DHCP Server Configuration on a Cisco Router

Every device on an IP network needs four pieces of information to communicate: an IP address, a subnet mask, a default gateway, and a DNS server. Manually assigning these to every host in an organisation is time-consuming and error-prone. DHCP (Dynamic Host Configuration Protocol) automates this — a DHCP server maintains pools of available addresses and automatically leases them to hosts when they connect to the network.

Cisco IOS routers can act as full DHCP servers — an efficient solution for branch offices and small networks that do not have a dedicated server. The router assigns IP addresses, subnet masks, default gateways, DNS servers, and lease durations to hosts. When hosts on remote subnets need to reach a DHCP server on a different subnet, a DHCP relay agent (ip helper-address) forwards the broadcast requests as unicast across router boundaries. For a conceptual overview see DHCP Overview and How DHCP Works.

Before starting, complete Inter-VLAN Routing — Layer 3 Switch and Inter-VLAN Routing — Router-on-a-Stick to understand how hosts on different VLANs reach the router. For default gateway redundancy with DHCP, see HSRP — First Hop Redundancy.

1. DHCP — Core Concepts

The DORA Process

DHCP uses a four-step exchange called DORA to assign an address to a client. Understanding each step is essential for troubleshooting DHCP failures:

Step	Message	Direction	Purpose	Transport
D	DHCP Discover	Client → Network	Client broadcasts to find available DHCP servers	Broadcast (255.255.255.255)
O	DHCP Offer	Server → Client	Server offers an available IP address and configuration	Broadcast or unicast
R	DHCP Request	Client → Network	Client formally requests the offered address (also broadcast to notify other servers)	Broadcast (255.255.255.255)
A	DHCP Acknowledge	Server → Client	Server confirms the lease — client may now use the IP address	Broadcast or unicast

Why DHCP uses broadcasts: The client does not have an IP address yet — it cannot send a unicast packet. Both Discover and Request are sent as Layer 2 broadcasts. This is also why DHCP does not cross router boundaries by default — routers do not forward broadcasts. The DHCP relay agent (ip helper-address) converts these broadcasts into unicast packets and forwards them to the DHCP server. See DHCP Relay and DHCP Relay Agent — ip helper-address for the dedicated relay lab.

DHCP Pool Parameters

Parameter	IOS Command	Description	Required?
Network	`network [IP] [mask]`	The subnet this pool serves — defines the range of available addresses	✅ Yes
Default Gateway	`default-router [IP]`	The gateway address sent to clients — should match the router's interface IP on that subnet	✅ Recommended
DNS Server	`dns-server [IP]`	DNS resolver address sent to clients — can specify up to 8 servers	✅ Recommended
Lease Duration	`lease [days] [hours] [minutes]`	How long a client may keep the address before renewing. Default: 1 day	Optional
Domain Name	`domain-name [name]`	DNS domain suffix appended to hostnames — e.g., `netstuts.local`	Optional
NetBIOS Server	`netbios-name-server [IP]`	WINS server for Windows NetBIOS name resolution (legacy)	Optional

Excluded Addresses

Before defining any pool, exclude addresses that are already statically assigned — router interfaces, servers, printers, switches, and the HSRP virtual IP. Excluded addresses are configured globally (not inside a pool) and apply across all pools:

Address Type	Example	Why Exclude?
Router interface (gateway)	192.168.10.1	Already assigned — DHCP must not hand it to a host
HSRP virtual IP	192.168.10.254	Used by HSRP — not available for hosts
Servers and printers	192.168.10.2–10	Static assignments — must be consistent
Managed switch IP	192.168.10.11	Static management address on the switch

2. Lab Topology & Scenario

NetsTuts_R1 serves as the DHCP server for two VLANs — VLAN 10 (Staff, 192.168.10.0/24) and VLAN 20 (Guest, 192.168.20.0/24). NetsTuts_R2 is on a remote subnet (192.168.30.0/24) and uses ip helper-address to relay DHCP requests from its LAN clients back to R1 across the routed network.

