Technical Documentation

Example: Configuring DHCP Snooping, DAI , and MAC Limiting on an EX Series Switch with Access to a DHCP Server Through a Second Switch

You can configure DHCP snooping, dynamic ARP inspection (DAI), and MAC limiting on the access interfaces of EX Series switches to protect the switch and the Ethernet LAN against address spoofing and Layer 2 denial-of-service (DoS) attacks. To obtain those basic settings, you can use the switch's default configuration for port security, configure the MAC limit, and enable DHCP snooping and DAI on a VLAN. You can configure those features when the DHCP server is connected to a different switch from the one to which the DHCP clients (network devices) are connected.

This example describes how to configure port security features on an EX Series switch whose hosts obtain IP addresses and lease times from a DHCP server connected to a second switch:

Requirements

This example uses the following hardware and software components:

  • One EX3200-24P switch—“Switch 1” in this example.
  • An additional EX Series switch—”Switch 2” in this example. You will not configure port security on this switch.
  • JUNOS Release 9.0 or later for EX Series switches.
  • A DHCP server connected to Switch 2. You will use the server to provide IP addresses to network devices connected to Switch 1.
  • At least two network devices (hosts) that you will connect to access interfaces on Switch 1. These devices will be DHCP clients.

Before you configure DHCP snooping, DAI, and MAC limiting port security features, be sure you have:

Overview and Topology

Ethernet LANs are vulnerable to address spoofing and DoS attacks on network devices. To protect the devices from such attacks, you can configure:

  • DHCP snooping to validate DHCP server messages
  • DAI to protect against ARP spoofing
  • MAC limiting to constrain the number of MAC addresses the switch adds to its MAC address cache

This example shows how to configure these port security features on an EX3200 switch, which is Switch 1 in this example. (You could also use an EX4200 switch for this example.) Switch 1 is connected to a switch that is not configured with port security features. That second switch (Switch 2) is connected to a DHCP server. (See Figure 1. ) Network devices (hosts) that are connected to Switch 1 will send requests for IP addresses (that is, the devices will be DHCP clients). Those requests will be transmitted from Switch 1 to Switch 2 and then to the DHCP server connected to Switch 2. Responses to the requests will be transmitted along the reverse path of the one followed by the requests.

The setup for this example includes the VLAN employee-vlan on both switches.

Figure 1 shows the network topology for the example.

Figure 1: Network Topology for Port Security Setup with Two Switches on the Same VLAN

Image g020047.gif

The components of the topology for this example are shown in Table 1.

Table 1: Components of Port Security Setup on Switch 1 with a DHCP Server Connected to Switch 2

Properties Settings

Switch hardware

One EX3200-24P (Switch 1), and an additional EX Series switch (Switch 2)

VLAN name and ID

employee-vlan, tag 20

VLAN subnets

192.0.2.16/28
192.0.2.17 through 192.0.2.30
192.0.2.31 is subnet's broadcast address

Trunk interface on both switches

ge-0/0/11

Access interfaces on Switch 1

ge-0/0/1, ge-0/0/2, and ge-0/0/3

Access interface on Switch 2

ge-0/0/1

Interface for DHCP server

ge-0/0/1 on Switch 2

Switch 1 is initially configured with the default port security setup. In the default configuration on the switch:

  • Secure port access is activated on the switch.
  • The switch does not drop any packets, which is the default setting.
  • DHCP snooping and dynamic ARP inspection (DAI) are disabled on all VLANs.
  • All access interfaces are untrusted and trunk interfaces are trusted; these are the default settings.

In the configuration tasks for this example, you configure a VLAN on both switches.

In addition to configuring the VLAN, you enable DHCP snooping on Switch 1. In this example, you will also enable DAI and a MAC limit of 5 on Switch 1.

Because the interface that connects Switch 2 to Switch 1 is a trunk interface, you do not have to configure this interface to be trusted. As noted above, trunk interfaces are automatically trusted, so DHCP messages coming from the DHCP server to Switch 2 and then on to Switch 1 are trusted.

