- Index
- Preface
- Overview
- Using the Command-Line Interface
- Assigning the Switch IP Address and Default Gateway
- Configuring Cisco IOS Configuration Engine
- Administering the Switch
- Configuring SDM Templates
- Configuring Switch-Based Authentication
- Configuring IEEE 802.1x Port-Based Authentication
- Configuring Web-Based Authentication
- Configuring Interface Characteristics
- Configuring Smartports Macros
- Configuring VLANs
- Configuring VTP
- Configuring Voice VLAN
- Configuring Private VLANs
- Configuring IEEE 802.1Q and Layer 2 Protocol Tunneling
- Configuring STP
- Configuring MSTP
- Configuring Optional Spanning-Tree Features
- Configuring Flex Links and the MAC Address-Table Move Update Feature
- Configuring DHCP and IP Source Guard
- Configuring Dynamic ARP Inspection
- Configuring IGMP Snooping and MVR
- Configuring Port-Based Traffic Control
- Configuring CDP
- Configuring LLDP, LLDP-MED, and Wired Location Service
- Configuring UDLD
- Configuring SPAN and RSPAN
- Configuring RMON
- Configuring System Message Logging
- Configuring SNMP
- Configuring Network Security with ACLs
- Configuring QoS
- Configuring EtherChannels and Layer 2 Trunk Failover
- Configuring IP Unicast Routing
- Configuring IPv6 Host Functions
- Configuring IPv6 MLD Snooping
- Configuring IPv6 ACLs
- Configuring HSRP and Enhanced Object Tracking
- Configuring Cisco IOS IP SLAs Operations
- Troubleshooting
- Configuring Online Diagnostics
- Supported MIBs
- Working with the Cisco IOS File System, Configuration Files, and Software Images
- Unsupported Commands in Cisco IOS Release 12.2(52)SE
- Understanding DHCP Features
- Configuring DHCP Features
- Default DHCP Configuration
- DHCP Snooping Configuration Guidelines
- Configuring the DHCP Server
- Configuring the DHCP Relay Agent
- Specifying the Packet Forwarding Address
- Enabling DHCP Snooping and Option 82
- Enabling DHCP Snooping on Private VLANs
- Enabling the Cisco IOS DHCP Server Database
- Enabling the DHCP Snooping Binding Database Agent
- Displaying DHCP Snooping Information
- Understanding IP Source Guard
- Configuring IP Source Guard
Configuring DHCP Features and IP Source Guard
This chapter describes how to configure DHCP snooping and option-82 data insertion, and the DHCP server port-based address allocation features on the switch. It also describes how to configure the IP source guard feature.
Note For complete syntax and usage information for the commands used in this chapter, see the command reference for this release, and see the "DHCP Commands" section in the Cisco IOS IP Command Reference, Volume 1 of 3: Addressing and Services, Release 12.2 from the Cisco.com page under Documentation > Cisco IOS Software > 12.2 Mainline > Command References.
This chapter consists of these sections:
•Displaying DHCP Snooping Information
•Understanding IP Source Guard
•Displaying IP Source Guard Information
•Understanding DHCP Server Port-Based Address Allocation
•Configuring DHCP Server Port-Based Address Allocation
•Displaying DHCP Server Port-Based Address Allocation
Understanding DHCP Features
DHCP is widely used in LAN environments to dynamically assign host IP addresses from a centralized server, which significantly reduces the overhead of administration of IP addresses. DHCP also helps conserve the limited IP address space because IP addresses no longer need to be permanently assigned to hosts; only those hosts that are connected to the network consume IP addresses.
These sections contain this information:
•Cisco IOS DHCP Server Database
•DHCP Snooping Binding Database
For information about the DHCP client, see the "Configuring DHCP" section of the "IP Addressing and Services" section of the Cisco IOS IP Configuration Guide, Release 12.2 from the Cisco.com page under Documentation > Cisco IOS Software > 12.2 Mainline > Configuration Guides.
DHCP Server
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 forwards the request to one or more secondary DHCP servers defined by the network administrator.
DHCP Relay Agent
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 output interfaces.
DHCP Snooping
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" section.
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 switch 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 VLAN 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 VLAN 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 switch 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.
You cannot configure IP source guard and dynamic Address Resolution Protocol (ARP) inspection on the switch unless you use static bindings or ARP access control lists (ACLs).
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.
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.
Note The DHCP option-82 feature is supported only when DHCP snooping is globally enabled and on the VLANs to which subscriber devices using this feature are assigned.
Figure 21-1 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 Catalyst switch) is configured with a helper address to enable broadcast forwarding and to transfer DHCP messages between the clients and the server.
Figure 21-1 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 Blade Server (DHCP client) generates a DHCP request and broadcasts it on the network.
•When the blade switch receives the DHCP request, it adds the option-82 information in the packet. By default, the The remote-ID suboption is the switch MAC address, and the circuit-ID suboption is the port identifier, vlan-mod-port, from which the packet is received.
