Need Help...Please

[samirshaikh52]

Hi All,

I need the help  of our experts. Its very urgent and important.

We have two buildings, 1 km distance you can say. There is a point to point connectivity with a wireless bridge.Both the buildings has there own Core Switch. And there is routing configured on both switches. B2 staff can access B1 servers

B1 has cisco 6500 switch and B2 has 3560,

B1 Switch has vlans configured from 1-10

B2 switchs has vlans configured from 20-30

Both of them has management has ip's in different subnets

I hope that the above details was informative

Now my question is that, I had some AP in B2 and these will be used by clients and I had DHCP server in B1 and place int vlan 1.

How is it possible that AP clients will get the ip from said dhcp server

I will appreciate any help.

[Alexander Pai]

You can use the "ip helper-address" command.  This command forwards UDP broadcast packets (like DHCP) to a specific IP address (your DHCP server IP).  Configure it on the L3 ports or SVI's (int vlan ...) of switch B2, and have it forward packets to the IP address of your DHCP server.

Posted from my mobile device.

[samirshaikh52]

Thanks for your prompt response.

IActually  have an ADSL modem acting as DHCP server. This modem is connected to B1 Switch port and the port is in vlan 5.

VLAN ip address is 10.1.5.1. The DHCP ip's leased by modem is 192.168.1.*

What ip should I configured in "ip helper-address" command ?

Big Thanks

[carl_townshend]

hi there

if the wireless clients are in vlan 1,

then go to your vlan 1 interface

interface vlan 1

then type ip helper-address x.x.x.x  (This will be the ip address of the ADSL modem)

give that a try

cheers

Carl

[samirshaikh52]

Hi Carl,

For eg.

I have my WAP clients in vlan 10 ( 10.1.10.1)

My DSL modem is connected to B1 Switch port and this port is vlan 20. IP address for this vlan is 10.1.20.1

Modem has default subnet configured (192.168.1.1)

Here my question is that what ip should i give in ip-helper address command.?

[Alexander Pai]

Hi Samir,

So, the main question is, what is the IP address of your DSL modem?  If it's in vlan 20, on the 10.1.20.1 subnet, I would assume it would be 10.1.20.1.  Whatever device serves as your DHCP server, you need to find the IP address of that device.  That's the IP you will want to use with the command.

-Alex

[samirshaikh52]

Hi Alexander,

I would like to let you  know that 10.1.5.1 is not the DHCP server, its a vlan IP. My DHCP server is placed in this vlan. and it has subnet 192.168.1.0.

So what should i do in this case

Thanks for your replies

[Alexander Pai]

Hi Samir,

Sorry, but I'm a bit confused now.  You stated your DHCP server is on vlan 5.  You also stated your ADSL modem is on vlan 20, and that it serves as your DHCP server.  Can you clarify?  What is the exact IP address of your DHCP server?  What is the exact IP address of your ADSL modem?

-Alex

[samirshaikh52]

Ok I'll be very clear. and sory for that.

1. WAP clients in the vlan 20

2. DSL modem connected to a core switch port and that port is in the vlan 5 (10.1.5.0).

3. VLAN 5 has mangaegment ip address 10.1.5.1

4. Linksy DSL modem has a DHCP subnet 192.168.1.0

So my ambition is that

when a wap client associciates to WAP, he should an ip address from DSL modem (dhcp server) and hes Ip should be such like that 192.168.1.2 and so

on.

How can i achieve this.

Please let me know if something not clear.

[Alexander Pai]

So, on your SVI for Vlan 20 on B2, you need to configure the ip helper-address command:

interface Vlan 20

     ip helper-address 10.1.5.1

Your WAP clients will send out a DHCP UDP broadcast packet on Vlan 20.  The SVI of the switch will see that it received a UDP broadcast on Vlan 20, and forward that packet directly to 10.1.5.1, the IP address of your DHCP server.  The DHCP server will respond accordingly and send the response back to Vlan 20.  The host should receive the packet, which will give them their assignment of an IP address from the 192.168.1.X subnet, assuming your DHCP server is configured correctly.

-Alex

[samirshaikh52]

Hi Alexander,

Really big thanks for such a great explanation.

I'll give a try and keep you inform For the time being i'll rate your reply 5 stars.

[samirshaikh52]

Hi Alexander,

I have tried putting ip helper address but its not getting

interface vlan 20

ip helper-address 10.1.5.1 ( this ip is for vlan 5 not dhcp server)

However dhcp modem is places in vlan 5. Ip leased in the subnet 192.168.1.0

Please need your suggestion

[samirshaikh52]

I will provide vlan configuration on switches for your info.

B1

interface Vlan 5
ip address 10.1.5.1 255.255.255.0

B2

interface Vlan 20

ip address 10.1.20.1 255.255.255.0

ip helper-address 10.1.5.1 (new command)

[samirshaikh52]

I am really in need of our experts help.

Please let me know if something not clear.

[Alexander Pai]

What is the exact IP address of your DHCP server?