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Webcast-Catalyst9k
New Member

DHCP issue between Switch and router

Hello all.

I need some help.

I have a cisco 3560 24PS and its connected to two ADSL broard band routers.

one is a personal broadband line using a Billion ADSL broadband router, and the other is a business broardband line using BT's 2wire broadband line.

on the Billion routers i have various things attached like a NAS and a printers, both wired connections. then i have laptops and phones that connect over wifi, so its configured to act as a DHCP server

the only thing conncted to my 2wire router is my company's laptop (wired or wifi depending on where i'm working from), so again i have it working as a dhcp server.

The switch is configured with multiple vlans, with dhcp scopes assigned for each vlan.

I have a static route pointing all traffic to my Billion ADSL for internet connectivity.

The problem i'm having is that when i turn on the cisco switch, all wifi conected devices loose their conection.

only 2 things get it working again, a reboot of the router, or disabling then enabling the DHCP service on the router.

upon further analysis i was able to find out that the devices were not able to pick up an address from the router.

again i looked deeper into this and i can see the following on logs of my router:

Jul 24 09:32:34 DHCP SERVER: DHCP ack to 7c:61:93:30:79:29

Jul 24 11:53:31 DHCP SERVER: DHCPINFORM from 192.168.1.9

Jul 24 11:53:46 DHCP SERVER: DHCPINFORM from 192.168.1.9

Jul 24 11:54:01 DHCP SERVER: DHCPDISCOVER from 00:11:bb:b7:fe:00 via 192.168.1.9

Jul 24 11:54:02 DHCP SERVER: DHCP offer to 00:11:bb:b7:fe:00

Jul 24 11:54:02 DHCP SERVER: Exiting as another DHCP server is found

so it seems that the router tuns off its DHCP capabilities because it detects that my Cisco switch is running DHCP services.

I'm at a loss here. I need to figure out how to keep the billion routers DHCP running when ever the switch is turned on.

can someone help out here? is there a way of filtering out any DHCP chat from the switch to the router?

Attached is my current config.

Everyone's tags (3)
7 REPLIES
Purple

Re: DHCP issue between Switch and router

Hi,

The question is how do the router knows there is another dhcp server and is it the 3560 they see?

if the clients and the router are in different subnet then without any dhcp relay agent the discover messages can't hit the router.

Regards.

Alain.

Don't forget to rate helpful posts.

Don't forget to rate helpful posts.
New Member

DHCP issue between Switch and router

Hi,

i'v checked the log on the router and i see the following message:

Jul 24 11:53:31 DHCP SERVER: DHCPINFORM from 192.168.1.9

192.168.1.9 is the layer 3 interface on my 3560 that connects to the router.

so it seems that the switch is advertising its DHCP capabilities out to my router.

does this make sense?

Purple

DHCP issue between Switch and router

Hi,

according to rfc 2131 the DHCP INFORM message is from a client(having already an ip address configured by other means) to a server for requesting options. and the DHCP DISCOVER confirms this is considered a client but it shouldn't be

I'm gonna take a closer look at your config and post your topology also.

Regards.

Alain.

Don't forget to rate helpful posts.

Don't forget to rate helpful posts.
New Member

DHCP issue between Switch and router

Thanks in advace Alain.

here is a layer2/layer3 topology i put together for my own benefit when i started the troubleshooting.

New Member

DHCP issue between Switch and router

Alain,

Did you get a chance to review my config? any ideas whats going on?

the DHCPINFORM is still causing me problems.

Regards


Sailesh

New Member

DHCP issue between Switch and router

Hi,

If I understood your first post correctly, there is 2 DHCP server (your two routers). What address are they offering ?

Why you don't configure the DHCP on 3560 and turn off DHCP on your routeurs ? (at least to test if it's work).

You can check "ip helper-address", to activate DHCP relay between different subnet or network.

Regards,

New Member

DHCP issue between Switch and router

Hi Jonathan

In reality there are 3 DHCP servers, 2 routers + 3560 switch.

the home router (Billion) is offering 192.168.1.0/24

the Business router (BT 2WIRE) is offereing 192.168.100.0/24

the 3560 switch has 4 DHCP scopes configured as follows:

192.168.2.0/24 (VLAN20)

192.168.3.0/24 (VLAN30)

192.168.4.0/24 (VLAN40)

192.168.100.0/24 (VLAN100)

I need to make sure that the Billion router continues to allocate addresses so that my personel devices can connect to the internet/printer/NAS etc.

i need to make sure that the BT 2wire router contines to allocate addresses so that my business laptop and phone can connected to the internet.

I'm not sure what you mean but check IP helper-address. I'm not using this funtions as the scopes have been defined on the local switch.

Thanks

Sailesh

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