DHCP in Layer 3 Switch

[Ahmed Malik]

hello,

       I have made a topology by using one 3560 switch and 2 2950 switches. I have also made 2 vlans name Clients and other Servers and vlan 1 is for anagement purposes. The left 2950 switch is for clients and the right is for servers. Clients is vlan 2 and servers is vlan3 . Now what i want is that my dhcp is should assign ips to clients in vlans 2 provided that servers are in vlan 3. I am also using a border router and i have introduced a default route on the 3560 to the border router.

Now when i assign static ips to my clients pc and server dhcp then i can ping between vlans but when i try to assign ip through dhcp then it wont work. Also the default route on the switch to the border router doesnt seem to work. I can ping only the border router when i put a default route on the border router instead of the 3560 switch.

I am attaching my pkt fiule along with this question. PLease help me out here.

[Rolf Fischer · Accepted solution]

The DHCP Discover messages send by the client use Layer 2 broadcast as destination address. The scope of this broadcast is your VLAN 2. You have to configure a feature which relays the DHCP messages into your VLAN 3 where you placed the DHCP Server. I'm sure you'll find it on your own ;-)

[smehrnia · Accepted solution]

Hi,

I cant view your pkt file (Mac OS), but i can guide you with regards to your question.

you have 2 different VLANs, that means you have TWO different Broadcast Domains. DHCP clients use BOOTP broadcasts to request IP Addresses, so when a client starts and sends a request for IP, it goes all the way up to the 3560 and NO further.

i didnt quite get ur setup without seeing the pkt file, but what you have to do is:

1* if you are using your 3560 to do inter-VLAN routing. you sure created VLAN interfaces on the switch for each vlan. you have to add the ip helper-address x.x.x.x [dhcp server ip] under the SVI interface.

2* if you are using the border router for inter-Vlan routing (router on a stick) add the ip helper-address command under the subinterfaces u created for 802.1q trunk (routing).

PS: you can ping the default gateways on the switch when you enable L3 function of the switch by creating VLAN interfaces (SVI) for each vlan.

i dunno if i got u right thou

plz Rate if it helped.

Soroush.

[Ahmed Malik]

I figured out the command. i was using an older version of pkt tracer and t wasnt allowing the ip helper-address command and i got confused but its all good now. time to move onto HSRP and VVRP. But thanks for the guidance fischer and soroushm.

Regards Ali