Multiple DHCP Servers

[rickycancio]

Greetings

We have 2 DHCP Servers that are serving same scopes but different addresses. I have entered this on the cisco router

ip helper-address 10.0.0.1

ip helper-address 10.0.0.2

My question is: How does the IOS work this? Will it try one and then the other the next time...or will it only try the second one when the first one fails (active/failover) ?

Thanks

Ricardo

[peterledwidge]

See,

http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_command_reference_chapter09186a0080087387.html#wp1018606

HTH

[rickycancio]

Yes thank you HTH, I have already checked the documentation various times, but it only tells me that I can input multiple ip helper-address commands for multiple servers, but not how it works.

Does it Load Balance? Active/Fail mode?

Ricardo

[lgijssel]

The IOS handles this as follows:

Whenever a qualifying broadcast arrives it is forwarded (as unicast) to all adresses that are listed as helper-adress on the interface where the packet arrives.

Also, by default not only DHCP qualifies but quite a few other UDP-based protocols as well. Please check the command reference for more details:

http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_command_reference_chapter09186a0080087387.html#wp1018606

Regards,

Leo

[amit-singh]

Hi Ricardo,

The router takes the DHCp broadcast request adds the DHCP server address and send a unicast request to DHCP server. The DHCP broadcast ( which is now unicast ) will be forwarded to both the DHCP servers and depending on which server receives it first will lease the Ip address. Now when your machine has already got any IP from one DHCP then the IP address from the other DHCP server will be discarded. There request can never reach both the server at one time, there will atleast be a nano-second differnce so dont get confused with the request reaching the servers at the same time.

HTH,

-amit singh

[rickycancio]

HTH

Ok! Thanks!.

Ricardo