VLAN not getting DHCP address using Router and L3 Switch

[johnblack123]

I've been trying to get DHCP from the router which is holding the DHCP pool and a L3 switch for the vlans, but the client is not receiving a DHCP address. It works if I configure the switch to trunk mode, but I need to configure EIGRP for NAT and to ping another VLAN in a different building.

Router

en

conf t

int gig 0/1

ip address 205.165.85.5 255.255.255.252

no shut

int gig 0/1.5

encapsulation dot1q 5

ip address 192.168.86.30 255.255.255.240

!

ip dhcp pool vlan5

network 192.168.86.16 255.255.255.240

default-router

end

Switch

en

conf t

ip routing

vlan 5

int fa0/1

switchport access vlan 5

!

int gig 0/2

no switchport

ip address 205.165.85.6 255.255.255.252

no shut

int vlan 5

ip address 192.168.86.30 255.255.255.240

ip helper-address 205.165.85.5

end

[Richard Burts]

The issue is that you have configured a vlan sub interface on the router for vlan 5. So the router will believe that all devices in vlan 5 are locally connected. This means that to communicate with any device in vlan 5 it will send an arp request and depend on receiving an arp response to be able to communicate. But if the switch configured its connection to the router as an access port rather than as a trunk then the devices in vlan 5 on the switch will not see the arp request from the router.

I believe that you have two choices in this situation:

1) configure the switch to router interface as a trunk (and in this case you do not need the vlan interface vlan 5 on the switch. 

2) let the switch route for vlan 5 and remove the vlan 5 sub interface from the router.

HTH

Rick

[Richard Burts · Accepted solution]

In reading the discussion through again I realize that there is another way to explain this issue and perhaps it might be easier to understand.

You have configured the router with a vlan sub interface. This implies that you intend the router to route for the subnet. This requires that the connection from router to switch be a trunk and it means that in DHCP the default gateway should be the router address.

You have configured with switch with a vlan interface. This implies that you intend the switch to route for the subnet. This implies that the switch to router connection is not a trunk and it means that in DHCP the default gateway should be the switch address.

These two things conflict with each other. 

Either you should have a vlan sub interface to route for the subnet

or 

you should have a vlan interface to route for the subnet.

Having both creates a conflict.

HTH

Rick

[Richard Burts]

I am glad that my suggestion was helpful. Thank you for marking this question as solved. This will help other participants in the community to identify discussions that have helpful information. These communities  are excellent places to ask questions and to learn about networking. I hope to see you continue to be active in the community.

HTH

Rick