DHCP Issues across networks

wilcd003 · ‎06-19-2023

Hi Guys,

I have a uni assignment and for the life of me I cannot figure out why DHCP isn't working across routers. I have a multi-layer Switch configured as ML_D1 that acts as a DHCP server that two other locations on separate networks will use ML_D1 their DHCP server. I have configured everything I can think of to make sure that the other locations can get their IP addresses through DHCP.

PC's that are connected to basic Switches that are directly connected to ML_D1 can get their IP addresses through DHCP but PC's that are connected to ML_D1 through routers are the ones that can't get their IP addresses.

Attached is the packet tracer and an Excel spreadsheet with IP addressing information included.

Any pointers would be much appreciated.

Cheers

Valerio Passeri · ‎06-19-2023

I didn't check your file, but reading your question there's something that sounds weird to me.

"...

I have a uni assignment and for the life of me I cannot figure out why DHCP isn't working across routers.

...

PC's that are connected to basic Switches that are directly connected to ML_D1 can get their IP addresses through DHCP but PC's that are connected to ML_D1 through routers are the ones that can't get their IP addresses.

..."

The first phase of the DORA process (the D step - DHCP DISCOVERY) happens with a broadcast message sent by a PC to any DHCP server present on its subnet.

Do L2 switches forward broadcast messages? Yes.

Do routers forward broadcast messages, to a DHCP server an a different subnet? No.

Did you configure the "ip helper-address" feature on your router?

Hope this helps.

Ciao,

Valerio

Ciao,

Valerio

wilcd003 · ‎06-19-2023

Thank you for the reply I appreciate it

Flavio Miranda · ‎06-19-2023

Hi @wilcd003

I looked at you file and there are a few things that is not right. But, I believe it will not work like this on PacketTracer. It might be some restriction in using the switch as remote DHCP server. As you already pointed out, the local DHCP service is working but the remote is not. You should consider to use a DHCP server instead.

Well, as you already pointed out, the local DHCP service is working. I will not comment on that.

For remote, DHCP -

ML_D1:

You had to have excluded IP address for every remote network. You only have for local netowrk.

default-router is wrong. The default gateway for host on vlan 70 must be 10.2.70.1 and not 10.254.2.2.

i.e:

ip dhcp pool AdminKensington

network 10.2.120.0 255.255.255.0

default-router 10.2.120.1

dns-server 8.8.8.8

KEN_D1#

You need the command "ip helper-address 10.254.3.2" for each Vlan.

You need to advertise your interfaces vlan on the OSPF. Meaning, you need to ping the IP address 10.254.3.2 from each vlan.

I fix this with the command "redistribut connect subnets" but you can also add every interface vlan on the OSPF process.

After fixing all that, I am able to ping from IT Support Kensington to the "so called" DHCP server:

C:\>ping 10.254.3.2

Pinging 10.254.3.2 with 32 bytes of data:

Reply from 10.254.3.2: bytes=32 time=1ms TTL=252

Reply from 10.254.3.2: bytes=32 time=10ms TTL=252

Reply from 10.254.3.2: bytes=32 time=1ms TTL=252

Which mean, the PC has connectivity with the "so called" DHCP server and it should be able to get IP address. But, if you change the PC to DHCP option, it fail getting IP address.

My advise is, Build up a DHCP server and use it. Switch as DHCP server is not right either in simulator or in real network.

wilcd003 · ‎06-19-2023

Hi Flavio,

Thank you so much for your help. I didn't chose for the multi layer switch to be the DHCP sever thats just what the assignment told me to do but once again thank you for your help.

Kind regards,

Flavio Miranda · ‎06-19-2023

You are welcome.

Well, if you have no alternative, then, try to fix those points I mentined. I will try to take look further and see If I missed something.

wilcd003 · ‎06-19-2023

Hi Flavio,

Just following up on this comment. Wondering if you figured out anything else.

Cheers,

Flavio Miranda · ‎06-22-2023

Hello @wilcd003

Yes, I did some dig on this case and it seems the Layer3 is not a good alternative. On this file attached, I installed a DHCP server on the Vlan 130 in Mawson Lakes HQ (10.1..130.4) and we can get IP on Vlan130 from the others sites just fine.

But, if the DHCP is from Layer3 the network does not work. Sometimes work sometimes not. I gave up to understand why. I believe must be some bug or it is not supported at all. I believe they built this to provide IP only on the attached Vlans, not remotelly.