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Dynamic Host Configuration Protocol (DHCP) Snooping

The DHCP Snooping feature provides network protection from rogue DHCP servers. It creates a logical firewall between untrusted hosts and DHCP servers. The switch builds and maintains a DHCP snooping table (also called DHCP binding database), shown in Figure 4-4a. In addition, the switch uses this table to identify and filter untrusted messages from the network. The switch maintains a DHCP binding database that keeps track of DHCP addresses that are assigned to ports, as well as filtering DHCP messages from untrusted ports. For incoming packets received on untrusted ports, packets are dropped if the source MAC address does not match MAC in the binding table entry.

Figure 4-4a

Figure 4-4a DHCP Snooping Table

Figure 4-4b illustrates the DHCP Snooping feature in action, showing how the intruder is blocked on the untrusted port when it tries to intervene by injecting a bogus DHCP response packet to a legitimate conversation between the DHCP client and server.

Figure 4-4b

Figure 4-4b DHCP Snooping in Action

The DHCP Snooping feature can be configured for switches and VLANs. When enabled on a switch, the interface acts as a Layer 2 bridge, intercepting and safeguarding DHCP messages going to a Layer 2 VLAN. When enabled on a VLAN, the switch acts as a Layer 2 bridge within a VLAN domain.

For DHCP Snooping to function correctly, all DHCP servers connected to the switch must be configured as trusted interfaces. A trusted interface can be configured by using the ip dhcp snooping trust interface configuration command. All other DHCP clients connected to the switch and other ports receiving traffic from outside the network or firewall should be configured as untrusted by using the no ip dhcp snooping trust interface configuration command.

To configure the DHCP Snooping feature, first enable DHCP Snooping on a particular VLAN by using the ip dhcp snooping vlan [vlan-id] command in global configuration mode. (Repeat this command for multiple VLANs.) Next, enable DHCP Snooping globally by using the ip dhcp snooping command from the global configuration mode. Both options must be set to enable DHCP snooping.

In Example 4-8, the DHCP server is connected to the FastEthernet0/1 interface and is configured as a trusted port with a rate limit of 100 packets per second. The rate limit command ensures that a DHCP flood will not overwhelm the DHCP server. DHCP Snooping is enabled on VLAN 5 and globally activated.

Example 4-8. DHCP Snooping Configuration Example

Switch(config)# interface Fastethernet0/1
Switch(config-if)# ip dhcp snooping trust
Switch(config-if)# ip dhcp snooping limit rate 100
Switch(config-if)# exit
Switch(config)# ip dhcp snooping vlan 5
Switch(config)# ip dhcp snooping
Switch(config)# ip dhcp snooping information option

Use the show ip dhcp snooping command to display DHCP snooping settings. Use the show ip dhcp snooping binding command to display binding entries corresponding to untrusted ports.

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