zNetLaB › Guided labs › Security
Dynamic ARP Inspection — a host that is not who it says it is
Security Advanced 4 devices 5 checked tasks 100 points Plan lab
ARP has no authentication, so any host can claim any address. The switch already learns who really owns which address from DHCP snooping; Dynamic ARP Inspection checks every ARP against that table. Build the chain — snooping, then inspection, with the uplink trusted — and see the statically-addressed intruder lose its voice while the legitimate host carries on. You build it in a browser, type real commands at each device, and every task below is
checked against the running configuration — so you find out it is wrong before anybody else does.
Open this lab in the simulator
The simulator runs in your browser with nothing to install. This lab is part of the full simulator: open it free for 7 days with a new account, then with a plan. It needs an account, which keeps your progress.
The scenario
ARP has no authentication, so any host can claim any address. The switch already learns who really owns which address from DHCP snooping; Dynamic ARP Inspection checks every ARP against that table. Build the chain — snooping, then inspection, with the uplink trusted — and see the statically-addressed intruder lose its voice while the legitimate host carries on.
Your role: a security engineer. Level: Advanced Done when: all 5 goals show as passed (100 points).
Network status
Devices: 1 switch, 1 router, 2 PCs, cabled as drawn. No Layer 1 or Layer 2 faults: every link is connected and up. GW starts with addresses (G0/0 10.10.0.1/24) and a DHCP pool already configured. Intruder is a host with 10.10.0.50/24, gateway 10.10.0.1. SW-Access starts with an empty configuration. Known problems
None: nothing is misconfigured. The network is simply not finished: what the goals list is what you have to build or secure. Goals
DHCP snooping is on for VLAN 1 (it builds the address table inspection relies on) (20 pts) Dynamic ARP Inspection is on for VLAN 1 (25 pts) The uplink is trusted for both, or the gateway would be cut off (15 pts) The victim, which leased its address, still reaches the gateway (20 pts) The intruder, which took an address by hand, cannot reach the gateway (20 pts) The activity, in order
DHCP snooping is on for VLAN 1 (it builds the address table inspection relies on) Why: This is the task that checks your configuration directly: the line must really be there, not just look right. Verify with: show running-config Dynamic ARP Inspection is on for VLAN 1 Why: This is the task that checks your configuration directly: the line must really be there, not just look right. Verify with: show running-config The uplink is trusted for both, or the gateway would be cut off Why: This is the task that checks your configuration directly: the line must really be there, not just look right. Verify with: show running-config The victim, which leased its address, still reaches the gateway Why: If the host has no address, look at the pool, the exclusions and, across a router, the ip helper-address. Verify with: ipconfig The intruder, which took an address by hand, cannot reach the gateway Why: Blocking traffic is half of security. You prove a block by showing the same ping that worked before now fails. Verify with: ping 10.10.0.1 Tips
Work in the order given: later goals depend on the earlier ones. After every change, run the verify command before moving on; do not guess. Stuck? Read the verify output first. It names the interface, neighbour or route that is wrong. Common mistake: Enabling DAI without snooping — with no bindings every ARP is dropped, including the legitimate ones Common mistake: Not trusting the router-facing port Common mistake: Statically addressed servers that need an ARP ACL or a static binding Why this matters
ARP trusts anything. An attacker who claims the gateway’s address in an ARP reply becomes the man in the middle. Dynamic ARP Inspection checks every ARP packet against the trusted IP-to-MAC bindings that DHCP snooping built.
What you will be able to do
Build the dependency chain: DHCP snooping, then ARP inspection
Trust the uplink for both features
Explain why a statically addressed host is dropped by DAI and how a static ARP ACL would admit it
The network
Catalyst 2960-24TT, ISR 2911, PC — 4 devices in all, already cabled.
