The DHCP server assigns IP addresses from specified address pools on a switch or router to DHCP clients and manages them. If the DHCP server cannot give the DHCP client the requested configuration parameters from its database, it can forward the request to one or more secondary DHCP servers defined by the network administrator.

A DHCP relay agent is a Layer 3 device that forwards DHCP packets between clients and servers. Relay agents forward requests and replies between clients and servers when they are not on the same physical subnet. Relay agent forwarding is different from the normal Layer 2 forwarding, in which IP datagrams are switched transparently between networks. Relay agents receive DHCP messages and generate new DHCP messages to send on egress interfaces.

DHCP snooping is a DHCP security feature that provides network security by filtering untrusted DHCP messages and by building and maintaining a DHCP snooping binding database, also referred to as a DHCP snooping binding table. For more information about this database, see the Displaying DHCP Snooping Information.

DHCP snooping acts like a firewall between untrusted hosts and DHCP servers. You use DHCP snooping to differentiate between untrusted interfaces connected to the end user and trusted interfaces connected to the DHCP server or another switch.

Note	For DHCP snooping to function properly, all DHCP servers must be connected to the router through trusted interfaces.

An untrusted DHCP message is a message that is received from outside the network or firewall. When you use DHCP snooping in a service-provider environment, an untrusted message is sent from a device that is not in the service-provider network, such as a customer’s switch. Messages from unknown devices are untrusted because they can be sources of traffic attacks.

The DHCP snooping binding database has the MAC address, the IP address, the lease time, the binding type, the bridge-domain number, and the interface information that corresponds to the local untrusted interfaces of a switch. It does not have information regarding hosts interconnected with a trusted interface.

In a service-provider network, a trusted interface is connected to a port on a device in the same network. An untrusted interface is connected to an untrusted interface in the network or to an interface on a device that is not in the network.

When a switch receives a packet on an untrusted interface and the interface belongs to a bridge-domain in which DHCP snooping is enabled, the switch compares the source MAC address and the DHCP client hardware address. If the addresses match (the default), the switch forwards the packet. If the addresses do not match, the switch drops the packet.

The router drops a DHCP packet when one of these situations occurs:

• A packet from a DHCP server, such as a DHCPOFFER, DHCPACK, DHCPNAK, or DHCPLEASEQUERY packet, is received from outside the network or firewall.
• A packet is received on an untrusted interface, and the source MAC address and the DHCP client hardware address do not match.
• The switch receives a DHCPRELEASE or DHCPDECLINE broadcast message that has a MAC address in the DHCP snooping binding database, but the interface information in the binding database does not match the interface on which the message was received.
• A DHCP relay agent forwards a DHCP packet that includes a relay-agent IP address that is not 0.0.0.0, or the relay agent forwards a packet that includes option-82 information to an untrusted port.

If the switch is an aggregation switch supporting DHCP snooping and is connected to an edge switch that is inserting DHCP option-82 information, the switch drops packets with option-82 information when packets are received on an untrusted interface. If DHCP snooping is enabled and packets are received on a trusted 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 allowed-trust 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, can still be enabled on the aggregation switch while the switch receives packets with option-82 information on ingress untrusted 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.

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.

Note	The DHCP option-82 feature is supported only when DHCP snooping is globally enabled and on the bridge-domains to which subscriber devices using this feature are assigned.

Figure below is an example of 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 Cisco ASR 903 Router) is configured with a helper address to enable broadcast forwarding and to transfer DHCP messages between the clients and the server.

