вЂ“ Forwards ARP packets received on a trusted interface without any checks To ensure that only valid ARP requests and responses are relayed, DAI takes these actions: In addition, to handle hosts that use statically configured IP addresses, DAI can also validate ARP packets against user-configured ARP ACLs. This is used to prevent DHCP starvation attacks by limiting the rate of the DHCP requests on untrusted ports.Дynamic ARP Inspection (DAI) determines the validity of an ARP packet based on a valid MAC-address-to-IP-address bindings database built by DHCP snooping. Normally, the rate limit applies to untrusted interfaces. RateConfigure the number of DHCP pps that an interface can receive. Switch(config-if)# ip dhcp snooping limitВ rate Configure the number of DHCP packets per second (pps) that are acceptable on the port. Use the no keyword to revert to untrusted.By default, all ports are untrusted.Д¤. Switch(config-if)# ip dhcp snooping trustДЄt least one trusted port must be configured. Configure DHCP server interfaces or uplink ports as trusted. This is optional for the forwarded DHCP request packet to contain information on the switch port where it originated.ДЈ. Switch(config)# ipВ dhcpВ snooping information option

Trusted ports host a DHCP server or can be an uplink toward the DHCP server.

Trusted ports can source all DHCP messages, whereas untrusted ports can source requests only. Ports are identified as trusted and untrusted. The addresses are returned to the address pool by the DHCP server, to be reallocated as necessary.ДHCP snooping is a Cisco Catalyst feature that determines which switch ports can respond to DHCP requests. This lease mechanism ensures that hosts that have been moved or are switched off for extended periods of time do not hold on to addresses that they do not use. The address is only leased to the host, so the host will periodically contact the DHCP server to extend the lease. The DHCP server selects an address and allocates it to that host. As hosts come online, they contact the DHCP server and request address information. All that is required is a defined range of IP addresses on a DHCP server. Using DHCP, a host can obtain an IP address quickly and dynamically. DHCP consists of two components: a protocol for delivering host-specific configuration parameters from a DHCP server to a host, and a mechanism for allocating network addresses to hosts. DHCP is also used to provide other configuration information as necessary, including the length of time the address has been allocated to the host. Cisco CCNP SWITCH Dynamic Host Configuration ProtocolДHCP is used to assign IP addresses automatically and to set TCP/IP stack configuration parameters, such as the subnet mask, default router, and Domain Name System (DNS) servers.

Cisco show mac address bindings