dhcp response from two sources

[suthomas1]

Hi,

We have this network , where a particular vlan(eg. vlan 10-wireless) is being configured for dhcp ip allocation & this is done by a third party authentication box(similar to ACS).

This vlan10 is only applicable to wireless users.

Now in the same network, we want to create another vlan (eg. Vlan20-Inhouse). This vlan20 will also have dhcp allocation , although this vlan20 will be served ip's from a server which will take the role of dhcp server.

The layer 3 interface for this vlan20 will be on a firewall upstream & will be extended to the switches where users will connect themselves to.

The query is , having two different dhcp on the same big network, will it create any conflict on who answers first.

My understanding is since the two dhcp servers areon/server different vlans, the users ip request will be handled by the respective dhcp server, based on the vlan they are attached.

Please help in getting this clarified. Thanks in advance.

[cadet alain · Accepted solution]

Hi,

a DHCP Discover message in one vlan won't be seen by the dhcp server in the other vlan unless there was some dhcp relay agent involved.

Regards

Alain

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[Joseph W. Doherty]

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Posting

As Alain has noted, when you jump VLANs you normally need a DHCP relay agent.  Without that, the DHCP server won't see the request from a non-local VLAN.  But also don't forget, DHCP servers also won't provide an IP for a remote subnet if they haven't been allocated a pool of IPs for that subnet.

Conversely, you might intentionally have both DHCP servers setup with non-overlapping blocks of IPs for the same subnet.  This way, if one fails, you'll still have DHCP redundancy.  Client will normally accept the first DHCP offer it receives.