Complex DHCP Issue 1

[DanClark86]

I have 3 separate networks that are on MetroEhternet2 as their backbone. Each Location has its own DHCP Server on a Cisco 2950 Switch. The Main Location has a Cisco 4506 switch for a total of 4 locations. The problem is that Sites 2 Site 3 and site 4 keep picking up IP address from each site, so a computer or printer at site 2 might pick up an IP address from the DHCP Server at Site 4. This problem effects only the locations that have the 2950 switch. Computers at site 1, which is the 4506 dose, not suffer from this problem nor has site 2, 3, or 4, picked up an IP address from site 1. Any Help would be great.

 

[Saugilsr]

What is the routing setup? Also are there any vlans involved?

 

[GlenJohnson]

Have you thought about setting up reservations for each dhcp server? This way, the servers know about each other, but don't pass ip's out to each other's network.

Glen A. Johnson
Johnson Computer Consulting
Tek-Tips in Chicago IL

 

[5jgibbs]

if these networks are connected .... i would look at the way the DHCP is giving out the addresses. Also, remember, dhcp is a broadcast packet.. so if you block port 67 and 68 "UDP" at the router / switch, you can stop the dhcp from transmitting its broadcast to any of the other sections of the network..

 

[DanClark86]

so how exactly would you block those ports? Use an access list to block those? The DHCP servers have not changed, the only change is the topology that we use to interconnect the offices, previously we were using a Nimly and/or a Frame T1, Now we are using MetroEthernet.

 

[5jgibbs]

well.. im not expert with the cisco IOS, but i would assum you would do something like

acces-list 92 deney UDP 67 UDP 68

something like that.. BUT PLEASE NOTE.. i dont know much about CISCO IOS.. .. i am trying to learn more though .. .good luck..

 

[5jgibbs]

ohh.. you could also do this..

access-list 33 deny 172.17.207.17 255.255.0.0

just substitute your IP for the one i put in//

you could also do without the subnett mask..

but you may need it depending on your situation and ip class

ex.. class a 255.0.0.0 class b 255.255.0.0 class c 255.255.255.0 have fun..

 

[5jgibbs]

geesh.. i forgot to say.. use the IP's of the other DHCP servers..