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tg1

join:2003-08-16
New Hyde Park, NY

[OOL] Cisco 2950 and OOL

Hey Everyone,

I was wondering if anyone had problems with getting DHCP leases from OOL with a 2950 between a router and a Arris modem on a VLAN? I'm trying to create a lab at home but I keep running into this issue of not receiving an IP.



jaa
Premium
join:2000-06-13
kudos:2
Reviews:
·Vonage
·Optimum Online

I doubt anyone has that configuration. Is there a reason you want a switch between the router and modem?

I'm not surprised you are having trouble. The Arris will only give out one DHCP lease, and does it by "learning" the MAC address of the client device - may be having difficulty with the VLAN.
--
NOTHING justifies terrorism. We don't negotiate with terrorists. Those that support terrorists are terrorists.


tg1

join:2003-08-16
New Hyde Park, NY

said by jaa:

I doubt anyone has that configuration. Is there a reason you want a switch between the router and modem?

I'm not surprised you are having trouble. The Arris will only give out one DHCP lease, and does it by "learning" the MAC address of the client device - may be having difficulty with the VLAN.

Mainly to learn but also to see if I could VLAN it out to a PFsense VM that threw on a Xenserver box I made. If PFsense doesn't work too well...this setup should be flexible enough try another OS.

I did find this though
»hardforum.com/showthread.php?t=1625683
»www.networkworld.com/community/node/25576

I'm going to try to disable STP on my WAN VLAN to see if it does the trick.


jaa
Premium
join:2000-06-13
kudos:2
Reviews:
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·Optimum Online

If I understand correctly you are trying to use the VLAN to change whether the modem is connected to your router or the Xenserver box. Not sure how it would work, even if you resolve the DHCP issue. Won't you have to powercycle the Arris to switch the attached device?

Not sure what you have tried, but when it does not get a DHCP address did you try powercycling the modem, and see if that allows it to pull an address through the VLAN?
--
NOTHING justifies terrorism. We don't negotiate with terrorists. Those that support terrorists are terrorists.


Network Guy
Premium
join:2000-08-25
New York
kudos:2
reply to tg1

Cisco 2950 is a layer 2 switch, not layer 3. You will never get an IP address passed from the Arris modem to any device connected to that switch considering it does not forward layer 3 traffic without a router directly connected to intervene.


vabello

join:2011-05-05
Hackettstown, NJ
reply to tg1

Be certain you don't have DHCP snooping enabled on the switch ports.


vabello

join:2011-05-05
Hackettstown, NJ
reply to Network Guy

said by Network Guy:

Cisco 2950 is a layer 2 switch, not layer 3. You will never get an IP address passed from the Arris modem to any device connected to that switch considering it does not forward layer 3 traffic without a router directly connected to intervene.

He stated he was using a PFsense VM for routing.

TheWiseGuy
Dog And Butterfly
Premium,MVM
join:2002-07-04
East Stroudsburg, PA
kudos:3
Reviews:
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reply to tg1

The Modem will pick up the MAC address of the switch as the connected device- CPE(customer premise equipment). When your CPE, router/computer, attempts to do DHCP it will send a different MAC address inside the DHCP packet. I could be wrong, but for security purposes I suspect the DHCP server will not allocate an IP address, if the MAC of the CPE that it has from the modem, does not match the MAC address of the CPE in the DHCP packet.
--
Warning, If you post nonsense and use misinformation and are here to argue based on those methods, you will be put on ignore.


Network Guy
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New York
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reply to vabello

said by vabello:


He stated he was using a PFsense VM for routing.

Correct. And for intents and purposes of layer 3 traffic, PFsense will do it's job.

But the modem is still only going to recognize the burned-in MAC address of the switch port it connects to, and it will go nowhere as it attempts to hand it an IP address that the switch port cannot "speak" to.

With a 2950, at most all you will accomplish is assigning VLAN1 a dynamic IP address. He will be able to remotely manage the switch from anywhere on the Internet. That's about it.

TheWiseGuy
Dog And Butterfly
Premium,MVM
join:2002-07-04
East Stroudsburg, PA
kudos:3
Reviews:
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The Modem is a transparent Bridge. It also does not assign the IP address and should pass on Broadcast DHCP responses from the CMTS which I am fairly certain acts as a DHCP relay server. The switch should also pass along the Broadcast DHCP responses.

»support.microsoft.com/kb/169289
--
Warning, If you post nonsense and use misinformation and are here to argue based on those methods, you will be put on ignore.


majortom1029

join:2006-10-19
Lindenhurst, NY
kudos:1
reply to tg1

If you only have a dhcp address from cablevision why do you need a switch between the router and the cablemodem?

If it was a static ip I can see by why if you only have the dhcp address?



Thinkdiff
Premium,MVM
join:2001-08-07
Bronx, NY
kudos:11
reply to tg1

Just to clear up some info - I used a DPC2100 modem with a VLAN capable switch to do exactly what the OP is trying to do. One port on my switch was set to VLAN1, another to VLAN2, and a third to the trunk of VLAN1/2. The modem plugged into VLAN1 and I assigned a virtual interface to pfSense on ESXi for each VLAN. it had no trouble picking up an IP address with the correct MAC address.

I don't see how this is any different than using a regular switch between the modem and your PC, which should work. If you're having trouble, make sure the switch is not running DHCP of its own for some reason on that VLAN. You can also assign a static IP on the pfSense interface in the 192.168.100.0/24 space and connect to the modem's internal page. It should list the "learned" CPE MAC address.
--
University of Southern California - Fight On!


Network Guy
Premium
join:2000-08-25
New York
kudos:2

I guess there's one way to find out. I tried this very same thing long time ago and had difficulty getting the modem to assign me an address.


vabello

join:2011-05-05
Hackettstown, NJ
reply to Network Guy

said by Network Guy:

said by vabello:


He stated he was using a PFsense VM for routing.

Correct. And for intents and purposes of layer 3 traffic, PFsense will do it's job.

But the modem is still only going to recognize the burned-in MAC address of the switch port it connects to, and it will go nowhere as it attempts to hand it an IP address that the switch port cannot "speak" to.

With a 2950, at most all you will accomplish is assigning VLAN1 a dynamic IP address. He will be able to remotely manage the switch from anywhere on the Internet. That's about it.

That's not how it works.

Seeing as the switch is only capable of layer 2, you cannot assign an IP address to an SVI as SVI's don't exist, just a management IP. In a pure layer 2 topology on a VLAN with both PFsense and the cable modem attached to ports on that VLAN, the Ethernet frames would be bridged by the 2950 between devices. PFsense would broadcast a DHCP request which would be forwarded to the cable modem via the switch, the cable modem would in turn bridge the DHCP request to the CMTS or whatever acts as the DHCP server on the cable headend... which would assign an IP to pfSense and associate the MAC address of PFsense with the DHCP lease. The switch MAC address doesn't come into play since it is a transparent bridge and no packets are originating or terminating on it. Likewise the switch will learn and populate it's own MAC address table with the MAC addresses of both the modem and PFsense learned in the process.

So this should work fine. The modem is none the wiser with any number of layer 2 bridge between it and the DHCP lease requester... they're just bridged Ethernet frames.

vabello

join:2011-05-05
Hackettstown, NJ
reply to Network Guy

I can try it here... I have a 2821 router and a 3750x switch I could configure in a similar setup. I don't see why it wouldn't work.


vabello

join:2011-05-05
Hackettstown, NJ
reply to majortom1029

port mirroring on the switch to an IPS or other device for packet capturing and/or classification and troubleshooting is one thought...