07-21-2011 01:59 AM - edited 03-07-2019 01:19 AM
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.
07-21-2011 02:15 AM
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.
07-21-2011 03:32 AM
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
07-21-2011 03:48 AM
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.
07-21-2011 04:01 AM
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.
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