•You can configure the remote ID and circuit ID. For information on configuring these suboptions, see the "Enabling DHCP Snooping and Option 82" section.
•If the IP address of the relay agent is configured, the switch adds this IP address in the DHCP packet.
•The blade 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 blade 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.
When the described sequence of events occurs, the values in these fields in Figure 21-2 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 remote-ID type
In the port field of the circuit ID suboption, the port numbers start at 1. For example, on a Cisco Catalyst Blade Switch 3020 for HP, which as 24 ports, port 1 is the Gigabit Ethernet 0/1 port, port 2 is the Gigabit Ethernet 0/2 port, port 3 is the Gigabit Ethernet 0/3 port, and so on. Ports 17 to 20 are dual-purpose SFP module/RJ-45 copper Ethernet uplink ports Gi0/17 to Gi0/20. Ports 21x and 22x are copper 10/100/1000BASE-T ports Gi0/21 and Gi0/22. Ports 23x and 24x are dual-purpose external/internal 10/100/1000BASET copper uplink ports Gi0/23 and Gi0/24.
Figure 21-2 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 you globally enable DHCP snooping and enter the ip dhcp snooping information option global configuration command.
Figure 21-2 Suboption Packet Formats
Figure 21-3 shows the packet formats for user-configured remote-ID and circuit-ID suboptions. The switch uses these packet formats when DHCP snooping is globally enabled and when the ip dhcp snooping information option format remote-id global configuration command and the ip dhcp snooping vlan information option format-type circuit-id string interface configuration command are entered.
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 21-3 User-Configured Suboption Packet Formats
Cisco IOS DHCP Server Database
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 "Configuring DHCP" chapter of the Cisco IOS IP Configuration Guide,
Release 12.2.
DHCP Snooping Binding Database
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 VLAN 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 that accounts for all the bytes from the start of the file through 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 or IP source guard is enabled, 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 switch reads the binding file to build the DHCP snooping binding database. The switch updates the file when the database changes.
When a switch learns of new bindings or when it loses bindings, the switch immediately updates the entries in the database. The switch 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 with 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 switch 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 switch starts and the calculated checksum value equals the stored checksum value, the switch reads entries from the binding file and adds the bindings to its DHCP snooping binding database. The switch ignores an entry when one of these situations occurs:
•The switch 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 switch 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.
Configuring DHCP Features
These sections contain this configuration information:
•DHCP Snooping Configuration Guidelines
•Configuring the DHCP Relay Agent
•Specifying the Packet Forwarding Address
•Enabling DHCP Snooping and Option 82
•Enabling DHCP Snooping on Private VLANs
•Enabling the Cisco IOS DHCP Server Database
•Enabling the DHCP Snooping Binding Database Agent
Default DHCP Configuration
Table 21-1 shows the default DHCP configuration.
|
|
---|---|
DHCP server |
Enabled in Cisco IOS software, requires configuration1 |
DHCP relay agent |
Enabled2 |
DHCP packet forwarding address |
None configured |
Checking the relay agent information |
Enabled (invalid messages are dropped)2 |
DHCP relay agent forwarding policy |
Replace the existing relay agent information2 |
DHCP snooping enabled globally |
Disabled |
DHCP snooping information option |
Enabled |
DHCP snooping option to accept packets on untrusted input interfaces3 |
Disabled |
DHCP snooping limit rate |
None configured |
DHCP snooping trust |
Untrusted |
DHCP snooping VLAN |
Disabled |
DHCP snooping MAC address verification |
Enabled |
Cisco IOS DHCP server binding database |
Enabled in Cisco IOS software, requires configuration. Note The switch gets network addresses and configuration parameters only from a device configured as a DHCP server. |
DHCP snooping binding database agent |
Enabled in Cisco IOS software, requires configuration. This feature is operational only when a destination is configured. |
1 The switch responds to DHCP requests only if it is configured as a DHCP server. 2 The switch relays DHCP packets only if the IP address of the DHCP server is configured on the SVI of the DHCP client. 3 Use this feature when the switch is an aggregation switch that receives packets with option-82 information from an edge switch. |
DHCP Snooping Configuration Guidelines
These are the configuration guidelines for DHCP snooping.
•You must globally enable DHCP snooping on the switch.
•DHCP snooping is not active until DHCP snooping is enabled on a VLAN.
•Before globally enabling DHCP snooping on the switch, make sure that the devices acting as the DHCP server and the DHCP relay agent are configured and enabled.
•When you globally enable DHCP snooping on the switch, these Cisco IOS commands are not available until snooping is disabled. If you enter these commands, the switch returns an error message, and the configuration is not applied.
–ip dhcp relay information check global configuration command
–ip dhcp relay information policy global configuration command
–ip dhcp relay information trust-all global configuration command
–ip dhcp relay information trusted interface configuration command
•Before configuring the DHCP snooping information option on your switch, be sure to configure the device that is acting as the DHCP server. For example, you must specify the IP addresses that the DHCP server can assign or exclude, or you must configure DHCP options for these devices.
