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New Member

DHCP helper address on a Switch with a Local DHCP pool

 Device: Cisco IOS Software, C3560 Software (C3560-IPBASEK9-M), Version 12.2(55)SE7, RELEASE SOFTWARE (fc1)

 DHCP Pool:

ip dhcp pool ITS_Voice
   network 10.xxx.4.xxx 255.255.255.0
   default-router 10.xxx.xxx.xxx
   dns-server 10.xxx.xxx.xxx 10.xxx.xxx.xxx 10.xxx.xxx.xxx
   domain-name xxxx.com
   option 150 ip 10.xxx.xxx.xxx

VLAN Interface:

interface Vlan2
 ip address 10.xxx.4.xxx 255.255.255.0
 ip helper-address <ISE-Node>
 ip helper-address <ISE-Node>
 no ip redirects
 no ip proxy-arp

The question is why does the layer 3 interface not pass the DHCP broadcast to the helper address? Devices get ip assignements.

I did a "debug ip DHCP server packets". I do not see the broadcast sent to the helper addresses. I need to send those DHCP Packets to the Identity Services Engine for profiling.

I confirmed the problem by adding another switch and trunking it to the first one. Then I added a L3 interface on the second switch with the helper addresses and that works. Again using the DHCP debug I can see the broadcast being forwarded to the ISE nodes.However this is not a fix, more like a workaround wasting address space.

Anyone have any Ideas?

 

3 REPLIES

Hello

Hello Dhcp relay is designed to relay dhcp requests towards the dhcp server and in your case the dhcp server is on the router as svi you are applying helper address to, So in this case it isn't required Res Paul
Please don't forget to rate any posts that have been helpful. Thanks.
New Member

Since we are using ISE

Since we are using ISE (Identity Services Engine) we do need that relay address so we can profile devices on those switches.

I have found the resolution. The problem is when a DHCP binding exists already for a device. If I have a new device access the switch for the first time the relay address works. If I remove that device and reconnected it with out removing the DHCP binding the relay address will not be used.

 

New Member

I found this on TCPIPGUIDE

I found this on TCPIPGUIDE.com that supports my findings.

 

"One difference between BOOTP and DHCP is that certain communications from the client to the server are unicast. The most noticeable instance of this is when a client tries to renew its lease with a specific DHCP server. Since it sends this request unicast, it can go to a DHCP server on a different network using conventional IP routing, and the relay agent does not need to be involved."

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