Configuring a VLAN, Interfaces, and Port Security Features on Switch 1

To configure a VLAN, interfaces, and port security features on Switch 1:

CLI Quick Configuration

To quickly configure a VLAN, interfaces, and port security features, copy the following commands and paste them into the switch terminal window:


[edit]
set ethernet-switching-options secure-access-port interface ge-0/0/1 mac–limit 5
set ethernet-switching-options secure-access-port vlan employee-vlan arp–inspection
set ethernet-switching-options secure-access-port vlan employee-vlan examine–dhcp
set interfaces ge-0/0/1 unit 0 family ethernet-switching vlan members 20
set interfaces ge-0/0/2 unit 0 family ethernet-switching vlan members 20
set interfaces ge-0/0/3 unit 0 family ethernet-switching vlan members 20
set interfaces ge-0/0/11 unit 0 family ethernet-switching port-mode trunk
set interfaces ge-0/0/11 unit 0 family ethernet-switching vlan members 20
set vlans employee–vlan vlan-id 20

Step-by-Step Procedure

To configure MAC limiting, a VLAN, and interfaces on Switch 1 and enable DAI and DHCP on the VLAN:

  1. Configure the VLAN employee-vlan with VLAN ID 20:

    [edit vlans]
    user@switch1# set employee-vlan vlan-id 20
  2. Configure an interface on Switch 1 as a trunk interface:

    [edit interfaces]
    user@switch1# set ge-0/0/11 unit 0 family ethernet-switching port-mode trunk
  3. Associate the VLAN with interfaces ge-0/0/1, ge-0/0/2, ge-0/0/3, and ge-0/0/11:

    [edit interfaces]
    user@switch1# set ge-0/0/1 unit 0 family ethernet-switching vlan members 20
    user@switch1# set ge-0/0/2 unit 0 family ethernet-switching vlan members 20
    user@switch1# set ge-0/0/3 unit 0 family ethernet-switching vlan members 20
    user@switch1# set ge-0/0/11 unit 0 family ethernet-switching vlan members 20
  4. Enable DHCP snooping on the VLAN:

    [edit ethernet-switching-options secure-access-port]
    user@switch1# set vlan employee-vlan examine-dhcp
  5. Enable DAI on the VLAN:

    [edit ethernet-switching-options secure-access-port]
    user@switch1# set vlan employee-vlan arp-inspection
  6. Configure a MAC limit of 5 on ge-0/0/1 and use the default action, drop (packets with new addresses are dropped if the limit has been exceeded):

    [edit ethernet-switching-options secure-access-port]
    user@switch1# set interface ge-0/0/1 mac-limit 5

Results

Display the results of the configuration:

[edit]
user@switch1# show
ethernet-switching-options {
secure-access-port {
interface ge-0/0/1.0{
mac-limit 5 action drop;
}
vlan employee-vlan {
arp-inspection;
examine-dhcp;
}
}
}
interfaces {
ge-0/0/1 {
unit 0 {
family ethernet-switching {
vlan {
members 20;
}
}
}
}
ge-0/0/2 {
unit 0 {
family ethernet-switching {
vlan {
members 20;
}
}
}
}
ge-0/0/3 {
unit 0 {
family ethernet-switching {
vlan {
members 20;
}
}
}
}
ge-0/0/11 {
unit 0 {
family ethernet-switching {
port-mode trunk;
vlan {
members 20;
}
}
}
}
}
vlans {
employee-vlan {
vlan-id 20;
}
}

Configuring a VLAN and Interfaces on Switch 2

To configure the VLAN and interfaces on Switch 2:

CLI Quick Configuration

To quickly configure the VLAN and interfaces on Switch 2, copy the following commands and paste them into the switch terminal window:


[edit]
set interfaces ge-0/0/1 unit 0 family ethernet-switching port-mode trunk
set interfaces ge-0/0/11 unit 0 family ethernet-switching port-mode trunk
set vlans employee-vlan vlan-id 20

Step-by-Step Procedure

To configure the VLAN and interfaces on Switch 2:

  1. Configure an interface on Switch 2 as a trunk interface:

    [edit interfaces]
    user@switch2# set ge-0/0/11 unit 0 ethernet-switching port-mode trunk
  2. Associate the VLAN with interfaces ge-0/0/1 and ge-0/0/11:

