DHCP Relay with ip helper-address for VLAN 10

Configure and verify DHCP client and relay Explain the role of DHCP and DNS within the network

Answer Correct answer: C — configure ip helper-address 192.168.20.2 on the SW1 VLAN 10 interface so DHCP requests from VLAN 10 clients are relayed to the remote DHCP server.

exhibit Refer to the exhibit. The DHCP server is configured with a DHCP pool for each of the subnets represented. Which command must be configured on switch SW1 to allow DHCP clients on VLAN 10 to receive dynamic IP addresses from the DHCP server?

SW1(config-if)#ip helper-address 192.168.10.1
SW1(config-if)#ip helper-address 192.168.20.1
SW1(config-if)#ip helper-address 192.168.20.2 Correct Answer
SW1(config-if)#ip helper-address 192.168.10.2

Community Insight

The question tests DHCP relay: ip helper-address must reference the DHCP server itself, and the common trap is confusing the server's address (192.168.20.2) with the subnet's default gateway (192.168.20.1) or with the local VLAN 10 SVI (192.168.10.x).

DHCP clients on VLAN 10 sit in a different broadcast domain from the DHCP server, so SW1 must act as a relay agent and forward BOOTP/DHCP requests with the ip helper-address command configured on the VLAN 10 SVI. The correct helper address is the DHCP server's own interface address, 192.168.20.2, not the VLAN 20 gateway address.

Option D (ip helper-address 192.168.10.2) is the most frequent wrong pick, especially from learners reasoning that "VLAN 10 clients need a VLAN 10 address." 192.168.10.2 is inside the clients' own subnet, so there is nothing to relay — a local server on the same subnet is reached by broadcast without a helper address at all.

Community Discussion (15 comments)

7fa6509 👍 1 Selected: C

The correct answer is C. Given that the DHCP server interface IP address is 192.168.20.2, there is no DHCP server with the IP address 192.168.10.2. Consider this: if you choose option D, where would the DHCP relay agent obtain assistance?

Tamunodiepiriye 👍 2 Selected: C

C is correct because the ip helper-address command will be for an address on another subnet

Lse 👍 1 Selected: C

When configuring the ip helper-address command on the switch, you should use the IP address of the router's interface that is directly connected to the subnet where the client devices reside, not the default router IP.: The ip helper-address command tells the switch (or router) where to forward DHCP requests.

matass_md 👍 3

So IP helper-address command must be on the Default-Gateway of our respective VLAN. So in this scenario our host is in VLAN 10 has a pool Created named Net10, we have two DHCP servers created for VLAN 10 and VLAN 20 Net 20 (READ THE commands inserted on R1 not the 192.168.20.2 IP address that you see on the picture , that's a trap ). We can't tell PC 1 get your IP address from DHCP Server on VLAN 20 because then you will have no internet connectivity because the our port F0/1 is in VLAN 10! We need our PC to get a valid IP address from 192.168.10.2 Then it will have connectivity on Port FastEthernet 0/1 access port vlan 10. Right answer D, 192.168.10.2 THIS is where we want our broadcast Discovery message to go as a UNICAST , to this address, because we have a DHCP server for each VLAN !!!

matass_md 👍 2

Ignore my above message, the right answer is 192.168.20.2 . our SW is the DG to our PC , we put there the IP helper then it sends it to another network (that is normal) and we receive an Offer with 192.168.10.x IP back . Final answer C .

eb63e5a 👍 1

D can't be answer, because 192.168.10.2 address is not given for any interface, it is just configured on router. We don't know on which interface it is given, since we have only one connected interface, which is even access port. But 192.168.20.2 is given for router interface. This is why I pick C

ebachka 👍 1

Ok this going to be long. Correct me if I am wrong please. 1st we have a multilayer switch with interswitch vlan routing going on. Ip helper should be applied on the routing interface closest to the clients, because the discover messages cant pass layer 3 interface without ip helper. On the switch we have Interface VLAN 10 192.168.10.1. So any device trying to exit the broadcast domain from vlan 10 has to go thru this interface. So that's where we should apply the IP Helper command on. Then we look for the IP Address of the DHCP Server and it looks like the IP Address of the server is Int g0/0 192.168.20.2. So the correct answer should be SW1(config)int vlan 10 , SW1(config-if)ip helper-address 192.168.20.2

