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New Member

DHCP - Subnets

Remote Routers. Central DHCP Server.

Can the Cisco IOS forward (via Helper-Address) DHCP

requests that let the Central DHCP Server know what IP's in different subnets to assign?

Need the Central DHCP Server to assign IP's to remote PC's on different IP Subnets depending on which Remote Subnet the request comes from.

Thanks,

Bob

1 ACCEPTED SOLUTION

Accepted Solutions
Silver

Re: DHCP - Subnets

This happens automatically, the router acts as a BootP relay agent and places the interface address the DHCP request arrived on in the gateway address field (giaddr) of the DHCP request, the DHCP server then uses this field to determine which address pool to assign the address out of.

There is a good guide to DHCP on CCO:

http://www.cisco.com/en/US/customer/tech/tk648/tk361/technologies_tech_note09186a00800f0804.shtml

3 REPLIES
Silver

Re: DHCP - Subnets

This happens automatically, the router acts as a BootP relay agent and places the interface address the DHCP request arrived on in the gateway address field (giaddr) of the DHCP request, the DHCP server then uses this field to determine which address pool to assign the address out of.

There is a good guide to DHCP on CCO:

http://www.cisco.com/en/US/customer/tech/tk648/tk361/technologies_tech_note09186a00800f0804.shtml

Gold

Re: DHCP - Subnets

Previous link is an excellent one.

In the middle of it, it makes mention of all the UDP broadcasts that get forwarded by default to the specified "ip helper-address". The only ones you need for DHCP are the BOOTP ones, ports 67 and 68.

If you are in a Microsoft Network environment, the default forwarding of ports 137 and 138 can cause browser election problems. (Not HTTP web browsers but Microsoft's Computer Browser service, which gets into master browsers, backup browsers, domain master browsers, etc.) In this case, it is best to modify the behavior of the "ip helper-address" interface command, by disabling the forwarding of those ports using the "no ip forward-protocol" global command.

As a rule, if I enable "ip helper-address" on an interface, 99%+ of the time it is just to do exactly what you want to do: forward only DHCP address requests and renewals to a centralized DHCP server. So I disable the default forwarding of all the other UDP ports except for the BOOTP ones.

Not sure if you knew this, but you can also put multiple "ip helper-address" commands on an interface. If you have two or more centralized DHCP servers, you can forward an address request to as many as you want. Just configure the scopes on each server so that they don't overlap addresses, and assign each server a unique subset of the range of addresses you want handed out to the clients of a particular LAN, VLAN or subnet. This provides you with redundancy, in case one of the servers goes off-line for an extended period, or the DHCP Server services stop.

As for what happens at the DHCP client end, it receives multiple address offers, accepts the first one, and completes negotiations with the server that offered it.

Hope this helps.

New Member

Re: DHCP - Subnets

Thanks!

Bob

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