Fa0/1 Fa0 Fa0/2 Fa0 Fa0/24 Gi0/0 10.10.0.1/24 SW-Access GW Victim Intruder 10.10.0.50/24 · gw 10.10.0.1 Drawn from this lab: devices, cables, ports and the addresses already set. Device Model Address already set
SW-Access Catalyst 2960-24TT configure it GW ISR 2911 configure it Victim PC configure it Intruder PC 10.10.0.50 / 255.255.255.0 gw 10.10.0.1
Cabling
SW-Access Fa0/1 ↔ Victim Fa0 SW-Access Fa0/2 ↔ Intruder Fa0 SW-Access Fa0/24 ↔ GW Gi0/0
What is configured at the start
These are already configured when the lab opens — including, where this is a troubleshooting lab, the fault itself:
GW interface g0/0
ip address 10.10.0.1 255.255.255.0
no shutdown
exit
ip dhcp excluded-address 10.10.0.1 10.10.0.9
ip dhcp pool LAN
network 10.10.0.0 255.255.255.0
default-router 10.10.0.1
exit Tasks, and how each one is checked
Every task is decided by reading the live network — a neighbour in FULL, a ping that succeeds, a line in the running configuration — not by looking for a keyword you typed. The same command shows you what the lab sees.
# Task Pts How to verify it yourself
1 DHCP snooping is on for VLAN 1 (it builds the address table inspection relies on) 20 On SW-Access: show running-config contains the line(s) matching ip dhcp snooping vlan 1. 2 Dynamic ARP Inspection is on for VLAN 1 25 On SW-Access: show running-config contains the line(s) matching ip arp inspection vlan 1. 3 The uplink is trusted for both, or the gateway would be cut off 15 On SW-Access: show running-config contains the line(s) matching interface FastEthernet0/24 … ip dhcp snooping trust … ip arp inspection trust|interface FastEthernet0/24 … ip arp inspection trust … ip dhcp snooping trust. 4 The victim, which leased its address, still reaches the gateway 20 On Victim: ipconfig shows an address. and On Victim: ping 10.10.0.1 must succeed. 5 The intruder, which took an address by hand, cannot reach the gateway 20 On Intruder: ping 10.10.0.1 must fail — the policy blocks it.
Stuck? Ask the coach
Tell the coach what you see or what you tried. It gives you a hint, not the answer, the way a senior engineer would. Included with a free trial or any plan.
Ask the coach
Common mistakes
Enabling DAI without snooping — with no bindings every ARP is dropped, including the legitimate ones
Not trusting the router-facing port
Statically addressed servers that need an ARP ACL or a static binding
Reference solution
Try it yourself first — the lab checks your work either way. This is one configuration that makes every task pass, device by device:
SW-Access ip dhcp snooping
ip dhcp snooping vlan 1
ip arp inspection vlan 1
interface Fa0/24
ip dhcp snooping trust
ip arp inspection trust
exit Victim ipconfig /release
ipconfig /renew Where this is examined
CCNA security fundamentals, CCNP SCOR (Layer 2 attacks), Security+ (ARP poisoning), CEH (sniffing and MITM).
This lab practises these topics in the zNetLaB certification courses:
Port security, DHCP snooping, DAI — CCNA · Security Fundamentals AAA, uRPF, CoPP, IPv6 first-hop security — CCNP Enterprise · ENARSI · Infrastructure Security CoPP, uRPF, IPv6 FHS, 802.1X, ZBFW — CCIE Enterprise Infrastructure · Infrastructure Security and Services Attacks and their defences (spoofing, flooding, rogue devices) — CompTIA Network+ · Network Security Network attacks: spoofing, flooding, rogue devices, VLAN hopping — CompTIA Security+ · Threats, Vulnerabilities and Mitigations Common threats, vulnerabilities and attack techniques — CCNP Security Core · Security Concepts Sniffing and man-in-the-middle: rogue DHCP, ARP spoofing — Ethical Hacking (network level) · Network Attacks and Defences
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