HelloPeople 👍 1

DHCP Network command not configured

ds0321 👍 1 Selected: D

vlan 10 , must be D

Muziaws 👍 3

The question is asking about clients on VLAN 10 so how can C be correct

Da_Costa 👍 2 Selected: C

Looking at the server address which is used as helper address 192.168.20.2

Friday_Night 👍 3

yeah it's C and if cisco wants us to be more confused they would show same commands in different SVIs :))

Rynurr 👍 13 Selected: C

Why "D"? The correct answer is "C". ip helper-address 192.168.20.2

sdmejia01 👍 2 Selected: D

D is correct!

oatmealturkey 👍 13

It is C. You are thinking of the default gateway, we need the DHCP server address which is 192.168.20.2. The exhibit with the long output is meant to throw us off, just look at the topology diagram and it is there. If we only needed 192.168.10.2 for DHCP for VLAN 10, then we would not need an IP helper address.

Comments & Corrections

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Expert Analysis

Why the Answer Is Correct

Clients in VLAN 10 send DHCP Discover as a limited broadcast, which routers do not forward, so SW1 must relay the request to a DHCP server on another network. The ip helper-address parameter is the unicast address of the DHCP server itself — and on this topology the server reachable in the 192.168.20.0/24 network is 192.168.20.2. The command is applied on the interface (the VLAN 10 SVI, SW1's gateway for that VLAN) facing the clients, and the relayed Discover lets the server issue a lease from its Net10 pool while the offer returns with a 192.168.10.x address to the client. This matches option C.

Why the Other Options Are Wrong

192.168.20.1 (B) is the gateway/first usable address of the server subnet, not the DHCP server's address, so relayed requests would be sent to a device that cannot answer DHCP. 192.168.10.1 (A) is a local VLAN 10 address — a helper address that points back into the clients' own broadcast domain is pointless and provides no relay function. 192.168.10.2 (D) is in the clients' own subnet; as oatmealturkey noted, "if we only needed 192.168.10.2 for DHCP for VLAN 10, then we would not need an IP helper address," because a same-subnet server is discovered by broadcast directly.

Community Comment Notes

Most commenters converged on C, with Rynurr and Da_Costa both pointing to "192.168.20.2" as the server interface used as the helper address. oatmealturkey captured the core concept by warning learners that they are "thinking of the default gateway" instead of the DHCP server, and matass_md revised an initial wrong reading of the exhibit, concluding "our SW is the DG to our PC, we put there the IP helper then it sends it to another network." The dissenting votes for D (from Muziaws and sdmejia01) rest on the intuition that VLAN 10 clients need a VLAN 10 address, which confuses the leased address with the relay target.

Official Reference

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipaddr/dhcp/configuration/15-mt/ipaddr-dhcp-15-mt-book/ipaddr-dhcp-relay-agent.html

Exam Strategy

On any DHCP relay question, first separate the three addresses in play: the clients' gateway (VLAN 10 SVI), the server's own address, and the server subnet's gateway. ip helper-address always takes the server's address, configured on the client-facing interface; if a server existed in the same subnet as the clients, no helper address would be required at all.

Frequently Asked Questions

Why can't the ip helper-address be 192.168.10.2 for VLAN 10 clients?

192.168.10.2 is in the clients' own subnet, so no relay is needed or possible — a same-subnet DHCP server is reached by broadcast. The helper address must be the remote DHCP server, 192.168.20.2.

Why is 192.168.20.1 wrong instead of 192.168.20.2?

192.168.20.1 is the gateway address of the server subnet, not the DHCP server itself. ip helper-address must point to the unicast address of the device that actually answers DHCP requests.

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