  ┌─────────────────────────────────────────────────────────────────┐
  │                        NetsTuts_R1                              │
  │              DHCP Server for all three pools                    │
  │  Gi0/0.10: 192.168.10.1/24   Gi0/0.20: 192.168.20.1/24         │
  │  Gi0/1:    10.0.12.1/30 ──────────────────────────────────┐    │
  └─────────────────────────────────────────────────────────────────┘
        |                |                                      |
   VLAN 10             VLAN 20                           10.0.12.0/30
   192.168.10.0/24     192.168.20.0/24                         |
   [PC1][PC2]          [Laptop1]                    ┌─────────────────┐
                                                    │  NetsTuts_R2    │
                                                    │  Gi0/0:         │
                                                    │  10.0.12.2/30   │
                                                    │  Gi0/1:         │
                                                    │  192.168.30.1/24│
                                                    │  ip helper-addr │
                                                    └─────────────────┘
                                                         |
                                                    192.168.30.0/24
                                                    [PC3][PC4]
                                                    (DHCP relay clients)

DHCP Pool	Subnet	Gateway	Excluded Range	Served By
STAFF-VLAN10	192.168.10.0 /24	192.168.10.1	.1 – .20	R1 directly
GUEST-VLAN20	192.168.20.0 /24	192.168.20.1	.1 – .10	R1 directly
REMOTE-VLAN30	192.168.30.0 /24	192.168.30.1	.1 – .10	R1 via relay on R2

3. Step 1 — Exclude Static Addresses on R1

Always configure exclusions before defining the pools. If a pool is defined first and a client receives an address before the exclusion is added, that address will be handed out and must be manually cleared. Exclusions take a range — use a single IP for just one address:

NetsTuts_R1>en
NetsTuts_R1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.

! ── Exclude static addresses from VLAN 10 pool ───────────
! ── Router interface (.1) through reserved range (.20) ───
NetsTuts_R1(config)#ip dhcp excluded-address 192.168.10.1 192.168.10.20

! ── Exclude static addresses from VLAN 20 pool ───────────
NetsTuts_R1(config)#ip dhcp excluded-address 192.168.20.1 192.168.20.10

! ── Exclude static addresses from VLAN 30 pool ───────────
NetsTuts_R1(config)#ip dhcp excluded-address 192.168.30.1 192.168.30.10

The exclusion range 192.168.10.1–192.168.10.20 reserves the first 20 addresses of the VLAN 10 subnet for static assignments — the router's interface (.1), any servers (.2–.10), managed switches (.11–.15), printers (.16–.19), and the HSRP virtual IP (.20 or .254). DHCP will only hand out addresses from .21 onward.

4. Step 2 — Define DHCP Pools on R1

Pool 1 — STAFF VLAN 10

! ── Pool for VLAN 10 Staff network ───────────────────────
NetsTuts_R1(config)#ip dhcp pool STAFF-VLAN10
NetsTuts_R1(dhcp-config)#network 192.168.10.0 255.255.255.0
NetsTuts_R1(dhcp-config)#default-router 192.168.10.1
NetsTuts_R1(dhcp-config)#dns-server 8.8.8.8 8.8.4.4
NetsTuts_R1(dhcp-config)#domain-name netstuts.local
NetsTuts_R1(dhcp-config)#lease 7
NetsTuts_R1(dhcp-config)#exit

The prompt changes to (dhcp-config)# when inside a pool. lease 7 sets a 7-day lease — appropriate for office workstations that stay on the network continuously. Shorter leases (e.g., 1 hour) are better for high-turnover environments like guest Wi-Fi.

Pool 2 — GUEST VLAN 20

NetsTuts_R1(config)#ip dhcp pool GUEST-VLAN20
NetsTuts_R1(dhcp-config)#network 192.168.20.0 255.255.255.0
NetsTuts_R1(dhcp-config)#default-router 192.168.20.1
NetsTuts_R1(dhcp-config)#dns-server 8.8.8.8 8.8.4.4
NetsTuts_R1(dhcp-config)#domain-name guest.netstuts.local
NetsTuts_R1(dhcp-config)#lease 0 4
NetsTuts_R1(dhcp-config)#exit

Guest devices receive a 4-hour lease (lease 0 4 = 0 days, 4 hours). Short leases reclaim addresses quickly as guest devices come and go — preventing address exhaustion on the guest VLAN. A different domain name (guest.netstuts.local) also segregates guest name resolution from the staff domain.

Pool 3 — REMOTE VLAN 30 (served via relay)

NetsTuts_R1(config)#ip dhcp pool REMOTE-VLAN30
NetsTuts_R1(dhcp-config)#network 192.168.30.0 255.255.255.0
NetsTuts_R1(dhcp-config)#default-router 192.168.30.1
NetsTuts_R1(dhcp-config)#dns-server 8.8.8.8 8.8.4.4
NetsTuts_R1(dhcp-config)#domain-name netstuts.local
NetsTuts_R1(dhcp-config)#lease 1
NetsTuts_R1(dhcp-config)#exit
NetsTuts_R1(config)#end
NetsTuts_R1#wr
Building configuration...
[OK]
NetsTuts_R1#

R1 serves this pool for the 192.168.30.0/24 subnet even though no hosts on that subnet are directly connected to R1. The gateway (default-router 192.168.30.1) points to R2's interface — not R1. Clients on the 192.168.30.0/24 subnet will reach this pool via the relay agent configured on R2 in the next step.