•When configuring a large number of circuit IDs on a switch, consider the impact of lengthy character strings on the NVRAM or the flash memory. If the circuit-ID configurations, combined with other data, exceed the capacity of the NVRAM or the flash memory, an error message appears.
•Before configuring the DHCP relay agent on your switch, make sure to configure the device that is acting as the DHCP server. For example, you must specify the IP addresses that the DHCP server can assign or exclude, configure DHCP options for devices, or set up the DHCP database agent.
•If the DHCP relay agent is enabled but DHCP snooping is disabled, the DHCP option-82 data insertion feature is not supported.
•If a switch port is connected to a DHCP server, configure a port as trusted by entering the ip dhcp snooping trust interface configuration command.
•If a switch port is connected to a DHCP client, configure a port as untrusted by entering the no ip dhcp snooping trust interface configuration command.
•Follow these guidelines when configuring the DHCP snooping binding database:
–Because both NVRAM and the flash memory have limited storage capacity, we recommend that you store the binding file on a TFTP server.
–For network-based URLs (such as TFTP and FTP), you must create an empty file at the configured URL before the switch can write bindings to the binding file at that URL. See the documentation for your TFTP server to determine whether you must first create an empty file on the server; some TFTP servers cannot be configured this way.
–To ensure that the lease time in the database is accurate, we recommend that you enable and configure NTP. For more information, see the "Configuring NTP" section on page 5-3.
–If NTP is configured, the switch writes binding changes to the binding file only when the switch system clock is synchronized with NTP.
•Do not enter the ip dhcp snooping information option allow-untrusted command on an aggregation switch to which an untrusted device is connected. If you enter this command, an untrusted device might spoof the option-82 information.
•Starting with Cisco IOS Release 12.2(37)SE, you can display DHCP snooping statistics by entering the show ip dhcp snooping statistics user EXEC command, and you can clear the snooping statistics counters by entering the clear ip dhcp snooping statistics privileged EXEC command.
Note Do not enable Dynamic Host Configuration Protocol (DHCP) snooping on RSPAN VLANs. If DHCP snooping is enabled on RSPAN VLANs, DHCP packets might not reach the RSPAN destination port.
Configuring the DHCP Server
The switch can act as a DHCP server. By default, the Cisco IOS DHCP server and relay agent features are enabled on your switch but are not configured. These features are not operational.
For procedures to configure the switch as a DHCP server, see the "Configuring DHCP" section of the "IP addressing and Services" section of the Cisco IOS IP Configuration Guide, Release 12.2.
Configuring the DHCP Relay Agent
Beginning in privileged EXEC mode, follow these steps to enable the DHCP relay agent on the switch:
To disable the DHCP server and relay agent, use the no service dhcp global configuration command.
See the "Configuring DHCP" section of the "IP Addressing and Services" section of the Cisco IOS IP Configuration Guide, Release 12.2 for these procedures:
•Checking (validating) the relay agent information
•Configuring the relay agent forwarding policy
Specifying the Packet Forwarding Address
If the DHCP server and the DHCP clients are on different networks or subnets, you must configure the switch 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.
Beginning in privileged EXEC mode, follow these steps to specify the packet forwarding address:
To remove the DHCP packet forwarding address, use the no ip helper-address address interface configuration command.
Enabling DHCP Snooping and Option 82
Beginning in privileged EXEC mode, follow these steps to enable DHCP snooping on the switch:
To disable DHCP snooping, use the no ip dhcp snooping global configuration command. To disable DHCP snooping on a VLAN or range of VLANs, use the no ip dhcp snooping vlan vlan-range global configuration command. To disable the insertion and removal of the option-82 field, use the no ip dhcp snooping information option global configuration command. To configure an aggregation switch to drop incoming DHCP snooping packets with option-82 information from an edge switch, use the no ip dhcp snooping information option allow-untrusted global configuration command.
This example shows how to enable DHCP snooping globally and on VLAN 10 and to configure a rate limit of 100 packets per second on a port:
Switch(config)# ip dhcp snooping
Switch(config)# ip dhcp snooping vlan 10
Switch(config)# ip dhcp snooping information option
Switch(config)# interface gigabitethernet0/1
Switch(config-if)# ip dhcp snooping limit rate 100
Enabling DHCP Snooping on Private VLANs
You can enable DHCP snooping on private VLANs. If DHCP snooping is enabled, the configuration is propagated to both a primary VLAN and its associated secondary VLANs. If DHCP snooping is enabled on the primary VLAN, it is also configured on the secondary VLANs.
If DHCP snooping is already configured on the primary VLAN and you configure DHCP snooping with different settings on a secondary VLAN, the configuration for the secondary VLAN does not take effect. You must configure DHCP snooping on the primary VLAN. If DHCP snooping is not configured on the primary VLAN, this message appears when you are configuring DHCP snooping on the secondary VLAN, such as VLAN 200:
2w5d:%DHCP_SNOOPING-4-DHCP_SNOOPING_PVLAN_WARNING:DHCP Snooping configuration may not take effect on secondary vlan 200. DHCP Snooping configuration on secondary vlan is derived from its primary vlan.
