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ymj
New Member

VLAN & DHCP

Hello

now lets say we have VLAN1, VLAN2 and VLAN3

I put my DHCP server in say VLAN2.

Now let's say there's a router to route between vlans

1) ClientA is located within VLAN3

It sends out a broadcast (ff:ff:ff:ff:ff:ff of course for destination MAC

address ...)

this broadcast should NOT go over VLAN3 (thats one of the reason what we use

VLAN for...)

HOW will this broadcast reach the DHCP server (with ip helper address on the

router ??)

2) OK now lets say question 1 is OK

The DHCP server(on vlan2) receives the DHCP DISCOVER broadcast.

HOW will the DHCP server know what scope (IP addresses pool) it is going to

choose an IP address from ?

(if we want a different subnet for each VLAN...)??

3) Now lets say question 2 is OK

DHCP server replies

-- To what IP/MAC address ?

I mean : DHCP server will reply to the router, OK, BUT how will the router

know to "whom" it has got to send the DHCP OFFER back to ??

4) If I want to put the DHCP server behind the router (in that case we do not consider a Layer 3 Switch acting as "router")

How would you design the whole stuff ?

Regards;

YMJ

3 REPLIES
New Member

Re: VLAN & DHCP

#1: Yes, IP HELPER-ADDRESS is what takes the DHCP request packet and forwards it to the DHCP server. The router acts as a "DHCP proxy" for the server.

#2: The router adds a "hint" to the DHCP request packet, indicating, among other things, what subnet the request is coming from. By the way, if you're multinetting (i.e. using IP ADDRESS x.x.x.x SECONDARY), only the Primary IP Address subnet is hinted to the DHCP server.

#3: The DHCP server replies via unicast to the router, which is "proxying" for the client workstation. The router knows where to send it back to, because he is tracking the whole DHCP transaction. He knows who the requestors are (by MAC address) and who the DHCP servers are (by IP HELPER-ADDRESS).

#4: DHCP server goes into VLAN 2, IP HELPER-ADDRESS is applied to the interfaces where DHCP services will be offered (don't need to apply them to interfaces where DHCP services *won't* be offered), set up proper scopes on the DHCP server for all the subnets you have DHCP servers for. You're done! That's it!

Jim

ymj
New Member

Re: VLAN & DHCP

Thanks Jim !

Excellent !

Just one thing:

After having written that post I read a doc which explains DHCP<->Router<->Client communication process.

And it says that the router replies with *broadcast* on the LAN segment where the Client is located.

Thanks again !

YMJ

New Member

Re: VLAN & DHCP

Yow! You got me!

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