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New Member

Dhcp with Vlans

hi,

I have a small question, can i use dhcp with multiple vlans,if yes then how.

with regards

manish young

4 REPLIES

Re: Dhcp with Vlans

Yes, you can.

Use ip helper-address command in your router interface config if you want to forward DHCP requests to a DHCP server placed to different VLAN.

Regards,

Milan

New Member

Re: Dhcp with Vlans

yaa this can be used, but this will forward the DHCP request to a single dhcp

server , which may give ip address of single range to different vlan, for example

VLAN2 is running on 192.168.1.0 and VLAN3 is running on 192.168.2.0 .if a

host in vlan3 send a dhcp request is may get address from 192.168.1.0 range.

with regards

manish young

New Member

Re: Dhcp with Vlans

The client broadcasts (MAC and IP) a BOOTP request. If a Cisco router sees this, has a helper address coded, and is forwarding UDP port 67, it does the following:

The router places the IP address of the interface which received the broadcast in the "giaddr" field (meaning Gateway IP Address). This will be the address to which the BOOTP server will send the BOOTP response.

The router forwards this packet as an IP unicast to the IP helper address.

The BOOTP server gets the packet, finds the client's MAC address in its tables, and sends a reply with the client's IP address and boot file information.

The reply is unicast directly to the Cisco router's IP address (giaddr).

When the router receives the BOOTP response (UDP port 68), it retrieves the original client's MAC address and IP address from the data portion of the packet and sends the packet as a MAC and IP unicast out whichever interface is directly connected to the client's IP subnet (unless the "broadcast reply" bit was set by the client; in that case, it is a MAC-level broadcast).

Re: Dhcp with Vlans

You can configure your DHCP server to give different address ranges to particular VLANs (RFC 3046 based solution).

See http://cco-rtp-1.cisco.com/en/US/customer/tech/tk648/tk362/technologies_tech_note09186a0080114aee.shtml

for details.

Regards,

Milan

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