Devices not reaching the DHCP server... bad switch gateway to blame?
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I've got a situation where a couple of switches were configured with an old gateway that is no longer in service. We've been seeing devices unable to reach the DHCP server, so I am curious how that works. Could the switch's bad configuration prevent all attached devices from reaching their correct gateway, or would the device configuration (correct) still find its way?
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@art_of_shred said in Devices not reaching the DHCP server... bad switch gateway to blame?:
I've got a situation where a couple of switches were configured with an old gateway that is no longer in service. We've been seeing devices unable to reach the DHCP server, so I am curious how that works. Could the switch's bad configuration prevent all attached devices from reaching their correct gateway, or would the device configuration (correct) still find its way?
If you're not using an IP-helper on the switch, then the traffic should find its own way to the DHCP server.
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I should note that the gateway and DHCP server are one and the same.
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Still shouldn't matter. Check the VLANs and all that?
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Flat network. No VLANs.
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Assign a static and see if you can ping the DHCP server / gateway ?
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Depends on how the switch is running but if you have Macros on the switch or it's in Layer3 mode (even if on the same subnet) it can affect things.
Also if this is cisco switch you might want to enable RSTP (spanning-tree mode rapid-pvst)
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These are Netgear s3300 switches.
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@Jason said in Devices not reaching the DHCP server... bad switch gateway to blame?:
Depends on how the switch is running but if you have Macros on the switch or it's in Layer3 mode (even if on the same subnet) it can affect things.
Also if this is cisco switch you might want to enable RSTP (spanning-tree mode rapid-pvst)
what is macro on a switch?
What in a layer 3 mode would prevent a DHCP request from making it through a switch when we know there are no VLANs?
What does STP have to do with this? Clearly the PC isn't a loopback to the switch. -
@Dashrender said in Devices not reaching the DHCP server... bad switch gateway to blame?:
@Jason said in Devices not reaching the DHCP server... bad switch gateway to blame?:
Depends on how the switch is running but if you have Macros on the switch or it's in Layer3 mode (even if on the same subnet) it can affect things.
Also if this is cisco switch you might want to enable RSTP (spanning-tree mode rapid-pvst)
what is macro on a switch?
What in a layer 3 mode would prevent a DHCP request from making it through a switch when we know there are no VLANs?
What does STP have to do with this? Clearly the PC isn't a loopback to the switch.If the switch is a layer 3 device that means it has enough brains to route traffic on its own. DHCP traffic can be routed wherever it wants if it is programmed to do so.
@art_of_shred That said, I would just default the damned switches. They are not doing anything important based on your description. By default they will switch traffic so who cares if anything else was configured before.
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@Dashrender said in Devices not reaching the DHCP server... bad switch gateway to blame?:
@Jason said in Devices not reaching the DHCP server... bad switch gateway to blame?:
Depends on how the switch is running but if you have Macros on the switch or it's in Layer3 mode (even if on the same subnet) it can affect things.
Also if this is cisco switch you might want to enable RSTP (spanning-tree mode rapid-pvst)
what is macro on a switch?
What in a layer 3 mode would prevent a DHCP request from making it through a switch when we know there are no VLANs?
What does STP have to do with this? Clearly the PC isn't a loopback to the switch.@Dashrender said in Devices not reaching the DHCP server... bad switch gateway to blame?:
@Jason said in Devices not reaching the DHCP server... bad switch gateway to blame?:
Depends on how the switch is running but if you have Macros on the switch or it's in Layer3 mode (even if on the same subnet) it can affect things.
Also if this is cisco switch you might want to enable RSTP (spanning-tree mode rapid-pvst)
what is macro on a switch?
Some SDN switches run a full blown OS, you can do pretty much everything on them.
What in a layer 3 mode would prevent a DHCP request from making it through a switch when we know there are no VLANs?
A firewall? For example: https://www.juniper.net/documentation/en_US/junos13.3/topics/concept/firewall-filter-ex-series-overview.html
What does STP have to do with this? Clearly the PC isn't a loopback to the switch.
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I'm on the call looking at this. What a pain. I'll try to post what we are finding....
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DHCP Renewals appear to be working just fine from anywhere. New DHCP Leases appear to never work, possibly not for a week or more. It is only that today there are new things going onto the network.
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Looked at the logs, there does not appear to be any issues with the pool being exhausted. Both logs and manually looking at DHCP Service confirm that IP addresses are available.
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Could you pinpoint the place where the packets get dropped? Wireshark before / behind the switch?
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@thwr said in Devices not reaching the DHCP server... bad switch gateway to blame?:
Could you pinpoint the place where the packets get dropped? Wireshark before / behind the switch?
Potentially, I just mentioned that on the call as a potential next step.
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@scottalanmiller said in Devices not reaching the DHCP server... bad switch gateway to blame?:
@thwr said in Devices not reaching the DHCP server... bad switch gateway to blame?:
Could you pinpoint the place where the packets get dropped? Wireshark before / behind the switch?
Potentially, I just mentioned that on the call as a potential next step.
Probably your best bet. Don't know the Netgears or how smart (dumb) they are, but you never know.
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Is this a wired or wireless connection through this switch having a problem?
I wonder if there is a latency issue on that specif cable?
tried restarting the DHCP service - Gene said it's on the same device as the gateway, so probably not a windows box.. so might require a reboot of the whole gateway.
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@Dashrender said in Devices not reaching the DHCP server... bad switch gateway to blame?:
Is this a wired or wireless connection through this switch having a problem?
I wonder if there is a latency issue on that specif cable?
tried restarting the DHCP service - Gene said it's on the same device as the gateway, so probably not a windows box.. so might require a reboot of the whole gateway.
Wired for sure, but AFAIK eveyrthing has been tested at this point.
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@Dashrender said in Devices not reaching the DHCP server... bad switch gateway to blame?:
tried restarting the DHCP service - Gene said it's on the same device as the gateway, so probably not a windows box.. so might require a reboot of the whole gateway.
It's not. It's on the main AD server along with AD and DNS.
