Switch Ports not receiving DHCP

I have a hub and spoke network topology. I am trying to bring up a couple devices attached to a HP A5120-48G-PoE+ EI access switch. I have the IP subnet pointed to the dhcp server via dhcp relay. I had the sys admin check the dhcp server and all the information for vlan, subnet, mask, and dhcp range is correct.

Some ports are receving dhcp addresses while others are not (that are not shut down and have poe enabled).

What could possibly be the problem?
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Fred MarshallPrincipalCommented:
I'm a bit unclear what the situation and question are:

Do you mean that *devices* connected to certain switch ports aren't getting DHCP service?

Or, do you mean that "some ports [on the switch] are not receiving addresses via DHCP"?

If the former, then I understand the question.
If the latter then I don't understand the question since ports don't receive addresses.....
Mohammed KhawajaManager - Infrastructure:  Information TechnologyCommented:
Are the ports not receiving on different subnet or VLAN?  If so then you need to configure interfaces (sub-interfaces) for each subnet/VLAN and create routes.  Then for each route you have to specify ip-helper addresses.  What is your network configuration like?
AkinsdNetwork AdministratorCommented:
Some ports are receiving dhcp addresses while others are not

Are these ports on the same vlan?
If yes, have you tried swapping known good devices and connect them to ports that aren't working.

What could possibly be the problem?
There's not enough information to determine that answer
A little more detail is needed. Please start with a topology diagram
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jonathanrkleinAuthor Commented:
I apologize for any confusion. I meant that the devices connected to the switch ports are not getting DHCP service. The DHCP server is receiving DHCP requests and sending out the assigned DHCP addresses but the devices are not receiving them.
AkinsdNetwork AdministratorCommented:
Thanks for clarifying.
Are all the ports are on the same vlan?
jonathanrkleinAuthor Commented:
The ports are on different vlan but the intended vlan is pointing to the dhcp server via dhcp relay.
Not sure what you refer to regarding DHCP relay, but you should have a statement in your config under the vlan interface for ip helper pointing to the DHCP server.

I also assume, since this is a PoE switch, you may have dual-mode ports (data and voice vlans on a single port)?
jonathanrkleinAuthor Commented:
yes the ports do have dual-mode ports
So, ensure the voice vlan is tagged and the data is untagged for these ports.

Also make sure you have ip helper addresses on each vlan interface (layer 3) pointing to your DHCP server.

One more thing, make sure there is s a default gateway defined on your switch so packets can return.
AkinsdNetwork AdministratorCommented:
The ports are on different vlan but the intended vlan is pointing to the dhcp server via dhcp relay.
You mentioned that some ports work and some don't work. What vlan are the ports that work on, and what vlans the ones that don't work on.

I'm thinking you may need to assign appropriate vlans to the ports
jonathanrkleinAuthor Commented:
First off, thank you for the responses. I really appreciate them. There is one vlan that can't get a DHCP IP address There are other vlans that also can't get an IP address but it mostly seems to happen on 1 switch.
Fred MarshallPrincipalCommented:
I'm confused still.  
The purpose of a VLAN is to run a separate subnet.  
A DHCP server generally provides addresses in a range on a single subnet.
So, it seems almost assured that the CHCP service will only be effective on a single subnet.
How is this different?

I can envision a DHCP server on one of the subnets.  Thus, one of the VLANs.
But one would not want devices on a different subnet (VLAN) to be getting addresses for the other subnet (VLAN).
Why is it that one DHCP server will work for 2 or more subnets?
A single DHCP server can serve many vlans (subnets) - it's done all the time.

There are some basic things to check over to make sure you have everything configured properly:

1) Each switch should have an IP address.  It's best practise to designate a vlan (other than vlan 1) to be your management vlan.  Although vlan 1 will work, all untagged traffic that doesn't belong to any vlan and is not specifically dropped will end up travelling around on that vlan by default - thus why it's a security issue.
2)  This management vlan needs to have a layer 3 interface set on each switch (which is where your IP address is).
3)  This vlan needs a default gateway on the same subnet (which is generally the layer 3 address on the routing device).
4)  All other layer 2 vlans on your switches may not need layer 3 addresses until they reach the routing device.
5)  All layer 2 vlans must exist on every switch you want it to traverse.
6)  You can use multiple layer 2 interfaces to move the data from switch to switch within the same vlan, but the normal way is to use a single "trunk" port between switches that "tags" the vlans that will traverse this link.
7)  Once the vlans aggregate on the routing device there needs to be a layer 3 (ip address) for each vlan so that they can communicate.  This layer 3 address is normally the gateway for each vlan as specified in the DHCP scope option for "router".
8)  The routing device either needs to enable routing (basic) or use a routing protocol (advanced) to allow all the vlans to communicate.
9)  Each vlan's layer 3 interface (normally on the routing device end) needs an "ip helper" address that directs DHCP requests to your DHCP server's IP address.
10)  There must be a scope for each vlan you want to receive DHCP addressing and the scope options must be correct for the subnet associated with the vlans you are servicing.  This means the network, subnet mask and address scope must be correct or it will fail.

If all the above is proper, then it's possible the hardware is suspect, the interconnection cable is unreliable, or the config is corrupt and should be erased (reset) and rebuilt.

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AkinsdNetwork AdministratorCommented:
There is some information missing somewhere.
If you setup DHCP relay,  assigned ports to appropriate vlans and configured appropriate scope on the DHCP server, unless vlan pruning is configured on the trunk port, everything should work.

You are not providing enough information and everyone is just guessing here.
Post a diagram and the config of the switch that is not working
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