Multiple IP-helper address is possible???

[vipinrajrc]

Hi,

Is it possible to configure multiple IP helper address in core switches??/

Thanks

Vipin

[Richard Michael]

Hello Vipin,

Yes it is possible, what is your requirement?

Link,

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

Thanks,

Ricky Micky

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[Mark Williams]

Yes.. we do just that on all our user LANs globally.

If you have multiple IP helper addresses, a DHCP discover message is sent to all DHCP servers. The DHCP discovery process can then work out what server assigned the IP.. there are a lot of acknowledges and offers happening in the background that sorts this out.

[Mohamed Sobair]

Yes, its possible.

However, Keep in mind that , this case is not used for Primary/Backup or for a REDUNDANT Dhcp server address allocation, because once you configure both commands on the interface, one request goes to the first dhcp server and the second request will go to the second Dhcp server.

To avoid conflicting and have appropriate redundancy, you just need to create Two Different DHCP Scopes on the Servers, this can be deployed in certain Scenarios.

Regards,

Mohamed

[Somasundaram Jayaraman]

Hi,

Yes it is possible.

Whenever there were two helper address, the router sends the Dhcp request all servers. When we studied our network, we noticed that routers convert DHCP request to unicast, and sends to all configured DHCP servers. So, you cannot control which server will accept and give IP address.

Hope this helps

Cheers

Somu

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[Jon Marshall]

Vipin

Just to follow up on Mohammed's post. If you have 2 ip helper-addresses then the request for an IP address will be forwarded to both. So both will respond with an offer of an IP address. This means that each DHCP server cannot be responsible for the same IP addresses.

So if you had a subnet of 192.168.5.0/24 you would do -

DHCP Server 1

scope 192.168.5.4 -> 127    <-- note i have assumed .1/.2/.3 in use for HSRP

excluded adresses -> 192.168.5.1 -> 3, 192.168.5.128 -> 254

subnet mask 255.255.255.0

default-gateway 192.168.5.1  <-- this is the HSRP VIP on your core switches

DHCP Server 2

scope 192.168.5.128 -> 254

excluded addresses 192.168.5.1 -> 192.168.5.127

subnet mask 255.255.255.0

default-gateway 192.168.5.1

note in the above that both scopes use the same subnet mask and the same default-gateway.

This way each DHCP server is responsible for roughly half of the entire scope. This also means that if you wanted to ensure that if one of the DHCP servers was unavailable you still had enough addresses on the other that you limit the number of clients in each subnet to the smallest scope.

Jon