DHCP Pool Command Reference

Command	What It Does	Example
`ip dhcp pool [name]`	Creates a named DHCP pool and enters dhcp-config mode	`ip dhcp pool STAFF-VLAN10`
`network [IP] [mask]`	Defines the subnet this pool serves — addresses not excluded are available for lease	`network 192.168.10.0 255.255.255.0`
`default-router [IP]`	Sets the default gateway sent to clients (Option 3)	`default-router 192.168.10.1`
`dns-server [IP] [IP2]`	Sets the DNS resolver(s) sent to clients (Option 6)	`dns-server 8.8.8.8 8.8.4.4`
`domain-name [name]`	Sets the DNS domain name sent to clients (Option 15)	`domain-name netstuts.local`
`lease [days] [hours] [minutes]`	Sets the lease duration. Default is 1 day. Use `lease infinite` for permanent leases	`lease 7` / `lease 0 4`

5. Step 3 — Configure DHCP Relay Agent on R2

Hosts on 192.168.30.0/24 send DHCP Discover as a broadcast. By default, R2 drops broadcasts and does not forward them to R1. The ip helper-address command on R2's LAN interface converts the broadcast into a directed unicast sent to R1's IP (10.0.12.1), allowing DHCP to cross the router boundary:

NetsTuts_R2>en
NetsTuts_R2#conf t
Enter configuration commands, one per line.  End with CNTL/Z.

! ── Apply relay on the interface facing the remote clients
NetsTuts_R2(config)#interface GigabitEthernet0/1
NetsTuts_R2(config-if)#ip helper-address 10.0.12.1
NetsTuts_R2(config-if)#exit
NetsTuts_R2(config)#end
NetsTuts_R2#wr
Building configuration...
[OK]
NetsTuts_R2#

ip helper-address 10.0.12.1 is configured on the interface facing the clients (Gi0/1 — the 192.168.30.0/24 side), not the uplink to R1. The helper-address is R1's IP address on the link between R1 and R2 (10.0.12.1). When a client broadcasts a DHCP Discover, R2 intercepts it, replaces the source IP with its own interface IP (192.168.30.1), and forwards it as a unicast to 10.0.12.1. R1 uses the relay agent's IP (192.168.30.1) to identify which pool to use — matching it to the REMOTE-VLAN30 pool whose network includes 192.168.30.1.

How the Relay Process Works

Step	What Happens	Key Detail
1	PC3 sends DHCP Discover as broadcast (255.255.255.255)	Source IP: 0.0.0.0 (no IP yet)
2	R2 receives broadcast on Gi0/1 — `ip helper-address` is configured	R2 intercepts the broadcast instead of dropping it
3	R2 adds its interface IP (192.168.30.1) as the giaddr (gateway address field) and forwards the Discover as unicast to 10.0.12.1	giaddr tells R1 which subnet the client is on
4	R1 receives the unicast Discover, reads the giaddr (192.168.30.1), matches it to the REMOTE-VLAN30 pool, and sends an Offer back to R2	R1 selects the correct pool based on giaddr
5	R2 forwards the Offer to PC3. PC3 completes the DORA exchange and receives 192.168.30.x	PC3 also receives gateway 192.168.30.1 and DNS 8.8.8.8

ip helper-address placement: Always configure ip helper-address on the interface closest to the clients — the LAN-facing interface of the relay router. If configured on the wrong interface (e.g., the uplink), the router will not intercept client broadcasts and DHCP fails silently.

6. Step 4 — DHCP Static Binding (Optional)

For devices that need the same IP address every time (network printers, servers, IP cameras) but should still use DHCP rather than manual static configuration, IOS supports manual bindings — a fixed IP address permanently reserved for a specific MAC address. For securing DHCP against rogue servers and starvation attacks, see DHCP Snooping.

! ── Static DHCP binding for a network printer ────────────
NetsTuts_R1(config)#ip dhcp pool PRINTER-OFFICE
NetsTuts_R1(dhcp-config)#host 192.168.10.15 255.255.255.0
NetsTuts_R1(dhcp-config)#hardware-address 00A0.C9B4.3210
NetsTuts_R1(dhcp-config)#default-router 192.168.10.1
NetsTuts_R1(dhcp-config)#dns-server 8.8.8.8
NetsTuts_R1(dhcp-config)#exit

The host command replaces network for static bindings — it specifies a single host address rather than a subnet. The hardware-address ties the binding to a specific MAC. When the printer sends a DHCP Discover with that MAC, R1 always offers 192.168.10.15 — regardless of the excluded-address range. Static bindings are stored separately from dynamic leases.