The show ip dhcp snooping privileged EXEC command output shows all VLANs, including primary and secondary private VLANs, on which DHCP snooping is enabled.
Enabling the Cisco IOS DHCP Server Database
For procedures to enable and configure the Cisco IOS DHCP server database, see the "DHCP Configuration Task List" section in the "Configuring DHCP" chapter of the Cisco IOS IP Configuration Guide, Release 12.2.
Enabling the DHCP Snooping Binding Database Agent
Beginning in privileged EXEC mode, follow these steps to enable and configure the DHCP snooping binding database agent on the switch:
To stop using the database agent and binding files, use the no ip dhcp snooping database global configuration command. To reset the timeout or delay values, use the ip dhcp snooping database timeout seconds or the ip dhcp snooping database write-delay seconds global configuration command.
To clear the statistics of the DHCP snooping binding database agent, use the clear ip dhcp snooping database statistics privileged EXEC command. To renew the database, use the renew ip dhcp snooping database privileged EXEC command.
To delete binding entries from the DHCP snooping binding database, use the no ip dhcp snooping binding mac-address vlan vlan-id ip-address interface interface-id privileged EXEC command. Enter this command for each entry that you want to delete.
Displaying DHCP Snooping Information
To display the DHCP snooping information, use one or more of the privileged EXEC commands in Table 21-2:
Note If DHCP snooping is enabled and an interface changes to the down state, the switch does not delete the statically configured bindings.
Understanding IP Source Guard
IP source guard (IPSG) is a security feature that restricts IP traffic on nonrouted, Layer 2 interfaces by filtering traffic based on the DHCP snooping binding database and on manually configured IP source bindings. You can use IP source guard to prevent traffic attacks caused when a host tries to use the IP address of its neighbor.
You can enable IPSG when DHCP snooping is enabled on an untrusted interface. After IPSG is enabled on an interface, the switch blocks all IP traffic received on the interface, except for DHCP packets allowed by DHCP snooping. A port access control list (ACL) is applied to the interface. The port ACL allows only IP traffic with a source IP address in the IP source binding table and denies all other traffic.
The IP source binding table has bindings that are learned by DHCP snooping or are manually configured (static IP source bindings). An entry in this table has an IP address, its associated MAC address, and its associated VLAN number. The switch uses the IP source binding table only when IP source guard is enabled.
Note The port ACL takes precedence over any router ACLs or VLAN maps that affect the same interface.
The IP source binding table bindings are learned by DHCP snooping or are manually configured (static IP source bindings). An entry in this table has an IP address with its associated MAC address and VLAN number. The switch uses the IP source binding table only when IP source guard is enabled.
IPSG is supported only on Layer 2 ports, including access and trunk ports.You can configure IPSG with source IP address filtering or with source IP and MAC address filtering.
These sections contain this information:
•Source IP and MAC Address Filtering
•IP Source Guard for Static Hosts
Source IP Address Filtering
When IPSG is enabled with this option, IP traffic is filtered based on the source IP address. The switch forwards IP traffic when the source IP address matches an entry in the DHCP snooping binding database or a binding in the IP source binding table.
When a DHCP snooping binding or static IP source binding is added, changed, or deleted on an interface, the switch modifies the port ACL using the IP source binding changes, and re-applies the port ACL to the interface.
If you enable IPSG on an interface on which IP source bindings (dynamically learned by DHCP snooping or manually configured) are not configured, the switch creates and applies a port ACL that denies all IP traffic on the interface. If you disable IP source guard, the switch removes the port ACL from the interface.
Source IP and MAC Address Filtering
When IPSG is enabled with this option, IP traffic is filtered based on the source IP and MAC addresses. The switch forwards traffic only when the source IP and MAC addresses match an entry in the IP source binding table.
When IPSG with source IP and MAC address filtering is enabled, the switch filters IP and non-IP traffic. If the source MAC address of an IP or non-IP packet matches a valid IP source binding, the switch forwards the packet. The switch drops all other types of packets except DHCP packets.
The switch uses port security to filter source MAC addresses. The interface can shut down when a port-security violation occurs.
IP Source Guard for Static Hosts
Note Do not use IPSG for static hosts on uplink ports or trunk ports.
IPSG for static hosts extends the IPSG capability to non-DHCP and static environments. The previous IPSG used the entries created by DHCP snooping to validate the hosts connected to a switch. Any traffic received from a host without a valid DHCP binding entry is dropped. This security feature restricts IP traffic on nonrouted Layer 2 interfaces. It filters traffic based on the DHCP snooping binding database and on manually configured IP source bindings. The previous version of IPSG required a DHCP environment for IPSG to work.
IPSG for static hosts allows IPSG to work without DHCP. IPSG for static hosts relies on IP device tracking-table entries to install port ACLs. The switch creates static entries based on ARP requests or other IP packets to maintain the list of valid hosts for a given port. You can also specify the number of hosts allowed to send traffic to a given port. This is equivalent to port security at Layer 3.
