zNetLaB › Guided labs › Enterprise
DHCP server and relay — an address from one hop away
Enterprise Beginner 5 devices 4 checked tasks 100 points Free lab
R1 is the DHCP server for a LAN that hangs off R2. A DHCP Discover is a broadcast and routers do not forward broadcasts — so build the pool on R1, give it a route back, and make R2 relay the request with ip helper-address. 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 is a free lab: it stays open on the free plan. It needs an account, which keeps your progress.
The scenario
R1 is the DHCP server for a LAN that hangs off R2. A DHCP Discover is a broadcast and routers do not forward broadcasts — so build the pool on R1, give it a route back, and make R2 relay the request with ip helper-address.
Your role: a network engineer on the enterprise team. Level: Beginner Done when: all 4 goals show as passed (100 points).
Network status
Devices: 2 routers, 1 switch, 2 PCs, cabled as drawn. No Layer 1 or Layer 2 faults: every link is connected and up. R1-DHCP starts with addresses (G0/1 10.0.12.1/30). R2-Branch starts with addresses (G0/1 10.0.12.2/30, G0/0 10.20.0.1/24). SW1 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
R2 relays DHCP from its LAN to the server at 10.0.12.1 (25 pts) R1 can route back to the branch LAN (20 pts) PC1 leases an address from R1 (25 pts) PC2 leases one too, and reaches the DHCP server (30 pts) The activity, in order
R2 relays DHCP from its LAN to the server at 10.0.12.1 Why: DHCP discovery is a broadcast and routers do not forward broadcasts. The helper address turns it into a unicast. Verify with: show running-config interface GigabitEthernet0/0 R1 can route back to the branch LAN Why: If the prefix is not in the routing table, the router drops packets for it, whatever else is configured. Verify with: show ip route PC1 leases an address from R1 Why: If the host has no address, look at the pool, the exclusions and, across a router, the ip helper-address. Verify with: ipconfig PC2 leases one too, and reaches the DHCP server Why: If the host has no address, look at the pool, the exclusions and, across a router, the ip helper-address. Verify with: ipconfig 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. Why this matters
R1 is the DHCP server for a LAN that hangs off R2. A DHCP Discover is a broadcast and routers do not forward broadcasts — so build the pool on R1, give it a route back, and make R2 relay the request with ip helper-address.
What you will be able to do
R2 relays DHCP from its LAN to the server at 10.0.12.1
R1 can route back to the branch LAN
PC1 leases an address from R1
PC2 leases one too, and reaches the DHCP server
The network
ISR 2911, Catalyst 2960-24TT, PC — 5 devices in all, already cabled.
Gi0/1 10.0.12.1/30 Gi0/1 10.0.12.2/30 Gi0/0 10.20.0.1/24 Gi0/1 Fa0/1 Fa0 Fa0/2 Fa0 R1-DHCP R2-Branch SW1 PC1 PC2 Drawn from this lab: devices, cables, ports and the addresses already set. Device Model Address already set
R1-DHCP ISR 2911 configure it R2-Branch ISR 2911 configure it SW1 Catalyst 2960-24TT configure it PC1 PC configure it PC2 PC configure it
Cabling
R1-DHCP Gi0/1 ↔ R2-Branch Gi0/1 R2-Branch Gi0/0 ↔ SW1 Gi0/1 SW1 Fa0/1 ↔ PC1 Fa0 SW1 Fa0/2 ↔ PC2 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:
R1-DHCP interface g0/1
ip address 10.0.12.1 255.255.255.252
no shutdown
exit R2-Branch interface g0/1
ip address 10.0.12.2 255.255.255.252
no shutdown
exit
interface g0/0
ip address 10.20.0.1 255.255.255.0
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 R2 relays DHCP from its LAN to the server at 10.0.12.1 25 On R2-Branch: show running-config interface Gi0/0 has ip helper-address 10.0.12.1. 2 R1 can route back to the branch LAN 20 On R1-DHCP: show ip route lists 10.20.0.0/24. 3 PC1 leases an address from R1 25 On PC1: ipconfig shows an address. 4 PC2 leases one too, and reaches the DHCP server 30 On PC2: ipconfig shows an address. and On PC2: ping 10.0.12.1 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
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:
R1-DHCP ip dhcp excluded-address 10.20.0.1 10.20.0.9
ip dhcp pool BRANCH
network 10.20.0.0 255.255.255.0
default-router 10.20.0.1
dns-server 10.0.12.1
exit
ip route 10.20.0.0 255.255.255.0 10.0.12.2 R2-Branch interface g0/0
ip helper-address 10.0.12.1
exit PC1 ipconfig /renew PC2 ipconfig /renew Where this is examined
This lab practises these topics in the zNetLaB certification courses:
Related labs
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