7. Verification

show ip dhcp pool

NetsTuts_R1#show ip dhcp pool

Pool STAFF-VLAN10 :
 Utilization mark (high/low)    : 100 / 0
 Subnet size (first/next)       : 0 / 0
 Total addresses                : 254
 Leased addresses               : 3
 Pending event                  : none
 1 subnet is currently in the pool :
 Current index        IP address range                    Leased addresses
 192.168.10.21        192.168.10.1     - 192.168.10.254    3

Pool GUEST-VLAN20 :
 Total addresses                : 254
 Leased addresses               : 1
 Current index        IP address range                    Leased addresses
 192.168.20.11        192.168.20.1     - 192.168.20.254    1

Pool REMOTE-VLAN30 :
 Total addresses                : 254
 Leased addresses               : 2
 Current index        IP address range                    Leased addresses
 192.168.30.11        192.168.30.1     - 192.168.30.254    2

Key fields: Total addresses = all addresses in the subnet (254 for /24 — network and broadcast excluded). Leased addresses = currently active leases. Current index = the next address IOS will try to assign. Note the current index for STAFF-VLAN10 starts at .21 — matching the excluded range (.1–.20).

show ip dhcp binding

NetsTuts_R1#show ip dhcp binding
Bindings from all pools not associated with VRF:
IP address          Client-ID/              Lease expiration        Type       State      Interface
                    Hardware address/
                    User name
192.168.10.21       0100.5056.AB.12.34      Mar 12 2026 10:22 AM    Automatic  Active     GigabitEthernet0/0.10
192.168.10.22       0100.5056.AB.56.78      Mar 12 2026 10:24 AM    Automatic  Active     GigabitEthernet0/0.10
192.168.10.23       0100.5056.AB.9A.BC      Mar 12 2026 10:25 AM    Automatic  Active     GigabitEthernet0/0.10
192.168.20.11       0100.AABB.CC.DD.EE      Mar 05 2026 06:32 PM    Automatic  Active     GigabitEthernet0/0.20
192.168.30.11       0100.1234.56.78.9A      Mar 06 2026 10:30 AM    Automatic  Active     GigabitEthernet0/1
192.168.30.12       0100.FEDC.BA.98.76      Mar 06 2026 10:32 AM    Automatic  Active     GigabitEthernet0/1

show ip dhcp binding lists every active lease — the assigned IP, the client's MAC address (in Client-ID format: 01 + MAC), the lease expiry time, whether the assignment was automatic (dynamic) or manual (static binding), and the interface the lease was issued from. Remote relay clients (192.168.30.x) show the relay interface (Gi0/1) rather than the client's actual interface.

show ip dhcp conflict

NetsTuts_R1#show ip dhcp conflict
IP address        Detection method   Detection time          VRF
192.168.10.25     Ping               Mar 05 2026 08:14 AM

IOS pings each address before offering it. If a response is received, the address is marked as a conflict — a device already has that IP statically assigned. The conflicted address is removed from the pool and logged here. Clear conflicts with clear ip dhcp conflict * after resolving the duplicate assignment.

show ip dhcp server statistics

NetsTuts_R1#show ip dhcp server statistics
Memory usage         : 22137
Address pools        : 3
Database agents      : 0
Automatic bindings   : 6
Manual bindings      : 1
Expired bindings     : 0
Malformed messages   : 0
Secure arp entries   : 0

Message              Received
BOOTREQUEST          0
DHCPDISCOVER         12
DHCPREQUEST          10
DHCPDECLINE          0
DHCPRELEASE          2
DHCPINFORM           0

Message              Sent
BOOTREPLY            0
DHCPOFFER            12
DHCPACK              10
DHCPNAK              0

The statistics table shows the count of each DORA message. Healthy operation shows DHCPDISCOVER = DHCPOFFER and DHCPREQUEST = DHCPACK counts. DHCPNAK (Negative Acknowledge) indicates declined requests — typically because the requested IP is already in use or the client tried to renew an expired lease. High DHCPDECLINE counts indicate address conflicts.

Verify Relay on R2

NetsTuts_R2#show running-config interface GigabitEthernet0/1
Building configuration...

