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New Member

IP Helper in VLAN-Problem

Greetings All

I am getting problem in IP helper commands.I issue commands on Interfaces but did not get IP from DHCP Pool of Router.

Router acting as DHCP---3750---3750--Router as DHCP Client.

Vlan is Up/Up.Alreday done shut /no shut  on All interfaces.

Please check the attached image and guide me.

3 REPLIES
Silver

Re: IP Helper in VLAN-Problem

Hi Muhammad,

There are many flaws, in my opinion, so let's go through them all.

You need to configure ip helper-address only on interfaces pointing to your (V)LANs. In your case R3's interface vlan 20 and interface vlan 30. And that's it! Remove the command from other interfaces.

You don't need to configure it anywhere else, because interfaces configured with this command will accept broadcast messages for specific services and then forward them as an unicast to specific IP address. Not just to another hop as broadcast.

There is another flaw, however. On R1, you have dhcp pool configured with SAME subnet as you are already using, now how could that work? You are  already using that subnet for other purposes(it defines network between R3 and R2)!

One more thing, you have interfaces vlan20 and vlan30 configured with ip address dhcp. I'm not sure how that could work. You said that you have vlans up/up - sure you have, because there are ports associated with this vlan which are up and the vlan is in the vlan database. But there really isn't any way to tell R3 where he should send his own dhcp discover messages (I suppose he will try to sent it out the ports where there is vlan20 and vlan30 configured). He will only accept and forward dhcp messages received in this vlan, but not the ones he will try to generate.

And think of this, how does the DHCP router know, from which subnet does dhcp discover messages come from? Well, it is determined by ip address of that interface. And you have none addresses configured (I think that ip address dhcp won't work in this case)... and even if it would, the IPs are supposed to be globally unique but you are trying to allocate ONE subnet (192.168.80.0/24) to TWO interfaces (vlan20,vlan30) and this subnet is also already in use! So you would have 3subnets with same addressing. But as I said, it doesn't work for other reasons also.

Moreover, I don't know if ip helper-address works correctly in the packet tracer. But try to resolve these issues first.

  • Correct DHCP POOL - configure two dhcp pools with appropriate default-routers and subnets according to, what subnets do you want to use on your vlan20 and vlan30 interfaces
  • Remove unecessary ip helper-address from all interfaces except vlan20 and vlan30
  • Assign vlan20 and vlan30 some address, based on which the DHCP server will know, from which pool to assign addresses to hosts

Best regards,

Jan

Hall of Fame Super Silver

IP Helper in VLAN-Problem

After posting this issue in this forum the original poster then also posted the same issue in the WAN forum, where I have responded noting the same issues. I suggest that any further discussion be focused in this forum since it was the original posting.

HTH

Rick

New Member

IP Helper in VLAN-Problem

hi muhammad,

check out the below post. it was by me when i was facing a similar kind of an issue.

https://supportforums.cisco.com/thread/2245176

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