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New Member

DHCP Problem

Hi All, I have a problem with DHCP server..actually there is one DHCP server in my network which provides IPs to my clients in networks...but the thing is that some users bring their ADSL router & they connect it into the network..without disabling the DHCP some of my client takes the IP from that DHCP instead of mine DHCP server...can any one tell me how to block these type of Routers to provide IPs to client...

there r so many users which connect ADSL routers in I can not block with IPs...




Re: DHCP Problem


what type of LAN switches do you have?

You might want to enable some security features. You could f.e. allow only one MAC on a port and error disable it, when a second MAC shows up. This would pretty much stop user from connecting their DSL routers.

Also shutdown all unused ports (if this is feasible in your environment).

In some switches you could also block unwanted traffic like DHCP with access-lists applied to access-ports.

Hope this helps! PLease rate all posts.

Regards, Martin

New Member

Re: DHCP Problem

You might want to use Cisco VACLs to prevent rogue DHCP servers. For example, on a CAT os switch:

set security acl ip ROGUE-DHCP permit udp host 123.456.789.1 any eq 68

set security acl ip ROGUE-DHCP deny udp any any eq 68

set security acl ip ROGUE-DHCP permit ip any any

Another option is DHCP snooping. Some Cisco switches offer the ability to suppress certain types of DHCP information on certain ports. The primary feature enabling this functionality is DHCP snooping. DHCP snooping works by separating trusted from untrusted interfaces on a switch. Trusted interfaces are allowed to respond to DHCP requests; untrusted interfaces are not. The switch keeps track of the untrusted port’s DHCP bindings and rate limits the DHCP messages to a certain speed.

The first task in configuring DHCP snooping is to enable it:

Switch(config)#ip dhcp snooping

From here, DHCP snooping must be enabled for specific VLANs:

Switch(config)#ip dhcp snooping vlan number [number]

To set up the trusted ports at the interface level, ports must be defined as trusted or untrusted using the following command:

Switch(config-if)# ip dhcp snooping trust

Untrusted ports can be optionally configured with a rate limit on the amount of DHCP messages allowed per second:

Switch(config-if)# ip dhcp snooping limit rate rate