SG200-26 DHCP issue

[pedrick01]

Good Day,  I have a SG200-26 switch for a small business network that is giving me some odd issue.  We have a Cable Broadband connection for Internet service, (Cable Modem), that is plugged into a Gig port on the switch.  In our office space we have a Wireless Router for PC's connected to the SG200-26, and IP phones connected via Ethernet to the SG200-26.  The PC's in the office gain access via Wireless -> SG200-26 -> Cable Modem.

DHCP is provided by the cable modem for devices connected to the SG200-26, Devices on the Wireless get an IP from the wireless AP.

I have an IP phone and a laptop that will not pick up a DHCP address when connected to the SG200-26 switch.  I know that DHCP is working on the cable modem because the 6 - 10 devices are all working without issue.  The phone that will not get an address is new.  When I connect it to the Wireless router to test it, it gets an IP address with no issue.  I connect my laptop to the switch and do not get a DHCP address.  I rebooted the cable modem, all devices got an IP except this new phone.  I would blame the phone, but my laptop is not getting and address either.  I also checked with the cable company and they do not see any errors on the Cable Modem.

Help.  I am concerned that the switch is not passing the DHCP information and when the current IP address leases expire I could have a dead network.

Thank You

[Charles Hill]

Can you access the cable modem and verify the dhcp range?

Are there available ip addresses?

Have you tried plugging your laptop or phone into more than one port on the SG200-26 switch?

Plug your phone/laptop in a working port and test?

[pedrick01]

I was able to identify the DHCP range,  it is actually on a MAN with over a 1000 addresses 255.255.224.0 subnet mask.  I ran an IP scanner and there are only ~ 100 active IP addresses on that network, so I don't think it a range issue.

I did plug the phone in directly to a few open ports on the SG200-26 switch and got the same behavior.  

I connected my laptop into a working port, No IP address.

The other complication is that I don't know the IP of the switch because it got and address from the cable modem and I am not sure, without a console port, how to determine the IP.

Thanks for the assistance.

[cchamorr]

Hello,

I was reading your post and I think this is the best way to find out any information about your switch.

What you want to do is to download the CIsco FindIt tool. here is the link:

https://software.cisco.com/download/release.html?mdfid=282833583&softwareid=283453334&release=1.1.0&relind=AVAILABLE&rellifecycle=&reltype=latest&i=rs

If you are on windows, it will install a toolbar on Internet explorer that will allow you to find any Small Business Devices on your network (Even if not on the same subnet)

If you are on MAC you will need to install firefox before you can install the XPI add-on so you can use it.

I hope this is helpful.

[pedrick01]

Thank You for the tool,  that did help.  Now I need to figure out why DHCP requests are not making it to this laptop.  I had to statically assign the IP to connect to the switch.  Again, everything else on the network seems to be working correctly.

[cchamorr]

I'm glad you were able to use the software to find the address of the switch.

To be honest with you, I cannot imagine anything that could be failing on the switch for you to experience this behavior, there must be something else on the modem or the network that we are not considering.

In any case, as best practice, I would recommend to check the firmware and boot code version on the switch and to update them if they are not at the latest.

Here is a link to the download page for this switch:

https://software.cisco.com/download/release.html?mdfid=283771818&softwareid=282463182&release=1.4.0.88&relind=AVAILABLE&rellifecycle=&reltype=latest

Make sure to step up the upgrade and don't jump straight to the latest, that could break the switch.

Please let us know if you have any questions.

[pedrick01]

I am stumped also.  I am now able to get into the switch with the help of the findit tool.  The IP does not seem to be in my subnet though.  Does the switch automatically establish a NAT?  I would not think so.  The devices connected to the switch are on 72.35.128.1  - 255.255.240.0  and the switch got the IP via DHCP from the cable modem of 67.204.152.1  doesn't make sense to me.

it is on 1.3.0.62 so I can step through each update if you think that is the next step.

Thank You

[cchamorr]

This is very unusual.

I have a very strong feeling the issue is related to the modem. I can see that, according to the subnet mask, you will have many more addresses, but it appears to me you are getting public ip addresses on your entire network and that is very atypical.

Now, the second address you are getting is also a public IP address, and I believe that the modem gave it to you also, maybe because you ran out of your allotted addresses on the first subnet.

One again, all of this is speculation but I think that I may be right.

In regards to the switch, two things:

1- I really don't think this will fix your issue as I'm convinced the problem is not on the switch.

2- It is still a good idea to upgrade the firmware to the latest to make it more reliable and get new features.

With that being said, you can go to 1.3.5.58 and upgrade both firmware and boot code and then go to 1.4.0.8 and upgrade just firmware.

[pedrick01]

That makes sense.  Kind of.  Not sure why the modem would hand out a different network IP, and if it were, why my laptop and new phone would get No address as opposed to an IP from the "other" network.

I do have a question that may address this.  Will this work as an alternative.

Cable Modem (with a static IP for the Switch)  Wireless Cisco E2500 router connected to the switch with DHCP enabled.  Will it provide addresses to the other devices connected to the Cisco SG200-26?  Should it be plugged into the other Gig Interface Port to enable that?

Thank You,  I am more familiar with 6905 switches behind traditional routers as opposed to small business type config

[cchamorr · Accepted solution]

Hello,

If you have that option then the best setup is to connect your E2500 directly to the modem with a Static IP address, then connect the switch to the E2500 and connect all the device from the switch. This will work like a charm.

Please let me know if you have more questions.

[pedrick01]

That would be great.  But, the switch room is on the 1st floor and we are on the 3rd floor, and today the Cisco E2500 provides wireless connectivity to the suite.  So I was wondering if it could be an end point on the switch and still provide DHCP to the network?  All Ethernet comes Only from the 1st floor closet.  :(, which is where the Cable Demark is as well as the SG200-26

[cchamorr]

Hello, 

The configuration you are suggesting to implement it is not what I would suggest but I understand your limitations and I think that it will work, but I will only recommended as a temporary solution.

Ideally you would like to have a router in between your modem and the switch.

Please let us know what you decide to do.

[pedrick01]

Good Morning,  I was able to configure the network in a model similar to your suggestion.  I was able to locate a central location in the 3rd floor suite that had 2 network locations in close proximity.  This enabled strong WiFi signal.

The config ended up:  Cable Modem, Patch panel - from closet to 3rd floor (building wire Ethernet) - E2500 router Internet port, from E2500 Ethernet port back down stairs (Building wire Ethernet) - SG200-26 switch Interface port - all Ethernet connected devices connected to SG200-26 via building wire Ethernet.

So the only addresses via DHCP are now coming from the E2500.  The only thing that I will need to keep an eye on is ensuring the E2500 does not become a bottleneck as it is now in the path for all traffic.

Thank You for the suggestions and assistance.

[cchamorr]

It was a pleasure.

I'm glad it is working for you now.

Thank you for taking the time to let us know what you did to get it to work.

Keep posting if you have any more questions