IPSG for static hosts also supports dynamic hosts. If a dynamic host receives a DHCP-assigned IP address that is available in the IP DHCP snooping table, the same entry is learned by the IP device tracking table.When you enter the show ip device tracking all EXEC command, the IP device tracking table displays the entries as ACTIVE.
Note Some IP hosts with multiple network interfaces can inject some invalid packets into a network interface. The invalid packets contain the IP or MAC address for another network interface of the host as the source address. The invalid packets can cause IPSG for static hosts to connect to the host, to learn the invalid IP or MAC address bindings, and to reject the valid bindings. Consult the vender of the corresponding operating system and the network interface to prevent the host from injecting invalid packets.
IPSG for static hosts initially learns IP or MAC bindings dynamically through an ACL-based snooping mechanism. IP or MAC bindings are learned from static hosts by ARP and IP packets. They are stored in the device tracking database. When the number of IP addresses that have been dynamically learned or statically configured on a given port reaches a maximum, the hardware drops any packet with a new IP address. To resolve hosts that have moved or gone away for any reason, IPSG for static hosts leverages IP device tracking to age out dynamically learned IP address bindings. This feature can be used with DHCP snooping. Multiple bindings are established on a port that is connected to both DHCP and static hosts. For example, bindings are stored in both the device tracking database as well as in the DHCP snooping binding database.
Configuring IP Source Guard
These sections contain this configuration information:
•Default IP Source Guard Configuration
•IP Source Guard Configuration Guidelines
•Configuring IP Source Guard for Static Hosts
Default IP Source Guard Configuration
By default, IP source guard is disabled.
IP Source Guard Configuration Guidelines
These are the configuration guides for IP source guard:
•You can configure static IP bindings only on nonrouted ports. If you enter the ip source binding mac-address vlan vlan-id ip-address interface interface-id global configuration command on a routed interface, this error message appears:
Static IP source binding can only be configured on switch port.
•When IP source guard with source IP filtering is enabled on a VLAN, DHCP snooping must be enabled on the access VLAN to which the interface belongs.
•If you are enabling IP source guard on a trunk interface with multiple VLANs and DHCP snooping is enabled on all the VLANs, the source IP address filter is applied on all the VLANs.
Note If IP source guard is enabled and you enable or disable DHCP snooping on a VLAN on the trunk interface, the switch might not properly filter traffic.
•If you enable IP source guard with source IP and MAC address filtering, DHCP snooping and port security must be enabled on the interface. You must also enter the ip dhcp snooping information option global configuration command and ensure that the DHCP server supports option 82. When IP source guard is enabled with MAC address filtering, the DHCP host MAC address is not learned until the host is granted a lease. When forwarding packets from the server to the host, DHCP snooping uses option-82 data to identify the host port.
•When configuring IP source guard on interfaces on which a private VLAN is configured, port security is not supported.
•IP source guard is not supported on EtherChannels.
•You can enable this feature when IEEE 802.1x port-based authentication is enabled.
•If the number of ternary content addressable memory (TCAM) entries exceeds the maximum available, the CPU usage increases.
Enabling IP Source Guard
Beginning in privileged EXEC mode, follow these steps to enable and configure IP source guard on an interface.
To disable IP source guard with source IP address filtering, use the no ip verify source interface configuration command.
To delete a static IP source binding entry, use the no ip source global configuration command.
This example shows how to enable IP source guard with source IP and MAC filtering on VLANs 10
and 11:
Switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)# interface gigabitethernet1/0/1
Switch(config-if)# ip verify source port-security
Switch(config-if)# exit
Switch(config)# ip source binding 0100.0022.0010 vlan 10 10.0.0.2 interface gigabitethernet1/0/1
Switch(config)# ip source binding 0100.0230.0002 vlan 11 10.0.0.4 interface gigabitethernet1/0/1
Switch(config)# end
Configuring IP Source Guard for Static Hosts
•Configuring IP Source Guard for Static Hosts on a Layer 2 Access Port
•Configuring IP Source Guard for Static Hosts on a Private VLAN Host Port
Configuring IP Source Guard for Static Hosts on a Layer 2 Access Port
Note You must configure the ip device tracking maximum limit-number interface configuration command globally for IPSG for static hosts to work. If you only configure this command on a port without enabling IP device tracking globally or by setting an IP device tracking maximum on that interface, IPSG with static hosts rejects all the IP traffic from that interface. This requirement also applies to IPSG with static hosts on a private VLAN host port.
Beginning in privileged EXEC mode:
This example shows how to stop IPSG with static hosts on an interface.
Switch(config-if)# no ip verify source
Switch(config-if)# no ip device tracking max
This example shows how to enable IPSG with static hosts on a port.
