Configuring IP Source Guard

This chapter describes how to configure IP Source Guard on Cisco NX-OS devices.

This chapter includes the following sections:

IP source guard overview

IP Source Guard is a filter that operates on a per interface basis.

IP traffic is permitted only if the IP address and MAC address of each packet match an entry from either the DHCP snooping binding table or a static IP source entry.

  • Entries in the Dynamic Host Configuration Protocol (DHCP) snooping binding table

  • Static IP source entries that you configure

Filtering on trusted IP and MAC address bindings helps prevent spoofing attacks. In a spoofing attack, an attacker uses the IP address of a valid host to gain unauthorized network access. To circumvent IP Source Guard, an attacker would have to spoof both the IP address and the MAC address of a valid host.

IP source guard architecture

You can enable IP Source Guard on Layer 2 interfaces that are not trusted by DHCP snooping. It supports interfaces operating in access mode and trunk mode. When you first enable IP Source Guard, the interface blocks all inbound IP traffic except:

  • DHCP packets that DHCP snooping inspects and then forwards or drops, depending on the results of the inspection

  • IP traffic from static IP source entries you have configured on the NX-OS device.

The device permits IP traffic when DHCP snooping adds a binding table entry for the IP address and MAC address of a packet, or when a static IP source entry has been configured.

The device drops IP packets when neither the IP address nor the MAC address have a binding table entry or a static IP source entry.

For example, assume that the show ip dhcp snooping binding command displays this binding table entry: 

MacAddress         IpAddress    LeaseSec   Type           VLAN    Interface    
-----------------  ----------   ---------  -------------  ----    ---------  
00:02:B3:3F:3B:99  10.5.5.2     6943       dhcp-snooping  10      Ethernet2/3

If the device receives an IP packet with an IP address of 10.5.5.2, IP Source Guard forwards the packet only if the MAC address of the packet is 00:02:B3:3F:3B:99.

Prerequisites for IP Source Guard

IP Source Guard requires these prerequisites

  • You must enable the DHCP feature and DHCP snooping before you can configure IP Source Guard. See Configuring DHCP .

  • You must configure the ACL TCAM region size for IP Source Guard using the hardware access-list tcam region ipsg command. See Configuring ACL TCAM Region Sizes .


    Note

    By default, the ipsg region size is zero. Allocate enough entries to this region to store and enforce the SMAC–IP bindings.

Recommendation for IP source guard configuration

IP Source Guard has the following configuration guidelines and limitations:

General guidelines

  • When you enable IP Source Guard on an interface, only sources with an IP-MAC address binding table entry or a static IP source entry can send IP traffic. You may notice a disruption in IP traffic until hosts on the interface receive a new IP address from a DHCP server.

  • You must enable DHCP snooping to build and maintain the IP-MAC address binding table with IP Source Guard, or manually maintain static IP source entries.

  • Do not use IP Source Guard on fabric extender (FEX) ports or generic expansion module (GEM) ports because it is not supported.

  • Do not configure IP Source Guard on EoR, as it is not supported.

  • The following guidelines and limitations apply to the Cisco Nexus 9200 Series switches:

    • IPv6 adjacency is not formed with IPSG enabled on the incoming interface.

    • IPSG drops ARP packets at HSRP standby.

    • With DHCP snooping and IPSG enabled, if a binding entry exists for the host, traffic is forwarded to the host even without ARP.

  • Beginning with Cisco NX-OS Release 9.3(5), IP Source Guard is supported on Cisco Nexus 9364C-GX, Cisco Nexus 9316D-GX, and Cisco Nexus 93600CD-GX switches.

  • You do not need to configure TCAM carving when you use IP Source Guard on Cisco Nexus 9300-X Cloud Scale Switches.

  • If IPSG is enabled, port security can't be enabled on the interface.

Default settings for IP source guard

Use the table to see the default states and values for key IP Source Guard configuration parameters.

Table 1. Default IP source guard parameters

Parameters

Default

IP Source Guard

IP Source Guard is disabled on all interfaces.

IP source entries

There are no static or default IP source entries.

