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ip dhcp relay

Posted on 2013-06-18
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Last Modified: 2013-10-31
This is my setup:
DHCP server<-->core switch (L3) <--> sw1 (L2) <-->sw2 (L2) <---> PC1

PC1 is using dhcp. But it could not get the DHCP from the server. If I configure it with a static IP address, everything works. Somebody suggested enabling dhcp relay on sw1. But I disagree because from my understanding, the dhcp relay is used to forward DHCP traffic between 2 different subnets. Furthermore, I don't think the layer 2 switch can do dhcp relay. Any thoughts would be greatly appreciated. Thx
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Question by:leblanc
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gt2847c earned 1000 total points
ID: 39257066
DHCP relay would be enabled on the L3 switch.  The VLAN interface routing for the PC would have to forward it as it has knowledge of the origination subnet...
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Expert Comment

by:JoPeLo
ID: 39257162
Looking at your layout, I believe the DHCP server is in a different subnet (VLAN?) of PC1. If so, you should configure the DHCP relay on your core switch L3. In cisco gear, the DHCP relay agent is configured using the ip helper-address interface command.
If the DHCP server is in the same VLAN of your PC1, and supposing you are not using PVLANs, you should look for trunk misconfiguration in all trunk configurations in sw1, sw2 and core switch (is that VLAN allowed in all trunk interfaces?) and/or if that VLAN exists in all switches.

Hope it helps.

jopelo
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Author Comment

by:leblanc
ID: 39257310
I am not using PVLAN. I will check the trunk configuration.
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Assisted Solution

by:vivigatt
vivigatt earned 1000 total points
ID: 39259948
Are the DHCP server and the client(s) in the same subnet?
If so, you need a DHCP relay (ip helper in Cisco world).
If not, this is something else...
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Author Comment

by:leblanc
ID: 39262584
Yes dhcp server and the client are in the same subnet and same vlan.
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Expert Comment

by:vivigatt
ID: 39262657
OK, so you don't need DHCP relay.
You must make sure that broadcast packets are handled correctly from end-to-end
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Assisted Solution

by:leblanc
leblanc earned 0 total points
ID: 39266210
I found out that the vlan where the dhcp server is  connected to was not configured.
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Author Closing Comment

by:leblanc
ID: 39613558
i found out that the dhcp sever was no configured correctly
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