Switch(config)# ip device tracking
Switch(config)# ip device tracking max 10
Switch(config-if)# ip verify source tracking port-security
This example shows how to enable IPSG for static hosts with IP filters on a Layer 2 access port and to verify the valid IP bindings on the interface Gi0/3:
Switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)# ip device tracking
Switch(config)# interface gigabitethernet 0/3
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 10
Switch(config-if)# ip device tracking maximum 5
Switch(config-if)# ip verify source tracking
Switch(config-if)# end
Switch# show ip verify source
Interface Filter-type Filter-mode IP-address Mac-address Vlan
--------- ----------- ----------- --------------- ----------------- ----
Gi0/3 ip trk active 40.1.1.24 10
Gi0/3 ip trk active 40.1.1.20 10
Gi0/3 ip trk active 40.1.1.21 10
This example shows how to enable IPSG for static hosts with IP-MAC filters on a Layer 2 access port, to verify the valid IP-MAC bindings on the interface Gi0/3, and to verify that the number of bindings on this interface has reached the maximum:
Switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)# ip device tracking
Switch(config)# interface gigabitethernet 0/3
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 1
Switch(config-if)# ip device tracking maximum 5
Switch(config-if)# switchport port-security
Switch(config-if)# switchport port-security maximum 5
Switch(config-if)# ip verify source tracking port-security
Switch(config-if)# end
Switch# show ip verify source
Interface Filter-type Filter-mode IP-address Mac-address Vlan
--------- ----------- ----------- --------------- ----------------- ----
Gi0/3 ip-mac trk active 40.1.1.24 00:00:00:00:03:04 1
Gi0/3 ip-mac trk active 40.1.1.20 00:00:00:00:03:05 1
Gi0/3 ip-mac trk active 40.1.1.21 00:00:00:00:03:06 1
Gi0/3 ip-mac trk active 40.1.1.22 00:00:00:00:03:07 1
Gi0/3 ip-mac trk active 40.1.1.23 00:00:00:00:03:08 1
This example displays all IP or MAC binding entries for all interfaces. The CLI displays all active as well as inactive entries. When a host is learned on a interface, the new entry is marked as active. When the same host is disconnected from that interface and connected to a different interface, a new IP or MAC binding entry displays as active as soon as the host is detected. The old entry for this host on the previous interface is marked as INACTIVE.
Switch# show ip device tracking all
IP Device Tracking = Enabled
IP Device Tracking Probe Count = 3
IP Device Tracking Probe Interval = 30
---------------------------------------------------------------------
IP Address MAC Address Vlan Interface STATE
---------------------------------------------------------------------
200.1.1.8 0001.0600.0000 8 GigabitEthernet0/1 INACTIVE
200.1.1.9 0001.0600.0000 8 GigabitEthernet0/1 INACTIVE
200.1.1.10 0001.0600.0000 8 GigabitEthernet0/1 INACTIVE
200.1.1.1 0001.0600.0000 9 GigabitEthernet0/2 ACTIVE
200.1.1.1 0001.0600.0000 8 GigabitEthernet0/1 INACTIVE
200.1.1.2 0001.0600.0000 9 GigabitEthernet0/2 ACTIVE
200.1.1.2 0001.0600.0000 8 GigabitEthernet0/1 INACTIVE
200.1.1.3 0001.0600.0000 9 GigabitEthernet0/2 ACTIVE
200.1.1.3 0001.0600.0000 8 GigabitEthernet0/1 INACTIVE
200.1.1.4 0001.0600.0000 9 GigabitEthernet0/2 ACTIVE
200.1.1.4 0001.0600.0000 8 GigabitEthernet0/1 INACTIVE
200.1.1.5 0001.0600.0000 9 GigabitEthernet0/2 ACTIVE
200.1.1.5 0001.0600.0000 8 GigabitEthernet0/1 INACTIVE
200.1.1.6 0001.0600.0000 8 GigabitEthernet0/1 INACTIVE
200.1.1.7 0001.0600.0000 8 GigabitEthernet0/1 INACTIVE
This example displays all active IP or MAC binding entries for all interfaces:
Switch# show ip device tracking all active
IP Device Tracking = Enabled
IP Device Tracking Probe Count = 3
IP Device Tracking Probe Interval = 30
---------------------------------------------------------------------
IP Address MAC Address Vlan Interface STATE
---------------------------------------------------------------------
200.1.1.1 0001.0600.0000 9 GigabitEthernet0/1 ACTIVE
200.1.1.2 0001.0600.0000 9 GigabitEthernet0/1 ACTIVE
200.1.1.3 0001.0600.0000 9 GigabitEthernet0/1 ACTIVE
200.1.1.4 0001.0600.0000 9 GigabitEthernet0/1 ACTIVE
200.1.1.5 0001.0600.0000 9 GigabitEthernet0/1 ACTIVE
This example displays all inactive IP or MAC binding entries for all interfaces. The host was first learned on GigabitEthernet 0/1 and then moved to GigabitEthernet 0/2. the IP or MAC binding entries learned on GigabitEthernet 0/1 are marked as inactive.
