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DHCP relay 14 — the pool is one router away
Enterprise Intermediate 5 devices 5 checked tasks 100 points Plan lab
Scenario — Ranchi's PCs ask for an address but the DHCP server is the head-office router R-HQ, one hop away across 10.33.0.0/30. A DHCP discover is a broadcast and routers do not forward broadcasts. On R-HQ build pool PRINTERS-RANCHI for 10.33.9.0/24 (exclude .1 – .9, gateway .1) and give it a route back; on R-Branch relay the requests with a helper address; then renew both PCs. 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
Ranchi's PCs ask for an address but the DHCP server is the head-office router R-HQ, one hop away across 10.33.0.0/30. A DHCP discover is a broadcast and routers do not forward broadcasts. On R-HQ build pool PRINTERS-RANCHI for 10.33.9.0/24 (exclude .1 – .9, gateway .1) and give it a route back; on R-Branch relay the requests with a helper address; then renew both PCs.
Your role: a network engineer on the enterprise team. Level: Intermediate Done when: all 5 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. R-HQ starts with addresses (G0/1 10.33.0.1/30). R-Ranchi starts with addresses (G0/1 10.33.0.2/30, G0/0 10.33.9.1/24). SW 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
Pool PRINTERS-RANCHI serves 10.33.9.0/24 with gateway .1, .1 – .9 excluded (30 pts) R-HQ has a route back to 10.33.9.0/24 (15 pts) The branch router relays requests to 10.33.0.1 (20 pts) PC1 leases an address (15 pts) PC2 leases an address and reaches R-HQ (20 pts) The activity, in order
Pool PRINTERS-RANCHI serves 10.33.9.0/24 with gateway .1, .1 – .9 excluded 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 R-HQ has a route back to 10.33.9.0/24 Why: The next hop must be an address on a directly connected subnet, or the route never enters the table. Verify with: show ip route The branch router relays requests to 10.33.0.1 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 PC1 leases an address 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 an address and reaches R-HQ 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. Common mistake: Helper address on the wrong interface Common mistake: A pool for a subnet the server cannot route back to Common mistake: Forgetting to exclude the gateway Why this matters
DHCP discover is a broadcast, and a router does not forward broadcasts. A relay agent (ip helper-address) turns it into a unicast to the real server, which then picks the pool from the relay's own interface address — so the server also needs a route back.
What you will be able to do
Build a pool on a remote server
Configure ip helper-address on the client-facing interface
Explain why the server needs a return route
The network
ISR 2911, Catalyst 2960-24TT, PC — 5 devices in all, already cabled.
Gi0/1 10.33.0.1/30 Gi0/1 10.33.0.2/30 Gi0/0 10.33.9.1/24 Gi0/1 Fa0/1 Fa0 Fa0/2 Fa0 R-HQ R-Ranchi SW PC1 PC2 Drawn from this lab: devices, cables, ports and the addresses already set. Device Model Address already set
R-HQ ISR 2911 configure it R-Ranchi ISR 2911 configure it SW Catalyst 2960-24TT configure it PC1 PC configure it PC2 PC configure it
Cabling
R-HQ Gi0/1 ↔ R-Ranchi Gi0/1 R-Ranchi Gi0/0 ↔ SW Gi0/1 SW Fa0/1 ↔ PC1 Fa0 SW 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:
R-HQ interface g0/1
ip address 10.33.0.1 255.255.255.252
no shutdown
exit R-Ranchi interface g0/1
ip address 10.33.0.2 255.255.255.252
no shutdown
exit
interface g0/0
ip address 10.33.9.1 255.255.255.0
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 Pool PRINTERS-RANCHI serves 10.33.9.0/24 with gateway .1, .1 – .9 excluded 30 On R-HQ: show running-config contains the line(s) matching ip dhcp excluded-address 10\.33\.9\.1 10\.33\.9\.9. and On R-HQ: show running-config contains the line(s) matching ip dhcp pool PRINTERS-RANCHI[\s\S]*network 10\.33\.9\.0 255\.255\.255\.0[\s\S]*default-router 10\.33\.9\.1. 2 R-HQ has a route back to 10.33.9.0/24 15 On R-HQ: show ip route 10.33.9.0 shows next hop 10.33.0.2. 3 The branch router relays requests to 10.33.0.1 20 On R-Ranchi: show running-config interface Gi0/0 has ip helper-address 10.33.0.1. 4 PC1 leases an address 15 On PC1: ipconfig shows an address. 5 PC2 leases an address and reaches R-HQ 20 On PC2: ipconfig shows an address. and On PC2: ping 10.33.0.1 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
Common mistakes
Helper address on the wrong interface
A pool for a subnet the server cannot route back to
Forgetting to exclude the gateway
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:
R-HQ ip dhcp excluded-address 10.33.9.1 10.33.9.9
ip dhcp pool PRINTERS-RANCHI
network 10.33.9.0 255.255.255.0
default-router 10.33.9.1
dns-server 8.8.8.8
exit
ip route 10.33.9.0 255.255.255.0 10.33.0.2 R-Ranchi interface g0/0
ip helper-address 10.33.0.1
exit PC1 ipconfig /renew PC2 ipconfig /renew Where this is examined
CCNA 4.0 DHCP relay; Network+ 1.6.
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