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Unicast RPF — drop packets whose source cannot be real
Security Advanced 3 devices 3 checked tasks 100 points Plan lab
Spoofed source addresses are how reflection attacks and many floods hide. A router that checks whether it could reach a packet’s source back out the interface it arrived on drops the lie. Turn on strict unicast RPF on the customer-facing interface and see the check in the interface’s own statistics. 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
Spoofed source addresses are how reflection attacks and many floods hide. A router that checks whether it could reach a packet’s source back out the interface it arrived on drops the lie. Turn on strict unicast RPF on the customer-facing interface and see the check in the interface’s own statistics.
Your role: a security engineer. Level: Advanced Done when: all 3 goals show as passed (100 points).
Network status
Devices: 1 router, 1 PC, 1 server, cabled as drawn. No Layer 1 or Layer 2 faults: every link is connected and up. Edge starts with addresses (G0/0 192.168.10.1/24, G0/1 10.0.0.1/24). Customer-PC is a host with 192.168.10.10/24, gateway 192.168.10.1. Core-Server is a host with 10.0.0.10/24, gateway 10.0.0.1. Known problems
The network worked and now does not: the scenario above says what users report. The cause is a configuration fault, not a cable. Find it with show commands (interfaces, routes, neighbours, running-config) before you change anything; fix only what is wrong. Goals
Strict uRPF (rx) is on the customer-facing interface (40 pts) It is not on the core-facing interface (asymmetric routes would break) (20 pts) The customer still reaches the core server (a real source passes the check) (40 pts) The activity, in order
Strict uRPF (rx) is on the customer-facing interface 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 It is not on the core-facing interface (asymmetric routes would break) 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 customer still reaches the core server (a real source passes the check) Why: A ping needs a route in BOTH directions. This is the end-to-end proof that everything before it worked. Verify with: ping 10.0.0.10 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: Strict mode on a multi-homed or asymmetric path — legitimate traffic is dropped Common mistake: No default or specific route toward the source, so everything fails the check Common mistake: Enabling it on the core-facing side Why this matters
Many floods and reflection attacks hide behind forged source addresses. Unicast RPF asks, per packet, “could I reach this source back out of the interface it came in on?” and drops it if not.
What you will be able to do
Enable strict uRPF on a customer-facing edge
Explain strict versus loose mode and why asymmetric routing breaks strict
Show the check in the interface statistics
The network
ISR 2911, PC, Server — 3 devices in all, already cabled.
Gi0/0 192.168.10.1/24 Fa0 Gi0/1 10.0.0.1/24 Fa0 Edge Customer-PC 192.168.10.10/24 · gw 192.168.10.1 Core-Server 10.0.0.10/24 · gw 10.0.0.1 Drawn from this lab: devices, cables, ports and the addresses already set. Device Model Address already set
Edge ISR 2911 configure it Customer-PC PC 192.168.10.10 / 255.255.255.0 gw 192.168.10.1 Core-Server Server 10.0.0.10 / 255.255.255.0 gw 10.0.0.1
Cabling
Edge Gi0/0 ↔ Customer-PC Fa0 Edge Gi0/1 ↔ Core-Server Fa0
What is configured at the start
These are already configured when the lab opens — including, where this is a troubleshooting lab, the fault itself:
Edge interface g0/0
ip address 192.168.10.1 255.255.255.0
no shutdown
exit
interface g0/1
ip address 10.0.0.1 255.255.255.0
no shutdown
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 Strict uRPF (rx) is on the customer-facing interface 40 On Edge: show running-config contains the line(s) matching interface GigabitEthernet0/0 … ip verify unicast source reachable-via rx. 2 It is not on the core-facing interface (asymmetric routes would break) 20 On Edge: show running-config contains the line(s) matching interface GigabitEthernet0/1 … ip verify unicast must not . 3 The customer still reaches the core server (a real source passes the check) 40 On Customer-PC: ping 10.0.0.10 must succeed.
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
Strict mode on a multi-homed or asymmetric path — legitimate traffic is dropped
No default or specific route toward the source, so everything fails the check
Enabling it on the core-facing side
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:
Edge interface g0/0
ip verify unicast source reachable-via rx
exit Where this is examined
CCNP ENARSI/SCOR (infrastructure security), CCIE EI (uRPF), Security+ (anti-spoofing).
This lab practises these topics in the zNetLaB certification courses:
Related labs
Where to start
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