Switch# show ip device tracking all inactive
IP Device Tracking = Enabled
IP Device Tracking Probe Count = 3
IP Device Tracking Probe Interval = 30
---------------------------------------------------------------------
IP Address MAC Address Vlan Interface STATE
---------------------------------------------------------------------
200.1.1.8 0001.0600.0000 8 GigabitEthernet0/1 INACTIVE
200.1.1.9 0001.0600.0000 8 GigabitEthernet0/1 INACTIVE
200.1.1.10 0001.0600.0000 8 GigabitEthernet0/1 INACTIVE
200.1.1.1 0001.0600.0000 8 GigabitEthernet0/1 INACTIVE
200.1.1.2 0001.0600.0000 8 GigabitEthernet0/1 INACTIVE
200.1.1.3 0001.0600.0000 8 GigabitEthernet0/1 INACTIVE
200.1.1.4 0001.0600.0000 8 GigabitEthernet0/1 INACTIVE
200.1.1.5 0001.0600.0000 8 GigabitEthernet0/1 INACTIVE
200.1.1.6 0001.0600.0000 8 GigabitEthernet0/1 INACTIVE
200.1.1.7 0001.0600.0000 8 GigabitEthernet0/1 INACTIVE
This example displays the count of all IP device tracking host entries for all interfaces:
Switch# show ip device tracking all count
Total IP Device Tracking Host entries: 5
---------------------------------------------------------------------
Interface Maximum Limit Number of Entries
---------------------------------------------------------------------
Gi0/3 5
Configuring IP Source Guard for Static Hosts on a Private VLAN Host Port
Note You must globally configure the ip device tracking maximum limit-number interface configuration command globally for IPSG for static hosts to work. If you only configure this command on a port without enabling IP device tracking globally or setting an IP device tracking maximum on that interface, IPSG with static hosts will reject all the IP traffic from that interface. This requirement also applies to IPSG with static hosts on a Layer 2 access port.
Beginning in privileged EXEC mode, follow these steps to configure IPSG for static hosts with IP filters on a Layer 2 access port:
This example shows how to enable IPSG for static hosts with IP filters on a private VLAN host port:
Switch(config)# vlan 200
Switch(config-vlan)# private-vlan primary
Switch(config-vlan)# exit
Switch(config)# vlan 201
Switch(config-vlan)# private-vlan isolated
Switch(config-vlan)# exit
Switch(config)# vlan 200
Switch(config-vlan)# private-vlan association 201
Switch(config-vlan)# exit
Switch(config)# int gigabitethernet0/3
Switch(config-if)# switchport mode private-vlan host
Switch(config-if)# switchport private-vlan host-association 200 201
Switch(config-if)# ip device tracking maximum 8
Switch(config-if)# ip verify source tracking
Switch# show ip device tracking all
IP Device Tracking = Enabled
IP Device Tracking Probe Count = 3
IP Device Tracking Probe Interval = 30
---------------------------------------------------------------------
IP Address MAC Address Vlan Interface STATE
---------------------------------------------------------------------
40.1.1.24 0000.0000.0304 200 FastEthernet0/3 ACTIVE
40.1.1.20 0000.0000.0305 200 FastEthernet0/3 ACTIVE
40.1.1.21 0000.0000.0306 200 FastEthernet0/3 ACTIVE
40.1.1.22 0000.0000.0307 200 FastEthernet0/3 ACTIVE
40.1.1.23 0000.0000.0308 200 FastEthernet0/3 ACTIVE
The output shows the five valid IP-MAC bindings that have been learned on the interface Fa0/3. For the private VLAN cases, the bindings are associated with primary VLAN ID. So, in this example, the primary VLAN ID, 200, is shown in the table.
Switch# show ip verify source
Interface Filter-type Filter-mode IP-address Mac-address Vlan
--------- ----------- ----------- --------------- ----------------- ----
Fa0/3 ip trk active 40.1.1.23 200
Fa0/3 ip trk active 40.1.1.24 200
Fa0/3 ip trk active 40.1.1.20 200
Fa0/3 ip trk active 40.1.1.21 200
Fa0/3 ip trk active 40.1.1.22 200
Fa0/3 ip trk active 40.1.1.23 201
Fa0/3 ip trk active 40.1.1.24 201
Fa0/3 ip trk active 40.1.1.20 201
Fa0/3 ip trk active 40.1.1.21 201
Fa0/30/3 ip trk active 40.1.1.22 201
The output shows that the five valid IP-MAC bindings are on both the primary and secondary VLAN.
Displaying IP Source Guard Information
To display the IP source guard information, use one or more of the privileged EXEC commands in Table 21-3:
Understanding DHCP Server Port-Based Address Allocation
DHCP server port-based address allocation is a feature that enables DHCP to maintain the same IP address on an Ethernet switch port regardless of the attached device client identifier or client hardware address.
