DHCP-address on interface (Catalyst 6500)

Unanswered Question
May 22nd, 2007

Hi all,

We would like to connect an ISP (small DSL line) to a 6500 switch. We have 2 kinds of Catalysts and thus 2 possibilities:

1) the old one where the Switch runs on CatOS and the MSFC runs on IOS (12.1(8a)E4)

2) the new one runs on IOS 12.2(18)SXF

Possibility 1) we would configure an interface Vlan x and the address of this interface has to be assigned with DHCP (ISP requires this). On the switch then we would assign a port in this particular vlan and connect the modem to this port.

Problem: it is not possible to assign a DHCP-address to this interface.

Possibility 2) we would like to make an interface a L3 interface (no switchport) and give this interface an address through DHCP as in 1). But this is not possible, we can only assign an interface a static address.

Routing and NATing is also configured so the only problem is the dynamic address-assignment.

Is there any solution for this problem?

Thank you.


I have this problem too.
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bjornarsb Tue, 05/22/2007 - 06:03


It depends on your L3 switch.

On our WS-C3560G-24TS it is possible:

(config)#interface Vlan1000

(config-if)#ip add

(config-if)#ip address ?

A.B.C.D IP address

dhcp IP Address negotiated via DHCP

But non of our 6500s support it.

If you pay a small fee, they will give you a static one.



SDWorx_2 Tue, 05/22/2007 - 06:23


Another remark. We indeed have a static IP address and are paying this small fee. But this address is assigned through DHCP, the ISP knows our MAC-address and always gives us the same address.

I asked if it is possible to configure this address statically on our interface but they do not recommend this.



ankbhasi Tue, 05/22/2007 - 06:09

Hi Jan,

Can you give this a try

interface vlan x

ip address dhcp



no switchport

ip address dhcp

I believe it should work as it works on some lower end switch like 3750 but I will confirm this for you if Cat6k supports or not.



*Pls rate all helpfull post

SDWorx_2 Tue, 05/22/2007 - 06:16


Thanks for the quick responses.

The problem is there the IOS's seem to not support DHCP:


RANT_05_1(config)#int vlan 224

RANT_05_1(config-if)#ip address ?

A.B.C.D IP address

RANT_05_1(config-if)#ip address


SANT_45(config)#int gi11/27

SANT_45(config-if)#no switchport

SANT_45(config-if)#ip address ?

A.B.C.D IP address

SANT_45(config-if)#ip address

On routers we can configure the interfaces to receive their addresses through DHCP, but appearantly not on these Catalysts...



mohammedmahmoud Tue, 05/22/2007 - 06:18


I think that i've done this before on my 6K which is running IOS Release 12.2(18)SXF:

interface x

no switchport

ip address dhcp

HTH, please do rate all helpful replies,

Mohammed Mahmoud.

mohammedmahmoud Tue, 05/22/2007 - 06:23


Ok i remembered something about not all SPAs supporting the DHCP option under the interface.

HTH, please do rate all helpful replies,

Mohammed Mahmoud.


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