Configuring IP Source Guard

Enabling or Disabling IP Source Guard on a Layer 2 Interface

You can enable or disable IP Source Guard on a Layer 2 interface. By default, IP Source Guard is disabled on all interfaces.

Before you begin

Make sure that the DHCP feature and DHCP snooping are enabled.

Make sure that the ACL TCAM region size for IPSG (ipsg) is configured.

Procedure

  Command or Action Purpose

Step 1

configure terminal

Example:

switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 2

interface ethernet slot/port

Example:

switch(config)# interface ethernet 2/3
switch(config-if)#

Enters interface configuration mode for the specified interface.

Step 3

[no] ip verify source dhcp-snooping-vlan

Example:

switch(config-if)# ip verify source dhcp-snooping vlan

Enables IP Source Guard on the interface. The no form of this command disables IP Source Guard on the interface.

Step 4

(Optional) show running-config dhcp

Example:

switch(config-if)# show running-config dhcp
 (Optional)

Displays the running configuration for DHCP snooping, including the IP Source Guard configuration.

Step 5

(Optional) copy running-config startup-config

Example:

switch(config-if)# copy running-config startup-config
 (Optional)

Copies the running configuration to the startup configuration.

Adding or Removing a Static IP Source Entry

You can add or remove a static IP source entry on the device. By default, there are no static IP source entries.

Procedure

  Command or Action Purpose

Step 1

configure terminal

Example:

switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 2

[no] ip source binding ip-address mac-address vlan vlan-id interface interface-type slot/port

Example:

switch(config)# ip source binding 10.5.22.17 001f.28bd.0013 vlan 100 interface ethernet 2/3

Creates a static IP source entry for the current interface. The no form of this command removes the static IP source entry.

Step 3

(Optional) show ip dhcp snooping binding [interface interface-type slot/port]

Example:

switch(config)# show ip dhcp snooping binding interface ethernet 2/3
 (Optional)

Displays IP-MAC address bindings for the interface specified, including static IP source entries. Static entries appear with the term in the Type column.

Step 4

(Optional) copy running-config startup-config

Example:

switch(config)# copy running-config startup-config
 (Optional)

Copies the running configuration to the startup configuration.

Configuring IP Source Guard for Trunk Ports

When IP Source Guard is configured on a port, traffic coming on that port will be dropped unless there is a DHCP snooping entry to allow it in the TCAM. However, when IP Source Guard is configured on trunk ports and you do not want traffic coming on certain VLANs to undergo this check (even if DHCP snooping is not enabled on them), you can specify a list of VLANs to exclude.

Before you begin

Make sure that the DHCP feature and DHCP snooping are enabled.

Procedure

  Command or Action Purpose

Step 1

configure terminal

Example:

switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 2

[no] ip dhcp snooping ipsg-excluded vlan vlan-list

Example:

switch(config)# ip dhcp snooping ipsg-excluded vlan 1001-1256,3097

Specifies the list of VLANs to exclude from the DHCP snooping check for IP Source Guard on trunk ports.

Step 3

(Optional) show ip ver source [ethernet slot/port | port-channel channel-number]

Example:

switch(config)# show ip ver source
 (Optional)

Displays which VLANs are excluded.

Step 4

(Optional) copy running-config startup-config

Example:

switch(config)# copy running-config startup-config
 (Optional)

Copies the running configuration to the startup configuration.

Displaying IP Source Guard Bindings

Use the show ip ver source [ethernet slot/port | port-channel channel-number] command to display the IP-MAC address bindings.

Clearing IP Source Guard Statistics

To clear IP Source Guard statistics, use the commands in this table.

Command

Purpose

clear access-list ipsg stats [instance number | module number]

Clears IP Source Guard statistics.

IP Source Guard configuration example

This example demonstrates how to create a static IP source entry and enable IP Source Guard on an interface. 

ip source binding 192.0.2.17 001f.28bd.0013 vlan 100 interface ethernet 2/3
        interface ethernet 2/3
        no shutdown
        ip verify source dhcp-snooping-vlan
        show ip ver source
        
        IP source guard excluded vlans:
        ------------------------------------------------------ 
        None
        
        -----------------------------------
        IP source guard is enabled on the following interfaces:
        ------------------------------------------------------ 
        ethernet2/3