Figure 1. DHCP Relay Agent in a Metropolitan Ethernet Network

When you enable the DHCP snooping information option 82 on the switch, this sequence of events occurs:

• The host (DHCP client) generates a DHCP request and broadcasts it on the network.
• When the switch receives the DHCP request, it adds the option-82 information in the packet. By default, the remote-ID suboption is the switch MAC address, and the circuit-ID suboption is the port identifier from which the packet is received. You can also configure the remote ID and circuit ID. For information on configuring these suboptions, see the Enabling DHCP Snooping and Option 82.
• If the IP address of the relay agent is configured, the switch adds this IP address in the DHCP packet.
• The switch forwards the DHCP request that includes the option-82 field to the DHCP server.
• The DHCP server receives the packet. If the server is option-82-capable, it can use the remote ID, the circuit ID, or both to assign IP addresses and implement policies, such as restricting the number of IP addresses that can be assigned to a single remote ID or circuit ID. Then the DHCP server echoes the option-82 field in the DHCP reply.
• The DHCP server unicasts the reply to the switch if the request was relayed to the server by the switch. The switch verifies that it originally inserted the option-82 data by inspecting the remote ID and possibly the circuit ID fields. The switch removes the option-82 field and forwards the packet to the switch port that connects to the DHCP client that sent the DHCP request.

In the default suboption configuration, when the described sequence of events occurs, the values in these fields in figure below do not change:

• Circuit ID suboption fields

• Suboption type
• Length of the suboption type
• Circuit ID type
• Length of the circuit ID type
• Remote ID suboption fields

• Suboption type
• Length of the suboption type
• Remote ID type
• Length of the circuit ID type

Figure below shows the packet formats for the remote ID suboption and the circuit ID suboption when the default suboption configuration is used. The switch uses the packet formats when DHCP snooping is globally enabled and when the ip dhcp snooping information option global configuration command is entered.

Figure 2. Suboption Packet Formats

Figure below shows the packet formats for user-configured remote-ID and circuit-ID suboptions The switch uses these packet formats when you globally enable DHCP snooping and enter the ip dhcp snooping information option format remote-id global configuration command and the ip dhcp snooping bridge-domain information option format-type circuit-id string interface configuration command.

The values for these fields in the packets change from the default values when you configure the remote-ID and circuit-ID suboptions:

• Circuit-ID suboption fields

• The circuit-ID type is 1.
• The length values are variable, depending on the length of the string that you configure.
• Remote-ID suboption fields

• The remote-ID type is 1.
• The length values are variable, depending on the length of the string that you configure.

Figure 3. User-Configured Suboption Packet Formats

During the DHCP-based autoconfiguration process, the designated DHCP server uses the Cisco IOS DHCP server database. It has IP addresses, address bindings, and configuration parameters, such as the boot file.

An address binding is a mapping between an IP address and a MAC address of a host in the Cisco IOS DHCP server database. You can manually assign the client IP address, or the DHCP server can allocate an IP address from a DHCP address pool. For more information about manual and automatic address bindings, see the IP Addressing Configuration Guide Library, Cisco IOS XE Release 3S .

When DHCP snooping is enabled, the switch uses the DHCP snooping binding database to store information about untrusted interfaces. The database can have up to 8192 bindings.

Each database entry (binding ) has an IP address, an associated MAC address, the lease time (in hexadecimal format), the interface to which the binding applies, and the bridge-domain to which the interface belongs. The database agent stores the bindings in a file at a configured location. At the end of each entry is a checksum value that accounts for all the bytes associated with the entry. Each entry is 72 bytes, followed by a space and then the checksum value.

To keep the bindings when the switch reloads, you must use the DHCP snooping database agent. If the agent is disabled, dynamic ARP inspection, and the DHCP snooping binding database has dynamic bindings, the switch loses its connectivity. If the agent is disabled and only DHCP snooping is enabled, the switch does not lose its connectivity, but DHCP snooping might not prevent DHCP spoofing attacks.

When reloading, the router reads the binding file to build the DHCP snooping binding database. The switch keeps the file current by updating it when the database changes.

When a router learns of new bindings or when it loses bindings, the router immediately updates the entries in the database. The router also updates the entries in the binding file. The frequency at which the file is updated is based on a configurable delay, and the updates are batched. If the file is not updated in a specified time (set by the write-delay and abort-timeout values), the update stops.

