear All,

             I would like to inform you that we are going to deploy Wireless Network with redundancy of Wireless LAN Controller and we need to figure out how DHCP scope would work during fail over  any one of Wireless LAN controller and DHCP Server .I have two site where Two Differenet DHCP Server would be palced with different DHCP socpe.So My query is that if Site-A(according to attached diagram) DHCP Server and controller fails how Wireless  client will take the DHCP scope from Site-B(according to attached diagram) DHCP Server as it has been configured different DHCP Scope.Can any one tell me whether i can configure same DHCP Scope at Both Site's DHCP Server.

Is there any way to configure same DHCP Scope for two sites.For your infromation client wants two Different DHCP Server with different scope for Two Site and how redundancy would be ensured if any one WLC or DHCP goes down.

I am eagerly waiting for your reply as early as possible.Please help to figure out the solution.

I am going to attached Sample Design for your reference.

Thanks and regards

Erfan

12 REPLIES

How DHCP scope will work between two Wireless contoller

Salam Erfan,

Internal DHCP within the WLC works only for clients connected to that WLC. it can not be used by clinets connected to APs that are joining a different WLC. So if your WLC in site A goes down and APs join WLC in site B the users in location A will get an IP from location B becaue they are connected actually to WLC in location B.

You can use big subnet and devide it between the two sites to avoid overlapping. This is if you want to use same subnet on both locations.

If you are using same SSID on both locations, you can use AP groups. This feature will let you choose that the SSID on location A is mapped to VLAN X while the same SSID in location B is mapped to VLAN Y. Configure this on both controllers and if, for example, WLC in location A goes down the APs in location A will join WLC in location B. But WLC in B is configured to put the APs in location A in their own group that is mapped to a specific VLAN. For the DHCP in this case you also need to split the DHCP among both controllers to avoid overlapping in case of down situation of one of the WLCs.

HTH

Amjad

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Community Member

How DHCP scope will work between two Wireless contoller

Salam Amjad,

                   Thanks for your reply and information.My client are running two different DHCP  Server with different scope between two location.For example if one client of Site-A wants to get the same IP block from site-B if Site-A DHCP server goes down then how we can we provide the Solution.

As you said that we can configure AP gourp for Same SSID but it will get different IP block.My question  is that Client of Site-A  want access policy on specific IP Block then how we can ensure same policy for different IP Block if they connected with Site-B with different scope .I hope  that you got my point.

It would be higly appreciated if you reply on my query and need your assistance badly.

Thanks and regards

Erfan

How DHCP scope will work between two Wireless contoller

Erfan:
you can use a subnet on your location A for the A clients. utilize only half of the IPs on site A and half of them on site B.

For example,If you are using 172.32.0.0/24 for your clinets, what you can do is to create:

use range 172.16.0.10 - 172.16.0.128 on WLC A.

use range 172.16.0.129 - 172.16.0.254 on WLC B.

(I assumed first 10 IP addresses reserved).

Do the same for site B. split its subnet on both WLCs.

If you create the full range (172.16.0.10 - 172.16.0.254) on both WLCs then there will possibly be overlapping when the clinets fall back to the other site's WLC. If you split the scope you then eliminates the overlapping.

I hope this is clear and easy now.

Amjad

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Community Member

How DHCP scope will work between two Wireless contoller

Hi Amjad,

              Thanks for reply and suggestion.As per your reply we have to create two different scope for two site in the DHCP server.so client which got ip from DHCP SEVER of Site A, will get different ip from DHCP Server of site-B if site-A goes down.then how we can configure same SSID for two controller using two ip block with same Access-list.

One more things I need to clarify that AP will be connected to one VLAN and we will created one ip helper address for Specific Vlan or subnet. Is there any way to put two ip helper address for same layer 3 VLAN to get ip address for wireless client if one site goes down.

Looking forward to your cooperation and help.

Thanks and regards

Erfan

How DHCP scope will work between two Wireless contoller

Erfan:

you can use one access list that contains the "big" scope. Remember that it was only one scope and we divided it to two parts. right? so if you use the big scope you can use one ACL that targets both sites.

Just a note: I was talknig about about the internal DHCP. I don't know why I implicitly thought you talk about internal DHCP within the WLC.

Now we have an issue of the gateway. If you split the scope as per the above your gateway IP address should be different for both scopes? If yes then splitting the scope as per the above will not work because if the site A goes down and the APs on that site moved to join WLC in site B, the gateway for the clients in site A will be different than the actual gateway in site B.

we'll, it is a kind of complicated. I think it is better for you to use same subnet, same gateway if you want to utilize your scenario. If you want to use two different subnets things will be more complicated and your requirement may not be applicable.

HTH

Amjad

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Community Member

Re: How DHCP scope will work between two Wireless contoller

Hey guys,

I'm pretty new to this forum. So, this is my first posting ... might be useful.

