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Cisco SLM2008 filters out DHCP Discover


                  Hello all,

I'm new on this site and I came here for help with my SLM2008 switches. I've been trying to solve the problem for quite a while, but so far I was unable to solve it. So here it is:

I'm trying to send IP TV from ADSL box (aka Freebox) located in a small annexe of the house where the telephone line enters. Whole house is wired with 1Gb CAT6 cable. I'm using this wiring to get internet connectivity in different parts of the house and this works great. Now the idea was to create a separate VLAN on my 3 SLM2008 switches to get IP TV stream from ADSL box to the media box (also provided by ISP, aka Freebox HD). Different VLAN configs did not work for me: Freebox HD would stuck on boot claiming it doesn't "see" the network. So then I've decided to go from simple things to more complex and removed the VLAN (came back to default config) and connected ADSL box with Freebox HD with just one SLM2008 in between. Surprisingly it did not work! Same error on Freebox HD! So I plugged the hub and started my WireShark to see what is going on.

First tried without the switch (just with a hub in the middle) and I saw successful connection:

1) Freebox HD send DHCP Discover

2) ADSL box responds with DHCP Offer

3) HD sends DHCP Request

4) ADSL issues a DHCP lease

Freebox HD is ready to show any channel from ISP.

Secodnly I plugged SLM2008 back and looked at traffic right after ADSL box: No DHCP packets seen at all!

Then I moved hub next to FreeBox HD and I saw it keeped sending DHCP Discover every 30 seconds or so....

And it was not getting reply back.

So my conclusion is that SLM2008 for yet unknown reason wou;d not let DHCP discover pass.... I've tried unmanaged NetGear switch and *boom* Freebox HD booted without a problem. I looked through SLM2008 settings and I have not found something relevant.

Does anyone know how to make my switch pass DHCP discover? If yes my next question would be how to configure VLAN on 3 switches to make it deliver IP TV to my TV set

Best regards,


11 Replies 11


Hi Nikolai,

Are there any VLAN’s currently configured on the SLM2008 and are all the ports set as access on the same VLAN?

Thank you,

Jason Nickle

Yes, SLM2008 is now in default state. It has only one VLAN defined ( '1') and all ports belong to VLAN 1 now.

I've also tracked BOOTP for Windows startup through SLM2008 since Windows somehow is able to get IP address from another DCHP server. Trace shows that there is no DCHP Discover packet in Windows <-> Linksys router DHCP server. It's starting right from DHCP Request and this seems to pass through okay.

I'm really puzzled! Seems like only DCHP Discover is not passing through.



This does seem unusual.  If you set a static IP on the device is it able to communicate with the network or does this fail as well?  Also have you tried setting up the SLM directly connected via a small patch cable to the DSL modem with the media device connected to the SLM with a small cable as well?  Also what about trying different ports on the SLM.

If the SLM is under warranty you may want to open a case with Cisco Small Business.  If you are outside of your warranty feel free to post a capture of the DHCP process and I will be happy to look at it.   

Jason Nickle

Freebox HD (the media box) is not configurable (at least by user). This is a device provided by Internet service provider in France. Therefore I'm not able to set static address. However, if I connect Freebox HD without Cisco SLM2008 in between it will boot okay, and if I rewire it with SLM2008 afterwards (without powering down) it will continue to work fine.

In other words, if Freebox HD got IP address it may work through cisco without the problem. However it's not possible to move Freebox HD next to the TV without unplugging it from the outlet

So the only problem as it seems is to get DHCP Discover packet through the Cisco. I have 3 SLM2008 switches in my installation. The last one was bought this year I believe, so must be still under warranty.

May be it's a good idea to swap them. However, I feel that this is not a hardware issue, but a firmware. And all of them has same firmware version 2. something...

I'll attach my traces when I get home.



P.S. I've just found this articles on the internet:

Someone has similar problem with different Cisco switch. They suggest the problem is about STP. I've seen some settings about STP in my configuration pages....

This weekend I've tried to swap switches and play with STP settings of the switch. Unfortunately all was unsuccessful.

STP was off by default on all ports, and Port fast option was ON. I've tried all combinations of these, but it did not solve the problem.

Media box still sending DHCP Discover packets that are NOT received by ADSL box. It seems that Cisco switch doesn't like this type of packets - I can't figure out why.

Here my WIRESHARK traces attached along with screen shot of the settings page. I feel that there is a bug in the firmware, but I can't really trace what is going on inside the switch. My firmware version is

Please help! Is there any work around this problem. I really want to use my media box....


Are there any news on this?

Have anyone looked into my traces?

Any advice on what steps I can take to fix DHCP Discover issue?

It looks like the solution would be to get rid of 3 CISCO switches and install something else that supports VLAN and lets DHCP discover to pass through.


How and where in your topology are you doing the pcaps?

Are you setting up a port mirror, or capturing directly on a device?

What ports on the switch that your are testing are connecting to what device and if you are doing a port mirror what port are you mirroring?


Randy Manthey

Cisco SBSC

Hi Randy!

Thanks a lot for a reply!!!

I'm using a Ethernet Hub in order to capture the traffic. It's mirroring all the ports. I've put the hub between media device (Freebox HD) and the Cisco Switch. Also tried to plug it in between ADSL box and the switch (i.e. straight after the ADSL modem), but there it's less informative, as you are not seeing any DHCP Discover that is sent by Freebox HD.

All traces attached in this discussion...

As I see the problem DHCP Discover packet never reaches ADSL box when SLM2008 is in between:

I see these packets sent from Freebox HD, but I see none of these behind Cisco Switch..

I do not know why but it looks that Freebox HD is using some extended DHCP protocol features. Windows laptop, for example,  seems to send straight 'DHCP Request' without preceeding DHCP Discover. Therefore problem is only happening for this specific device.

Any ideas on what I can do? I'm loosing hope here


Have you checked Storm control this is under Security tab.

Randy Manthey

Cisco SBSC

I haven't checked Storm Control until now...  I never touched that settings and they are in default state:

Broadcast rate: NO LIMIT

Multicast rate: NO LIMIT

Is there any other value I should try?


I would recommend calling into the support center at 1-866-606-1866 and a support engineer can troubleshoot further with you.

We will want to setup a port mirror to a pc so we can see the traffic as it hits the switchport from the FreeBox.

Randy Manthey

Cisco SBSC

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