Unable to make DHCP relay work

[tschertel]

I'm trying to use the DHCP relay feature, with no success.

I've already tried on a SG300, on a SG350X and now on a CBS350, so clearly I'm letting something pass.
Today I have the CBS350 in my lab. It's connected to the network (port 2, connected to another switch) and I have a cable directly connected from my computer to it (port 47).
I'm doing everything through the web interface, as I'm not a network expert.

That's what I'm doing:
- I create a new VLAN (103);
- I add the port 47 to VLAN 103;
- change port 2 to trunk;
- enable DHCP relay and add the DHCP server (snooping and option 82 are disabled);

The DHCP server has two scopes: the regular one (xxx.xxx.93.0/24) and the new one (xxx.xxx.103.0/24);

Then, when I try to get a new IP, I get a "time out" message:
"An error occurred while renewing interface Ethernet 2 : unable to contact your DHCP server. Request has timed out."

I've tried all tutorials from internet
I've also tried some from Cisco:
https://www.cisco.com/c/en/us/td/docs/switches/lan/csbms/CBS_250_350/CLI/cbs-350-cli-/dhcp-relay-commands.html
https://www.cisco.com/c/en/us/support/docs/smb/switches/cisco-350-series-managed-switches/smb5406-configure-dynamic-host-configuration-protocol-dhcp-snooping.html

Any help would be appreciated.

[balaji.bandi]

can you post show run (from console to look)

where is your DHCP Server ? what IP address ?

[tschertel]

Hi, here is the "show run" output:

config-file-header
switchf27fed
v3.1.1.7 / RCBS3.1_930_871_081
CLI v1.0
file SSD indicator encrypted
@
ssd-control-start
ssd config
ssd file passphrase control unrestricted
no ssd file integrity control
ssd-control-end cb0a3fdb1f3a1af4e4430033719968c0
!
!
unit-type-control-start
unit-type unit 1 network gi uplink te
unit-type unit 2 network gi uplink te
unit-type unit 3 network gi uplink te
unit-type unit 4 network gi uplink te
unit-type-control-end
!
vlan database
vlan 103
exit
voice vlan oui-table add 0001e3 Siemens_AG_phone
voice vlan oui-table add 00036b Cisco_phone
voice vlan oui-table add 00096e Avaya
voice vlan oui-table add 000fe2 H3C_Aolynk
voice vlan oui-table add 0060b9 Philips_and_NEC_AG_phone
voice vlan oui-table add 00d01e Pingtel_phone
voice vlan oui-table add 00e075 Polycom/Veritel_phone
voice vlan oui-table add 00e0bb 3Com_phone
ip dhcp relay address 192.168.93.3
ip dhcp relay enable
bonjour interface range vlan 1
hostname switchf27fed
username hi_there_:) password encrypted RWZYpw== privilege 15
ip ssh server
!
interface vlan 103
 name Servers
 ip address dhcp
 ip dhcp relay enable
!
interface GigabitEthernet1/0/2
 ip dhcp snooping trust
 switchport mode trunk
!
interface GigabitEthernet1/0/47
 ip dhcp snooping trust
 switchport access vlan 103
!
exit

[Flavio Miranda]

You did not say where the DHCP server is. 

If the DHCP server is connected in the other switch you did right putting the port 2 as trunk. However, unless the DHCP is on the vlan 103 on the other switch, DHCP requests will not get to them.

 Also, the port 2 is connect to which port on the other switch? Does that port is in trunk as well?

If DHCP server is in a different vlan, you need to do inter-vlan routing in order your computer reach the DHCP server.

[tschertel]

Hi Flavio.

Are you from Brazil?

The DHCP server is in our "regular" network (93.0).

The diagram would be something like this:

DHCP <-> switch <-> switch in lab <-> my computer

Just my lab's switch has a trunk port.

The other switch has no VLAN configured on it.

---

@Flavio Miranda wrote:

If DHCP server is in a different vlan, you need to do inter-vlan routing in order your computer reach the DHCP server.

---

The DHCP servers is on VLAN1.

[Flavio Miranda]

Yeah, Brazil. You?

DHCP <-> switch <-> switch in lab <-> my computer

Well, in this case, either you create the vlan 103 on the switch, put the DHCP server port on this vlan 103 or you need to do inter-vlan routing.

[tschertel]

Yeah, Brazil. You?

---

Me too

DHCP <-> switch <-> switch in lab <-> my computer

Well, in this case, either you create the vlan 103 on the switch, put the DHCP server port on this vlan 103 or you need to do inter-vlan routing.

---

OK. But this is not DHCP relay, right?

Don't we "have" DHCP relay, exactly to avoid having one DHCP server on each VLAN?

[Flavio Miranda]

 Oh yeah, sorry. You not just trying to make it work right, you want to test DHCP relay.

Well, did you followed a guide like this and did not work?

https://9to5it.com/cisco-sg300-configure-dhcp-relay-to-allow-dhcp-server-to-support-multiple-subnets/ 

[tschertel]

---

@Flavio Miranda wrote:

 Oh yeah, sorry. You not just trying to make it work right, you want to test DHCP relay.

Well, did you followed a guide like this and did not work?

https://9to5it.com/cisco-sg300-configure-dhcp-relay-to-allow-dhcp-server-to-support-multiple-subnets/ 

---

That's exactly what I did.

Maybe the problem is that I have more than one switch between the DHCP server and my computer.

[MHM Cisco World]

interface vlan 103
 name Servers
 ip address dhcp
 ip dhcp relay enable

OK, the Broadcast will convert to unicast and send to DHCP server BUT it need IP source and I see the SVI ip address also assign IP form DHCP.
first check if the SVI have IP address and it better to manually add the IP address for SVI. 

[tschertel]

Hi MHM,

The switch gets an IP from DHCP indeed.

So if I understood, I need to a fixed IP on my switch to have DHCP relay working?

[MHM Cisco World]

Yes, try set ip then check dhcp

relay

[tschertel]

Thanks for your suggestion, but unfortunately setting a static IP on SVI did not work.
At first, I put a static IP on SVI VLAN 1. All good. All working.
After that, I put a static IP on SVI VLAN 103. Then I lost access to the switch using its hostname. I had to use its static IP (from VLAN 1) to access it.
And the worst is, I lost the DHCP relay configuration menu in options. I need to factory reset it to get the menu back.

Does it matter, having another switch between the DHCP server and the switch I'm trying to configure?

[MHM Cisco World]

can you share 
show ip route 
for this switch.

[tschertel]

Hi MHM,

This is the "show ip route" output:

Maximum Parallel Paths: 1 (1 after reset)
IP Forwarding: enabled
Codes: > - best, C - connected, S - static,
       R - RIP


C   192.168.93.0/24 is directly connected, vlan 1
C   192.168.103.0/24 is directly connected, vlan 103

Maybe I have something wrong in DHCP server, because the switch gets an IP from the DHCP server for network 93.0. Just network 103.0 is not working.