ip helper-address queries

[nigelb]

Hi

I've come a cross a client with a number of independent L3 switches (no HSRP/VRRP groups) with "ip helper-address" statements on similar VLAN's.

My queries are:-

1. Will this generate lots of duplicate DHCP request packets on the server VLAN - is this recommended?

2. What is the behaviour of the switch where (as in the example below) the ip helper-address is added to the same VLAN where the DHCP server resides?

3. Is it still recommended to prevent netbios broadcast across VLAN's with the no ip forward-protocol udp commands?

E.G.

***Switch-1***

interface Vlan1

  ip address 10.10.10.254 255.255.255.0

ip helper-address 10.10.10.100

ip helper-address 10.10.10.101

!

interface Vlan20

ip address 10.20.20.254 255.255.255.0

ip helper-address 10.10.10.100

ip helper-address 10.10.10.101

***Switch-2***

interface Vlan1

ip address 10.10.10.253 255.255.255.0

ip helper-address 10.10.10.100

ip helper-address 10.10.10.101

!

interface Vlan20

ip address 10.20.20.253 255.255.255.0

ip helper-address 10.10.10.100

ip helper-address 10.10.10.101

Many thanks for any replies.

[manju.cisco]

Hii

I can answer for

2. What is the behaviour of the switch where (as in the example below)  the ip helper-address is added to the same VLAN where the DHCP server  resides?

No., since its the same broadcast vlan, no need to add ip helper-address , devices in that vlan can get ip from dhcp without any issues.

[nigelb]

Thanks for the reply Manju

I understand that it isn't required, I just wondered action the switch will take?

Will it forward the dhcp request to the dhcp server, so that the dhcp server receives the original broadcast from the client and a copy forwarded from the switch; or just ignore and drop the broadcast?

Thanks again

Nigel

[Latchum Naidu]

Hi,

if a workstation plugged into the same VLAN in which the dhcp server also, sending out the dhcp broadcast requst and the DHCP server (on the same VLAN) would already have received a copy, in which case, you don’t need the “ip helper-address” command for the VLAN on which the dhcp server is.

Still if you configured the "ip  helper-address" it just drop the broadcast packet.

Please rate the helpfull posts.
Regards,
Naidu.

[ogamanya]

1) I would suppose it will generate a duplicate packet for every DHCP packer request sent out. The DHCP will recieve 2 copies for every request

2) It will forward the UDP packet...not tested (IMO)

3) no ip forward allows you to limit / scale down the protocols forwarded by defaut when you configure an IP helper.