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681
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4
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DHCP from Service Provider Not Working

mtnchaser
Beginner
Beginner

Good Day,

Fighting an issue with DHCP from the service provider. Just not getting it.

So I have  a 2600 that I have configured three sub interfaces on. FA0.0.1 is set for DHCP and supports VLAN 1. FA 0/0.2 for Voice, FA 0/0.3 for Data.

I have this router interface plugged into FA 0/24 on my 3550 and the 3550 is configured as a dot1Q trunk (I have attached configs for RTR and SW).

I have most ports configured as access VLAN 1 which is where i have my ISP connection plugged FA0/1 on the 3550.

When I connect the service provider link FA0/0.1 never picks up an address. If I take my internal DHCP server and connect it to FA0/1 of the 3550 it snags an address almost immediately.

In my mind this validates that my config is fine. I also took the same cable from the service provider cable modem and connected it to my laptop and the laptop is pulling DHCP.

Been pulling my hair out on this for a few hours now. Any advice? Am I missing something really stupid?

Thanks in advance.

Joel

4 Replies 4

Hello Joel,

This is possibly due to the ISP having registered the laptop mac address, try spoofing your router wan interface with the laptops mac.

int (wan interface)
mac  xxxx.xxxx.xxxx

res

Paul

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Please rate and mark as an accepted solution if you have found any of the information provided useful.
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Kind Regards
Paul

Paul,

Thanks for the reply man. Yeah, I had thought this might be the problem. Thing is, I can attach multiple devices to the cable modem and Comcast just hands out a different lease each time.

Do you think it has something to do with the cable modem seeing the 3550? I wouldn't think so but am going to test on another switch quick:  Comcast into switch and laptop into switch and see if the laptop still gets a public lease.

Hello Joel,

On 3550.

1)L2 vlans propergated? if not

vlan1,2,3

exit

2)int vlan 1

ip helper-addrress x.x.x.x ( shouldnt be need)

3) disable routing and add default-gateway

2600

ip route 0.0.0.0 0.0.0.0 dhcp

res

Paul

Please don't forget to rate this post if it has been helpful.


Please rate and mark as an accepted solution if you have found any of the information provided useful.
This then could assist others on these forums to find a valuable answer and broadens the community’s global network.

Kind Regards
Paul

Thanks Paul...

Problem solved :-) I went ahead and got a static IP. Got tired of fighting them. It's clearly a Mac issue. The cable modem snags the first mac attached (the switchport) and only will serve up DHCP to that mac. So, my test of going to another switch, plugging in the cable modem and my PC to that flat switch validated that it's the SP not dishing up the DHCP. My config was fine.  Appreciate the help on this man.

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