IP helper address subinterfaces

[henriquecadore]

Hello

I have an L2 switch connected to a Cisco router, the L2 has no VLANs associated to its ports and the router has a subinterface (0/1.1000) configured on the interface (0/1) that connects to the L2. The subinterface shouldn't be configured there, but for now we can't remove that. The problem is that the hosts connected to the L2 are not getting IP from the DHCP servers, which are working fine and Ican also ping them from the router and L2 as well as from hosts with fixed IPs. The configuration now is as follows:

interface GigabitEthernet0/1
ip address GATEWAY 255.255.255.0 secondary
ip helper-address DHCP_SERVER_1
ip helper-address DHCP_SERVER_2
ip flow ingress
duplex auto
speed auto
!
interface GigabitEthernet0/1.1000
encapsulation dot1Q 1000
no cdp enable
!

Both interfaces are up.

My question is : the router is not relaying the DHCP requests since it's not configured on interface 1.1000? 

Thank you.

[Mark Malone]

Hey you need to put the ip helper address under the SVI for the L2 vlan that will allow them get an ip address it wont work under physical when runnint router on stick setup

[devils_advocate]

On the switch that connects to this port, does it support Vlans or is it just a L2 unmanaged switch?

What is the purpose of the subinterface?

[henriquecadore]

Hi devils, it's managed and supports VLAN configuration, although there is no VLAN configured. The subinterface on the router is there by mistake and I cannot fix it by now, although it has been already requested. I wonder if if it will work when the subinterface is removed and it's the case that this subinterface is causing the DHCP problem.

The actual configuration of GI0/1 is:

interface GigabitEthernet0/1
ip address SUBNET1 255.255.255.0 secondary
ip address SUBNET2 255.255.255.0 secondary
ip address SUBNET3 255.255.255.0 secondary
ip address SUBNET4 255.255.255.0 secondary
ip address SUBNET5 255.255.255.0 secondary
ip address SUBNET6 255.255.255.0 secondary
ip address SUBNET7 255.255.255.0 secondary
ip address SUBNET8 255.255.255.0 secondary
ip address ROUTER_MANAGEMENT_IP 255.255.255.192
ip helper-address DHCP_SERVER1
ip helper-address DHCP_SERVER2
ip flow ingress
duplex auto
speed autointerface GigabitEthernet0/1.1000
encapsulation dot1Q 1000
no cdp enable

SUBNETs are all 172.29.X.X.

[paul driver]

Hello

Just like to add to marks post-

When using secondary addressing for DHCP you may need to apply a feature called

IP dhcp smart-relay -

Dhcp will default to the primary interface address  and will never utilise seconday interface address but with this feature if no dhcp offer is received for the primary it will try use the secondary

I used this feature when I had dhcp subnets related to secondary addressing during some transision work and the primary dhcp scopes were exhusted

Nice feature to be aware of!

res

Paul

[Richard Burts]

We have been told that the switch does not have vlans configured so all the ports are in the same vlan. We have not been told whether the switch port that connects to the router is configured as an access port or as a trunk port. Would the original poster provide clarification of this?

If the switch has no vlans (and no plans for vlans) then the subinterface on the router does look like a mistake. Until it can be corrected there should be helper addresses configured on the subinterface as well as on the physical interface. While some characteristics are inherited by subinterfaces from the physical interface helper address is not inherited. So you need helper address configured on the interface/subinterface where the traffic is arriving.

HTH

Rick