    [edit interfaces]
    user@switch2# set ge-0/0/1 unit 0 family ethernet-switching vlan members 20
    user@switch2# set ge-0/0/11 unit 0 family ethernet-switching vlan members 20

Results

Display the results of the configuration:

[edit]
user@switch2# show
interfaces {
ge-0/0/1 {
unit 0 {
family ethernet-switching {
vlan {
members 20;
}
}
}
}
ge-0/0/11 {
unit 0 {
family ethernet-switching {
port-mode trunk;
vlan {
members 20;
}
}
}
}
}
vlans {
employee-vlan {
vlan-id 20;
}
}

Verification

To confirm that the configuration is working properly:

Verifying That DHCP Snooping Is Working Correctly on Switch 1

Purpose

Verify that DHCP snooping is working on Switch 1.

Action

Send some DHCP requests from network devices (here they are DHCP clients) connected to the switch.

Display the DHCP snooping information when the interface through which Switch 2 sends the DHCP server replies to clients connected to Switch 1 is trusted. The server has provided the IP addresses and leases:


DHCP Snooping Information:
MAC Address         IP Address    Lease    Type     VLAN          Interface

----------------- ---------- ----- ---- ---- ---------
00:05:85:3A:82:77 192.0.2.17 600 dynamic employee—vlan ge-0/0/1.0
00:05:85:3A:82:79 192.0.2.18 653 dynamic employee—vlan ge-0/0/1.0
00:05:85:3A:82:80 192.0.2.19 720 dynamic employee—vlan ge-0/0/1.0
00:05:85:3A:82:81 192.0.2.20 932 dynamic employee—vlan ge-0/0/1.0
00:05:85:3A:82:83 192.0.2.21 1230 dynamic employee—vlan ge-0/0/1.0
00:05:85:3A:82:90 192.0.2.20 932 dynamic employee—vlan ge-0/0/2.0
00:05:85:3A:82:91 192.0.2.21 1230 dynamic employee—vlan ge-0/0/3.0

Meaning

The output shows, for each MAC address, the assigned IP address and lease time—that is, the time, in seconds, remaining before the lease expires.

Verifying That DAI Is Working Correctly on Switch 1

Purpose

Verify that DAI is working on Switch 1.

Action

Send some ARP requests from network devices connected to the switch.

Display the DAI information:


ARP inspection statistics:
Interface    Packets received  ARP inspection pass  ARP inspection failed
----------   –-------------–-  -------------------  ---------------------
ge-0/0/1.0                  7                    5                      2
ge-0/0/2.0                 10                   10                      0
ge-0/0/3.0                 18                   15                      3

Meaning

The sample output shows the number of ARP packets received and inspected per interface, with a listing of how many packets passed and how many failed the inspection on each interface. The switch compares the ARP requests and replies against the entries in the DHCP snooping database. If a MAC address or IP address in the ARP packet does not match a valid entry in the database, the packet is dropped.

Verifying That MAC Limiting Is Working Correctly on Switch 1

Purpose

Verify that MAC limiting is working on Switch 1.

Action

Display the MAC addresses that are learned when DHCP requests are sent from hosts on ge-0/0/1:


Ethernet-switching table:  6 entries, 5 learned

VLAN MAC address Type Age Interfaces employee-vlan 00:05:85:3A:82:77 Learn 0 ge-0/0/1.0 employee-vlan 00:05:85:3A:82:79 Learn 0 ge-0/0/1.0 employee-vlan 00:05:85:3A:82:80 Learn 0 ge-0/0/1.0 employee-vlan 00:05:85:3A:82:81 Learn 0 ge-0/0/1.0 employee-vlan 00:05:85:3A:82:83 Learn 0 ge-0/0/1.0 employee-vlan * Flood - ge-0/0/1.0

Meaning

The sample output shows that five MAC addresses have been learned for interface ge-0/0/1, which corresponds to the MAC limit of 5 set in the configuration. The last line of the output shows that a sixth MAC address request was dropped, as indicated by the asterisk (*) in the MAC address column.


Published: 2009-10-08