When Ethernet switches are deployed in the network, they offer connectivity to the directly connected devices. In some environments, such as on a factory floor, if a device fails, the replacement device must be working immediately in the existing network. With the current DHCP implementation, there is no guarantee that DHCP would offer the same IP address to the replacement device. Control, monitoring, and other software expect a stable IP address associated with each device. If a device is replaced, the address assignment should remain stable even though the DHCP client has changed.
When configured, the DHCP server port-based address allocation feature ensures that the same IP address is always offered to the same connected port even as the client identifier or client hardware address changes in the DHCP messages received on that port. The DHCP protocol recognizes DHCP clients by the client identifier option in the DHCP packet. Clients that do not include the client identifier option are identified by the client hardware address. When you configure this feature, the port name of the interface overrides the client identifier or hardware address and the actual point of connection, the switch port, becomes the client identifier.
In all cases, by connecting the Ethernet cable to the same port, the same IP address is allocated through DHCP to the attached device.
The DHCP server port-based address allocation feature is only supported on a Cisco IOS DHCP server and not a third-party server.
Configuring DHCP Server Port-Based Address Allocation
This section contains this configuration information:
•Default Port-Based Address Allocation Configuration
•Port-Based Address Allocation Configuration Guidelines
•Enabling DHCP Server Port-Based Address Allocation
Default Port-Based Address Allocation Configuration
By default, DHCP server port-based address allocation is disabled.
Port-Based Address Allocation Configuration Guidelines
These are the configuration guidelines for DHCP port-based address allocation:
•Only one IP address can be assigned per port.
•Reserved addresses (preassigned) cannot be cleared by using the clear ip dhcp binding global configuration command.
•Preassigned addresses are automatically excluded from normal dynamic IP address assignment. Preassigned addresses cannot be used in host pools, but there can be multiple preassigned addresses per DHCP address pool.
•To restrict assignments from the DHCP pool to preconfigured reservations (unreserved addresses are not offered to the client and other clients are not served by the pool), you can enter the reserved-only DHCP pool configuration command.
Enabling DHCP Server Port-Based Address Allocation
Beginning in privileged EXEC mode, follow these steps to globally enable port-based address allocation and to automatically generate a subscriber identifier on an interface.
After enabling DHCP port-based address allocation on the switch, use the ip dhcp pool global configuration command to preassign IP addresses and to associate them to clients. To restrict assignments from the DHCP pool to preconfigured reservations, you can enter the reserved-only DHCP pool configuration command. Unreserved addresses that are part of the network or on pool ranges are not offered to the client, and other clients are not served by the pool. By entering this command, users can configure a group of switches with DHCP pools that share a common IP subnet and that ignore requests from clients of other switches.
Beginning in privileged EXEC mode follow these steps to preassign an IP address and to associate it to a client identified by the interface name.
To disable DHCP port-based address allocation, use the no ip dhcp use subscriber-id client-id global configuration command. To disable the automatic generation of a subscriber identifier, use the no ip dhcp subscriber-id interface-name global configuration command. To disable the subscriber identifier on an interface, use the no ip dhcp server use subscriber-id client-id interface configuration command.
To remove an IP address reservation from a DHCP pool, use the no address ip-address client-id string DHCP pool configuration command. To change the address pool to nonrestricted, enter the no reserved-only DHCP pool configuration command.
In this example, a subscriber identifier is automatically generated, and the DHCP server ignores any client identifier fields in the DHCP messages and uses the subscriber identifier instead. The subscriber identifier is based on the short name of the interface and the client preassigned IP address 10.1.1.7.
switch# show running config
Building configuration...
Current configuration : 4899 bytes
!
version 12.2
!
hostname switch
!
no aaa new-model
clock timezone EST 0
ip subnet-zero
ip dhcp relay information policy removal pad
no ip dhcp use vrf connected
ip dhcp use subscriber-id client-id
ip dhcp subscriber-id interface-name
ip dhcp excluded-address 10.1.1.1 10.1.1.3
!
ip dhcp pool dhcppool
network 10.1.1.0 255.255.255.0
address 10.1.1.7 client-id "Et1/0" ascii
<output truncated>
This example shows that the preassigned address was correctly reserved in the DHCP pool:
switch# show ip dhcp pool dhcppool
Pool dhcp pool:
Utilization mark (high/low) : 100 / 0
Subnet size (first/next) : 0 / 0
Total addresses : 254
Leased addresses : 0
Excluded addresses : 4
Pending event : none
1 subnet is currently in the pool:
Current index IP address range Leased/Excluded/Total
10.1.1.1 10.1.1.1 - 10.1.1.254 0 / 4 / 254
1 reserved address is currently in the pool
Address Client
10.1.1.7 Et1/0
For more information about configuring the DHCP server port-based address allocation feature, go to Cisco.com, and enter Cisco IOS IP Addressing Services in the Search field to access the Cisco IOS software documentation. You can also access the documentation here:
http://www.cisco.com/en/US/docs/ios/ipaddr/command/reference/iad_book.html
Displaying DHCP Server Port-Based Address Allocation
To display the DHCP server port-based address allocation information, use one or more of the privileged EXEC commands in Table 21-4: