cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
500
Views
0
Helpful
7
Replies

DHCP helper address

badcop
Level 1
Level 1

Hi,

 

Can anyone confirm if i add a DHCP help address to a switch with a layer 3 interface in the same vlan (that is not the default gateway) would the switch pass on the DHCP discover request from the workstation? or does it have to be added to the router / def gate

 

We have a managed WAN with no access to the router itself, we have full access to the switches.

7 Replies 7

Jitendra Kumar
Spotlight
Spotlight

It would be better to share diagram for understanding topology.

 

you can configure DHCP on the L3 switch and can configure ip helper on the vlan. behind your switch will able to get a IP address. if other l2 switch's connected then make sure Uplink are made trunk.

 

Thanks,

Jitendra

Thanks,
Jitendra

1.drawio.png

make a default route on the switch to forward traffic to the Gateway router.

config mode

#ip route 0.0.0.0 0.0.0.0 10.230.130.1

 

make switch port trunk that is connected direct to router.

go to the interface vlan 10     and configure #ip helper-address 10.230.130.1

 

move your workstation switch port in vlan 10.

 

and check it should be work..

 

Thanks,

Jitendra

 

Thanks,
Jitendra

Hi

 It does not require a router. Layer 3 switch can work just fine. DHCP helper address is not necessary on the same vlan, if that´s realy the case. But, if your machine seats in one vlan and the DHCP server in a different vlan, then, DHCP helper address can help.

int vlan 100

ip add 192.168.1.1 255.255.255.0

ip heper access 192.168.2.2 255.255.255.0

 

int vlan 200

ip add 192.168.2.1 255.255.255.0

 

DHCP server with IP 192.168.2.2 on vlan 200.

 

The L3 switch will do the routing between vlans if you add the command "ip routing".

 

AS you may know, dhcp helper address is necessary as DHCP discovery is a broadcast and does not travel between different networks.

 

Jon Marshall
Hall of Fame
Hall of Fame

 

So the SVI is in the same subnet as the WAN router IP you want to set the default gateway to ? 

 

If so yes it should work ie. there is no need to make the SVI the default gateway. 

 

Jon

Hello
The dhcp helper address is used to rely a discovery request towards the dhcp server and because the dhcp server in your case resides off your LAN then dhcp helper address (relay) IS required.

 

Note :As/when the ip helper address are applied, this will enable and forward unwarranted UDP broadcast for additional traffic that may not be required, which inturn has a potential for high link/CPu utilization to be incurred as such its recommended to turn these additional udp broadcast off if you deem them not applicable.

Example:

 

int vlan xx
ip helper-address x.x.x.x
exit
no ip forward-protocol udp tftp
no ip forward-protocol udp nameserver
no ip forward-protocol udp domain
no ip forward-protocol udp time
no ip forward-protocol udp netbios-ns
no ip forward-protocol udp netbios-dgm
no ip forward-protocol udp tacacs


Please rate and mark as an accepted solution if you have found any of the information provided useful.
This then could assist others on these forums to find a valuable answer and broadens the community’s global network.

Kind Regards
Paul

It is not necessarily an issue that the default gateway is the router interface. The DHCP request is sent as a broadcast and so the switch vlan interface will see the broadcast packet. If helper address is configured then the switch will send a DHCP request to the server. What is required is that the switch have IP connectivity with the server (the switch must have routing information for the address of the server, and the server's default gateway must have routing information for the address of the switch SVI, and that there are not security policies along the path that would deny the traffic.

HTH

Rick
Getting Started

Find answers to your questions by entering keywords or phrases in the Search bar above. New here? Use these resources to familiarize yourself with the community: