configure 1841 as a dhcp relay agent(not a dhcp server)

[gwu]

I have a 1841 that I would like to relay all dhcp requests from all our vlans to our dhcp server(10.0.7.50). What are the correct command to do this?

thanks


Here is our config:

int fa0/1
ip address 10.0.1.49 255.255.255.240

int fa0/1
no ip address

int fa0/1.1
encap dot1Q 1 native
ip address 10.0.1.49 255.255.255.240

int fa0/1.2
encap dot1Q 2
ip address 10.0.3.1 255.255.255.0

int fa0/1.3
encap dot1Q 5
ip address 10.0.6.1 255.255.255.0

int fa0/1.4
encap dot1Q 6
ip address 10.0.7.1 255.255.255.0

int fa0/1.5
encap dot1Q 3
ip address 10.0.5.1 255.255.255.0

 

[KiscoKid]

All you *should* need to do is add:

ip helper-address 10.0.7.50

to each vlan except int fa0/1.4 attached to vlan 6

Naturally make sure you have IP pools defined on 10.0.7.50 for each IP subnet used otherwise it won't know what IP address to allocate (it uses the IP address of the router sub-interface to determine what a suitable IP address allocation is)

 

[gwu]

Thanks, I will try it!

Cisco definition of ip helper-address:

"The ip helper-address interface subcommand tells the router to forward UDP broadcasts, including BootP, received on this interface."

Are there any security problems with sending all udp broadcasts? Or do I set the acls to allow BOOTP ports?

Thanks

 

[rtfmdude]

gwu,

you can modify the forwarded ports if you'd like; nice little explanation here:

"Another issue deals with how Cisco routers deal with certain broadcast based traffic like NetBIOS or DHCP. When you configure the ip helper-address command, you forward every IP broadcast in the ip forward protocol command list. The no ip forward protocol command can be used to disable the forwarding for specific protocols. Here is an example to disable the forwarding of NetBIOS.
no ip forward-protocol udp netbios-ns
no ip forward-protocol udp netbios-dgm"

http://www.knowledgenet.com/em/kn/cisco/2003/news0703/index2.html

 

[plshlpme]

im pretty sure it takes the broadcast and sends it as a unicast to the dhcp server. so your not really broadcasting across any wans.