DHCP relay and ip-helper- what's the different?

[meirhasin]

Hello

What's the different betweem dhcp-relay and ip-helper?

if i enabled the option of dhcp-relay in my switch(HP 5406) for what i need to write ip-helper?

Thanks you ver much.

[Fredrik Lönnman]

the dhcp-relay command in global config is to enable or disable the use of ip-helper to relay DHCP packets. if you disable dhcp-relay, the ip-helper command will not relay DHCP anymore, but still be able to be used as UDP forwarder for other protocols.

[meirhasin]

thanks!

why there is need to forward other udp packets? other services like dns, need ip helper to serve devices from other subnets?

and "ip bootp" It’s necessary when there is no bootp server (for proper work of regular dhcp server) ? when configure bootp, there is need to write bootp server or just vlan default gateway?

thank you

[meirhasin]

Hi

I have other question, very important question:

Some department in our company ran out of ip address(in there VLAN) , there is no option to move them to other VLAN or create new VLAN for them, the only option was to do multinet= combine to subnets in 1 vlan.

Its work fine but the problem occurred when the servers try to get ip from DHCP, we have ip-helper and BOOTP on this VLAN, and according to HP manuals, in multinet VLAN, just 1 subnet can reach the dhcp server.

It’s make us a lot of problem, I can create no VLAN for this department, there is some option the “work around” and enable bootp&dhcp for 2 subnets in multinet?

vlan 11

   name "qa-vlan"

   tagged B1,B4,B8,B11,B23,C4,C8,D1

   untagged A5-A9,A11

   ip helper-address 10.4.1.121

   ip helper-address 10.4.1.122

   ip address 10.7.4.1 255.255.252.0

   ip address 10.7.80.1 255.255.248.0

   ip bootp-gateway 10.7.80.1

when to BOOTP is going to 10.7.80.1, the servers in subnet 10.7.80.1 can get ip from the dhcp, and the servers from 10.7.4.1 cant, and also opposite.

Anyone can give me an answer?

Thank you very much

[Fredrik Lönnman · Accepted solution]

I haven't tried your scenario so I cannot give you an answer unfortunately. But if the HP manual says it cant be done.. :/. 

Why can't you add another VLAN? I'd definately recommend against having that many clients (/21 + /22 == ~3000 IPs) in one segment.

[meirhasin]

you’re absolutely right, but we have some specific department that ran out of ip and we can’t change or add another VLAN, we need to make their vlan bigger in some way :)

thank you very much for the help