redundant ip helper-address

Unanswered Question
Nov 16th, 2009

hi,

In 3560 with IPbase IOS image how can we acheine DHCP redundancy?

If we give two commands

ip helper-address IP1

ip helper-address IP2

in the VLAN interface will it work?

If dhcp IP1 is down can client get IP from dhcp IP2 ?

Please share the experience.

Any link on cisco.com is highly appreciable.

thanks in advance.

Subodh

I have this problem too.
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Giuseppe Larosa Mon, 11/16/2009 - 04:37

Hello Subodh,

yes multiple ip helper address commands are supported but the router uses all of them at once: each broadcast is translated multiple times to each ip helper address unicast destination.

Faster anwer or some other form of control allows for managing the DHCP server redundancy.

if servers can talk to each other you can define the same scope on both.

We do so with Cisco network registrar.

other colleagues have reported that when the two servers are indipendent they configure two different scopes on the two servers taken from the same ip subnet.

so this works for servers in cluster and also for indipendent servers with the trick suggested above.

Hope to help

Giuseppe

jgross Mon, 11/16/2009 - 06:55

Ho Subodh,

redundancy is implemented in the DHCP Protocol. Check http://www.cisco.com/en/US/docs/ios/12_0t/12_0t1/feature/guide/Easyip2.html

You can also use a boadcast adress as helper adress when you enable ip directed-broadcast in the target VLAN of the helper address. So if your dhcp servers in vlan 100 with ip subnet 10.1.1.x/24, add ip helper-address 10.1.1.255 in the client vlans, and add ip directed-broadcast in the VLAN100 of the DHCP Servers.

bye

Jo

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