This is the format of the file that has the bindings:

<initial-checksum> 

TYPE DHCP-SNOOPING 

VERSION 1 

BEGIN 

<entry-1> <checksum-1> 

<entry-2> <checksum-1-2> 

... 

... 

<entry-n> <checksum-1-2-..-n> 

END

Each entry in the file is tagged with a checksum value that the router uses to verify the entries when it reads the file. The initial-checksum entry on the first line distinguishes entries associated with the latest file update from entries associated with a previous file update.

This is an example of a binding file:

2bb4c2a1

TYPE DHCP-SNOOPING

VERSION 1

BEGIN

192.1.168.1 3 0003.47d8.c91f 2BB6488E Fa1/0/4 21ae5fbb

192.1.168.3 3 0003.44d6.c52f 2BB648EB Fa1/0/4 1bdb223f

192.1.168.2 3 0003.47d9.c8f1 2BB648AB Fa1/0/4 584a38f0

END

When the router starts and the calculated checksum value equals the stored checksum value, the router reads entries from the binding file and adds the bindings to its DHCP snooping binding database. The router ignores an entry when one of these situations occurs:

• The router reads the entry and the calculated checksum value does not equal the stored checksum value. The entry and the ones following it are ignored.
• An entry has an expired lease time (the router might not remove a binding entry when the lease time expires).
• The interface in the entry no longer exists on the system.
• The interface is a routed interface or a DHCP snooping-trusted interface.

1.    configure terminal


2.    service dhcp


3.    end


4.    show running-config


5.    copy running-config startup-config

To disable the DHCP relay agent, use the no service dhcp global configuration command.

If the DHCP server and the DHCP clients are on different networks or subnets and the router is running the metro IP access image, you must configure the router with the ip helper-address address interface configuration command. The general rule is to configure the command on the Layer 3 interface closest to the client. The address used in the ip helper-address command can be a specific DHCP server IP address, or it can be the network address if other DHCP servers are on the destination network segment. Using the network address enables any DHCP server to respond to requests.

Note	To remove the DHCP packet forwarding address, use the no ip helper-addressaddress interface configuration command.

Beginning in privileged EXEC mode, follow these steps to specify the packet forwarding address:

1.    configure terminal


2.    interface bridge-domain id


3.    ip address ip-address subnet-mask


4.    ip helper-address address


5.    end


6.    show running-config


7.    copy running-config startup-config

1.    configure terminal


2.    ip dhcp snooping


3.    ip dhcp snooping bridge-domain id


4.    ip dhcp snooping information option


5.    ip dhcp snooping information option format remote-id [string ASCII-string | hostname]


6.    ip dhcp snooping information option allowed-untrusted


7.    interface interface-id


8.    no shutdown


9.    ip dhcp snooping bridge-domain id information option format-type circuit-id string [override] ASCII-string


10.    ip dhcp snooping trust


11.    ip dhcp snooping limit rate rate


12.    exit


13.    ip dhcp snooping verify mac-address


14.    end


15.    show running-config


16.    copy running-config startup-config

1.    configure terminal


2.    ip dhcp snooping database {flash:/filename | ftp://user:password@host/filename | http://[[username:password]@]{hostname | host-ip}[/directory] /image-name.tar | rcp://user@host/filename}| tftp://host/filename


3.    ip dhcp snooping database timeout seconds


4.    ip dhcp snooping database write-delay seconds


5.    end


6.    ip dhcp snooping binding [ip-address | mac-address | dynamic | static | bridge-domain id | interface interface]


7.    show ip dhcp snooping database [detail]


8.    copy running-config startup-config

1.    configure terminal


2.    ip dhcp use subscriber-id client-id


3.    ip dhcp subscriber-id interface-name


4.    interface interface-id


5.    ip dhcp server use subscriber-id client-id


6.    end


7.    show running config


8.    copy running-config startup-config