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Webcast-Catalyst9k
New Member

queery on ip helper address

It has been said that the ip helper-address command will forward broadcast of the 8 udp's (port 37,49,53, etc.). Say I have the DHCP (port 67 & 68) service on VLAN1 and my clients are on VLAN2.

Without the NO IP FORWARD-PROTOCOL COMMANDs, what will be the behavior of the rest of the UDP protocols (port 137,138, 37,53,49, 69)?

If I dont have this services running on the client machine, would this configuration create additional traffic on the network?

Another thing, how is the IP DIRECTED BROADCAST related to IP HELPER-ADDRESS?

Thanks a lot.

2 REPLIES
New Member

Re: queery on ip helper address

Hello,

if you don't switch of the other ports (37,49,53,...) all UDP broadcasts that are sent by the clients on vlan 2 on this ports, will also be forwarded to the ip helper-adress.

ip directed-broadcast means, that subnet broadcasts are allowed on the vlan.

You have to enable this in the destination vlan, if you enter the subnet as helpler-address.

So, if your clients are in VLAN 2, and you have one DHCP Server in VLAN1, and you know its ip address you can configure

interface vlan 1

description DHCP Server VLAN

ip address 10.1.1.1 255.255.255.0

interface vlan2

description Client VLAN

ip address 10.1.2.1 255.255.255.0

ip helper-address 10.1.1.100

no ip forward-protocol udp tftp

no ip forward-protocol udp nameserver

no ip forward-protocol udp domain

no ip forward-protocol udp time

no ip forward-protocol udp netbios-ns

no ip forward-protocol udp netbios-dgm

no ip forward-protocol udp tacacs

If you don't know the ip address of the DHCP server, or if you have 2 or more DHCP Servers in VLAN 1 for redundancy, you should configure

interface vlan 1

description DHCP Server VLAN

ip address 10.1.1.1 255.255.255.0

ip directed-broadcast

interface vlan2

description Client VLAN

ip address 10.1.2.1 255.255.255.0

ip helper-address 10.1.1.255

no ip forward-protocol udp tftp

no ip forward-protocol udp nameserver

no ip forward-protocol udp domain

no ip forward-protocol udp time

no ip forward-protocol udp netbios-ns

no ip forward-protocol udp netbios-dgm

no ip forward-protocol udp tacacs

Hope this helps

bye

Jo

New Member

Re: queery on ip helper address

Hi Jo,

Thanks a lot. That clears my mind.

Rey

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