IP Services Fix · Medium ~15 min · 3 devices · 4 tasks · CCNA v1.1 and v2.0

Branch clients get no DHCP address: fix the relay and the pool

R1 is the central DHCP server with pool BRANCH for 192.168.20.0/24. R2 is the branch router: its Gi0/1 (192.168.20.1) is the gateway for PCs on SW1, and it should relay their DHCP broadcasts to R1 at 10.0.12.1. No branch PC gets an address. There are four mistakes between the relay and the server.

R1 Gi0/0 10.0.12.1/30 (DHCP server) to R2 Gi0/0 10.0.12.2/30
R2 Gi0/1 192.168.20.1/24 (branch gateway) to SW1 Gi0/1
Pool BRANCH: 192.168.20.0/24, gateway 192.168.20.1, reserve .1 to .10 for devices

Topology

Tasks

○R2 relays DHCP from the LAN interface Gi0/1 to 10.0.12.1 (and not from Gi0/0)
○The pool hands out 192.168.20.1 as the default gateway
○Only 192.168.20.1 to 192.168.20.10 are excluded
○R1 has a route back to 192.168.20.0/24 and can ping 192.168.20.1

Hints (open one at a time)

Hint 1: where does the broadcast arrive?

The client broadcast arrives on the LAN side of R2. ip helper-address belongs on the interface that receives it. Check R2 with show running-config (press Space at --More--, or run terminal length 0 first).

Hint 2: the pool

On R1 run show running-config and read the ip dhcp pool BRANCH block. Press Space at --More-- to keep reading. The default router must be the real gateway, 192.168.20.1.

Hint 3: excluded addresses

An excluded range that covers the whole subnet leaves nothing to lease. Remove it and exclude only .1 to .10.

Hint 4: the reply path

The relay sends requests with R2’s LAN address as the gateway address (giaddr). R1 must have a route back to 192.168.20.0/24 to answer.

Full solution

R2# configure terminal
R2(config)# interface gigabitethernet0/0
R2(config-if)# no ip helper-address 10.0.12.1
R2(config-if)# interface gigabitethernet0/1
R2(config-if)# ip helper-address 10.0.12.1
R2(config-if)# end

R1# configure terminal
R1(config)# no ip dhcp excluded-address 192.168.20.1 192.168.20.254
R1(config)# ip dhcp excluded-address 192.168.20.1 192.168.20.10
R1(config)# ip dhcp pool BRANCH
R1(dhcp-config)# default-router 192.168.20.1
R1(dhcp-config)# exit
R1(config)# ip route 192.168.20.0 255.255.255.0 10.0.12.2
R1(config)# end
R1# ping 192.168.20.1

The DHCP server and the relay agent are often on different routers. Check both, plus the route between them.

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Branch clients get no DHCP address: fix the relay and the pool

Stuck on a lab, or not sure what to practice next?