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Community Member

VLANs and DHCP

We have a perimeter router. The perimeter router has connections to the Internet, VPN clients, two point-point connections to branch offices and the LAN. The LAN is one network-one IP scheme. We would like to segregate the LAN/network to minimize collision and for enhancing security. Our Cisco switch can be configured for VLANs. How do the VLANs get dynamic IP addresses?

Thanks.

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Gold

Re: VLANs and DHCP

Said

Win 2003 server is capable of assigning dynamic IPs for multiple subnets. You would configure multiple scopes on the Win 2003 DHCP server (one scope per subnet).

There are choices about how to implement DHCP. Some people configure the DHCP service on one (or more) of their routers. Some people implement DHCP on separate servers, one server per subnet. Most people implement a single server with multiple scopes.

You are correct that by default broadcasts are limited to the individual subnet. To get the DHCP request broadcast from the local subnet to the remote server you can configure the ip helper-address command which will look for DHCP broadcast requests (as well as a few other kinds of broadcasts) and will forward a copy of the request to the remote DHCP server.

HTH

Rick

7 REPLIES
Hall of Fame Super Gold

Re: VLANs and DHCP

Said

If you want your VLANs to get dynamic addresses then you need to have a DHCP server. Some people use their router or layer 3 switch to provide the DHCP service and some people choose to have a separate server to provide DHCP services.

HTH

Rick

Community Member

Re: VLANs and DHCP

Rick,

does each subnet/VLAN require its own resident DHCP server? Can the perimeter router be configured to enable one designated Win 2003 server DHCP server to send DHCP to other subnets?, is a Win 2003 capable of sending Dynamic IPs to different subnets?

Thanks.

Said

Community Member

Re: VLANs and DHCP

Hi Saidfrh,

If you have the DHCP server in your network then following VLAN configuration helps to get a dynamic IP from DHCP

Int VLAN

ip address

ip helper-address

Community Member

Re: VLANs and DHCP

The DHCP server will be on a different subnet. By default broadcasts are limited to each subnet. Each subnet has its own broadcast domain. In other words, the perimeter router has to be configured to enable DHCP to all subnets?

Hall of Fame Super Gold

Re: VLANs and DHCP

Said

Win 2003 server is capable of assigning dynamic IPs for multiple subnets. You would configure multiple scopes on the Win 2003 DHCP server (one scope per subnet).

There are choices about how to implement DHCP. Some people configure the DHCP service on one (or more) of their routers. Some people implement DHCP on separate servers, one server per subnet. Most people implement a single server with multiple scopes.

You are correct that by default broadcasts are limited to the individual subnet. To get the DHCP request broadcast from the local subnet to the remote server you can configure the ip helper-address command which will look for DHCP broadcast requests (as well as a few other kinds of broadcasts) and will forward a copy of the request to the remote DHCP server.

HTH

Rick

Community Member

Re: VLANs and DHCP

Rick,

Thank you.

Said

Hall of Fame Super Gold

Re: VLANs and DHCP

Said

I am glad that my explanations were helpful. Thank you for using the rating system to indicate that your question was resolved (and thanks for the rating). It makes the forum more useful when people can read a question and can know that they will read responses that resolved the question.

I encourage you to continue your participation in the forum.

HTH

Rick

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