Block Rogue DHCP Server’s on Cisco Equipment
August 5, 2011 in Cisco Switches, Infrastructure
Block Rogue DHCP Server’s on Cisco Equipment
Network loops are not the only issue that you may face in simple or complex LAN setup; you may face Client-Installed DHCP server.
Well, such issue can be overcome by implementing one of two:
- By enabling DHCP snooping as bellow:
- Enable the feature globally with the ip dhcp snooping global configuration command.
- Enable the feature for individual VLANs with the ip dhcp snooping #VLAN# global configuration command.
- Configure the trusted interfaces with the ip dhcp snooping trust interface configuration command.
- Rate-limit DHCP on untrusted interfaces with the ip dhcp snooping limit rate interface configuration command.
- By applying an ACL that prevent DHCP traffic, as known DHCP offers are flooded on port 68 from DHCP server to DHCP client; so, you can apply an inbound extended ACL on the interface facing the would-be rogue servers. This is the relevant configuration:
ip access-list 100 deny udp any any eq 68
ip access-list 100 permit ip any any
int [interface facing the would-be rogue]
ip access-group 100 in
That will block any DHCP server sending an offer or an acknowledgment into that interface, but it will not block a client sending a discover or a request into the interface.
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