Cisco Catalyst 2960 Series, Configuration Manual

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port, the aggregation switch does not learn the DHCP snooping bindings for connected devices and cannot
build a complete DHCP snooping binding database.
When an aggregation switch can be connected to an edge switch through an untrusted interface and you enter
the
ip dhcp snooping information option allow-untrusted global configuration command, the aggregation
switch accepts packets with option-82 information from the edge switch. The aggregation switch learns the
bindings for hosts connected through an untrusted switch interface. The DHCP security features, such as
dynamic ARP inspection or IP source guard, can still be enabled on the aggregation switch while the switch
receives packets with option-82 information on untrusted input interfaces to which hosts are connected. The
port on the edge switch that connects to the aggregation switch must be configured as a trusted interface.
Normally, it is not desirable to broadcast packets to wireless clients. So, DHCP snooping replaces destination
broadcast MAC address (ffff.ffff.ffff) with unicast MAC address for DHCP packets that are going from server
to wireless clients. The unicast MAC address is retrieved from CHADDR field in the DHCP payload. This
processing is applied for server to client packets such as DHCP OFFER, DHCP ACK, and DHCP NACK
messages. The
ip dhcp snooping wireless bootp-broadcast enable can be used to revert this behavior. When
the wireless BOOTP broadcast is enabled, the broadcast DHCP packets from server are forwarded to wireless
clients without changing the destination MAC address.
Related Topics
Prerequisites for Configuring DHCP Snooping and Option 82, on page 1277
Option-82 Data Insertion
In residential, metropolitan Ethernet-access environments, DHCP can centrally manage the IP address
assignments for a large number of subscribers. When the DHCP option-82 feature is enabled on the switch,
a subscriber device is identified by the switch port through which it connects to the network (in addition to
its MAC address). Multiple hosts on the subscriber LAN can be connected to the same port on the access
switch and are uniquely identified.
The DHCP option-82 feature is supported only when DHCP snooping is globally enabled on the VLANs
to which subscriber devices using option-82 are assigned.
Note
The following illustration shows a metropolitan Ethernet network in which a centralized DHCP server assigns
IP addresses to subscribers connected to the switch at the access layer. Because the DHCP clients and their
associated DHCP server do not reside on the same IP network or subnet, a DHCP relay agent (the Catalyst
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
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Information About DHCP