Solved: Re: Interface not accepting DHCP address - Page 2

Adam_S · ‎07-27-2023

Have configured an interface on cisco 2811 for DHCP, it is up/up but not accepting an address. I've attached a basic config and DHCP debug. Any advice would be appreciated, any further info can be provided.

Adam_S · ‎07-28-2023

Thanks for that. I will try changing the config register and the rest of
the config is only 1/2 built at present. I removed the

ip nat

outside just
for testing purposes. I will let everyone know how it goes.

Peter Paluch · ‎07-28-2023

Hello Adam,

Perfect.

One more thing: When you change the config register and reload, then after the router boots up again, it will still read the

ip broadcast-address 0.0.0.0

commands from its startup-config. You will need to remove those commands from your running-config manually, but this time, the router should finally allow you to do that.

Don't forget to do this, though.

Best regards,
Peter

MHM Cisco World · ‎07-30-2023

Register number how it relate to

dhcp

assing ?

If he have issue with register then router never run any config??

Peter Paluch · ‎07-30-2023

MHM,

Register number how it relate to dhcp assing ?

Adam's configuration register forced the limited broadcast address to be

0.0.0.0 instead of 255.255.255.255

DHCP servers usually don't listen to packets with destination IP address

0.0.0.0

- they listen to packets with destination IP address

255.255.255.255

This caused Adam's router to send out its DHCP Discover messages to a "broadcast" IP address that the DHCP server was not listening on, and it ignored them. That is why it never responded back. It was visible in the debugs Adam shared earlier - there are traces of the router sending out Discover packets but never getting back an Offer.

If he have issue with register then router never run any config??

This is not true. Only specific bits in the configuration register - if set - will cause the router to ignore the NVRAM configuration. These are bits

15 (0x8000) and 6 (0x0040)

I have referred to a document describing the meaning of the bits in the configuration register above - this one: https://www.cisco.com/c/en/us/support/docs/routers/10000-series-routers/50421-config-register-use.html#toc-hId--612512991 - it's an interesting reading I can recommend.

Best regards,
Peter

MHM Cisco World · ‎07-30-2023

Sure I will read it.

Thanks

MHM

johnlloyd_13 · ‎07-27-2023

hi,

can you do this?

interface FastEthernet0/0
no ip address dhcp
no ip broadcast-address 0.0.0.0
ip address dhcp client-id FastEthernet0/0

Peter Paluch · ‎07-27-2023

Hey John,

Good eye on the

ip broadcast-address 0.0.0.0

A friendful comment: The

client-id Fa0/0

forces the DHCP client to identify itself by deriving its identity from the MAC address of the

Fa0/0

interface. This is required when the DHCP server is offering a stable IP to the client based on its identification. It is very safe to try this out, but if the DHCP server is not intentionally configured in advance for this identifier, then any should do, in which case the

client-id Fa0/0

is not required.

Let's see what Adam comes back with.

Best regards,
Peter

Adam_S · ‎07-27-2023

Thanks. I tried this, no difference.

MHM Cisco World · ‎07-27-2023

You try what ?

The client ID must be same as what

dhcp server

use you need to contact ISP ask them the clinet ID format they use. Then config your router to send same client ID.

Peter Paluch · ‎07-27-2023

Hello buddy,

I touched that topic here already: https://community.cisco.com/t5/routing/interface-not-accepting-dhcp-address/m-p/4893494/highlight/true#M386912

Unless we have an evidence that this is even an ISP scenario and not a lab test, and that the ISP is expecting a specific DHCP client ID to allocate IP addresses, we cannot say that Adam "must" do anything. Let's wait for Adam to come back with more information.

Best regards,
Peter

MHM Cisco World · ‎07-27-2023

Thanks I just clarify point here.

Have a nice summer

MHM

Adam_S · ‎07-27-2023

Right you are. Sorry i probably should of mentioned that this is indded a
test lab, connected to a cisco 3845. With the DHCP pool configured. I will
provide the request config in the next couple of days.

MHM Cisco World · ‎07-27-2023

if it LAB then check
the interface receive the DHCP discover must be in same subnet of DHCP POOL, check the subnet and subnet MASK
also if you use excluded

255.255.255.255 or x.x.x.255

remove it.
and check again.

have a nice summer
MHM

Georg Pauwen · ‎07-27-2023

Hello,

as mentioned by others, you are running a very old IOS version, which requires the DHCP broadcast to go to

0.0.0.0 instead of 255.255.255.255

I somewhere recall that the global config command

no ip source-route disables ip directed broadcasts

can you try that command and check if the interface still adds

ip directed-broadcast 0.0.0.0

to the interface ?

Peter Paluch · ‎07-27-2023

Hi Georg,

A very humble correction: The IOS on Adam's 2811 router is admittedly old but not tragically so - it's 12.4(15)T9. Back in my days at the university and NetAcad, we'd use even older versions (12.2, 12.3, 12.4) which would already be using

255.255.255.255, not 0.0.0.0, as the default limited-broadcast address

So I am very confident that the IOS version itself isn't the problem.

As for the

no ip source-route

that would be an interesting although unexpected correlation. Source routing is about the packet's sender explicitly putting the list of transit hops (up to 9) into the packet's header. But the packet still needs to be unicast... so I'm not sure why would that command have any bearing on the limited-broadcast address. But doesn't harm to test anyway, certainly!

I keep my hopes high about the config register and/or the complete configuration having something odd there.

Best regards,
Peter