First of all, isn't there a problem with one subnet on both sides regarding to routing?

@erfan

Why don't you make two profiles on each wlc with the same SSID, different configurations for each site and bind them to apgroups ( site a and site b).

So, you can have the controllers on the same configurations.

Split the subnets in two /25

Let me know if this was helpful.

Best regards

Sebastian

Sent from Cisco Technical Support iPad App

Community Member

How DHCP scope will work between two Wireless contoller

Hi Amjad,

                 I am eagerly waiting for your valuable comments.Please help to solve the issue of DHCP for two sites.

Thanks and regards...

Erfan

Community Member

Re: How DHCP scope will work between two Wireless contoller

You can configure both DHCP server with same pool, because in case wich one WLC goes down, the Access-Points will connect to secondary controller and the clients associated to the AP will make a new DHCP Discover and take new IP address.

You can set for example:

WLC1 pool: 192.168.0.0/24

WLC1 GW: 192.168.0.1

WLC1 IP: 192.168.0.2

WLC2 pool: 192.168.0.0/24

WLC2 GW: 192.168.0.1

WLC2 IP: 192.168.0.3

This work fine

Hall of Fame Super Silver

How DHCP scope will work between two Wireless contoller

When using the WLC as dhcp and having more than one, you need to split the dhcp scopes.  The WLC is not smart enought to know that the other wlc handed out a dhcp address and will cause duplicate address in your network.

-Scott
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Community Member

How DHCP scope will work between two Wireless contoller

Hello Guys,

                  I am sorry for delay reply. I hope that you all got my problem as we have two External DHCP Server at both Locations. So we have two different IP block for that two DHCP Server as same IP block doesn’t support for Two DHCP Server.

My query was that if one wireless client already associated WLC-A with one IP block of Location-A (DHCP Server) using SSID named TEST. If DHCP-A or WLC-A goes down then how that same client would associate with WLC-B and get the IP Block from Location-B (DHCP Server) using same SSID name TEST.

Please have a look the below query:

1. How we will create same policy for that two different block using same SSID.

2. client would be connected with Access point but they will get the IP through VLAN interface of the Controller and we have to assign IP helper address in the interface vlan.So how the the client would get the IP block if we assign two IP helper address in the interface vlan in case of failure of DHCP and WLC of one location.

3. If we assign two IP helper address then which DHCP Server will release the IP for client and we can make delay between two DHCP Server to release the IP address.

I need all your help to solve the problem and your early response is highly appreciated.

Thanks and regards

Erfan

Re: How DHCP scope will work between two Wireless contoller

Erfan:

My query was that if one wireless client already associated

WLC-A with one IP block of Location-A (DHCP Server) using SSID named TEST. If DHCP-A or WLC-A goes down then how that same client would associate with WLC-B and get the IP Block from Location-B (DHCP Server) using same SSID name TEST.

In this case the clinets connected to the APs in location A will disconnect (if the WLC went down) and they have to connect again when the APs join WLC-B. They will get IP address from location B block then.

If DHCP went down when new clinets try to connect the primary DHCP is down so they are referred to the secondary DHCP. connected clinets will disconnect only when their DHCP lease times out. They will try to renew but the DHCP server is not available. They will disconnect then and connect again and getting IP from B site.

1. How we will create same policy for that two different block using same SSID.

what policy? where?..etc?

2. client would be connected with Access point but they will get the IP through VLAN interface of the Controller and we have to assign IP helper address in the interface vlan.So how the the client would get the IP block if we assign two IP helper address in the interface vlan in case of failure of DHCP and WLC of one location.

3. If we assign two IP helper address then which DHCP Server will release the IP for client and we can make delay between two DHCP Server to release the IP address.

If two IP helpers are configured the switch will send them both the client's request and they client receives two offers. The clients chooses one of the offers. Usually the clinet will accept the faster DHCP server to respond.

If you are using the DHCP on the WLC then it is different. The WLC will always try to reach the primary DHCP server as long as it is reachable. If the primary DHCP is not reachable then it falls back to the secondary DHCP server.

Note that when you use IP helper then you usually disable DHCP Proxy feature on the controller. Do you have it disabled?

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Community Member

How DHCP scope will work between two Wireless contoller

Hi Amjad,

               Many many thanks for your comments and help.

Few more questions are given below:

Policy means Access-List for SSID.

If we configure two IP helper address then faster DHCP Server will provide the IP as you said.So for this  do we need to configure External DHCP server such a way that delay would be configured during DHCP Server Configuration ???

Please can you tell me if we have assign Primary DHCP Server and Secondary DHCP Server IP address in the interface vlan of the controller,then Two IP helper address is required on the switch's interface VLAN  at time or not ???

Can  we configure any one of them ???

Your valuable comments are always important for me.Please help me on those issue and eagerly waiting for your mail

Thanks and regards

Erfan

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