Current configuration : 112 bytes
!
interface GigabitEthernet0/1
 description Remote-LAN-VLAN30
 ip address 192.168.30.1 255.255.255.0
 ip helper-address 10.0.12.1
 duplex auto
 speed auto
end

Confirm ip helper-address 10.0.12.1 is on the correct interface — Gi0/1, the LAN-facing interface. If the helper-address were on Gi0/0 (the uplink toward R1), R2 would not intercept client broadcasts and DHCP would fail for all hosts on the 192.168.30.0/24 subnet.

Verification Command Summary

Command	What It Shows	Primary Use
`show ip dhcp pool`	All pools — total addresses, leased count, current index, utilisation	Confirm pools are defined correctly and check address usage
`show ip dhcp binding`	All active leases — IP, client MAC, expiry time, type (auto/manual)	Identify which host has which IP — essential for troubleshooting
`show ip dhcp conflict`	Addresses removed from the pool due to conflict detection (ping response)	Diagnose address conflicts — clear with `clear ip dhcp conflict *`
`show ip dhcp server statistics`	Message counters — DISCOVER, OFFER, REQUEST, ACK, NAK, DECLINE	Verify DORA exchange is completing — check for NAK or high conflict counts
`show running-config \| section dhcp`	All DHCP-related config — pools, exclusions, helper-addresses	Audit entire DHCP configuration in one view
`show logging`	DHCP event log — address assignments, conflicts, and relay events	Post-event review of DHCP activity

8. Troubleshooting DHCP Issues

Problem	Symptom	Cause	Fix
Host receives 169.254.x.x address	PC shows APIPA address — cannot reach the network	DHCP Discover sent but no Offer received — DHCP server unreachable or pool exhausted	Check `show ip dhcp pool` — confirm leased count vs total. Check if server is reachable. On relay scenarios, confirm `ip helper-address` is correct.
Pool exhausted — no addresses available	`show ip dhcp pool` shows leased = total. New clients get no address	All available addresses are leased — either pool is too small, lease time too long, or stale bindings from disconnected devices	Clear expired bindings: `clear ip dhcp binding *`. Reduce lease time for dynamic environments. Add a larger subnet or expand the pool range.
Remote hosts not receiving DHCP (relay issue)	Direct hosts work but hosts on the remote subnet (via relay) get APIPA	`ip helper-address` not configured, configured on wrong interface, or pointing to wrong server IP	Verify `show running-config interface [LAN-int]` on relay router — confirm helper-address is on the LAN-facing interface and points to the DHCP server's correct IP
Address conflict logged	`show ip dhcp conflict` shows entries — some hosts fail to get addresses	A device has a static IP in the DHCP pool range — when DHCP pings the address before offering it, the device responds, and DHCP removes it from the pool	Exclude the conflicting address: `ip dhcp excluded-address [IP]`. Clear the conflict log: `clear ip dhcp conflict *`
Wrong default gateway assigned	Hosts receive an IP but cannot reach other networks	`default-router` in the pool is misconfigured — pointing to the wrong IP or not configured at all	Check pool with `show running-config \| section dhcp`. Correct the `default-router` statement. Hosts need to renew their lease to get the corrected value.
DHCP server assigning addresses from wrong pool	Relay clients receive addresses from the wrong subnet	The `network` statement in the pool does not match the giaddr sent by the relay router — pool matching uses the relay agent's interface IP	Verify the relay router's interface IP falls within the pool's `network` range. The pool's network must match the subnet of the relay router's LAN interface.

Key Points & Exam Tips

Always configure ip dhcp excluded-address before defining pools — this prevents DHCP from handing out addresses already statically assigned to routers, switches, servers, printers, or HSRP virtual IPs.
The DHCP DORA process (Discover → Offer → Request → Acknowledge) uses broadcasts for Discover and Request — routers drop broadcasts by default. DHCP only works across subnets with a relay agent. See How DHCP Works for the full process detail.
ip helper-address [server-IP] must be placed on the interface facing the clients — not the uplink toward the server. A common exam and real-world mistake is placing it on the wrong interface.
The DHCP server identifies which pool to use for relay clients based on the giaddr field — the relay router's LAN interface IP. The pool's network statement must include that IP address.
show ip dhcp binding shows every active lease — IP address, client MAC, expiry time, and whether it was dynamically assigned or a static binding. This is the primary command for tracing which host holds which IP.
show ip dhcp pool shows utilisation — total addresses vs leased count. Use this to detect pool exhaustion before hosts start failing to get addresses.
show ip dhcp conflict shows addresses that were removed from the pool because DHCP detected they were already in use (ping response before offering). Clear with clear ip dhcp conflict * after resolving the duplicate.
Static DHCP bindings use host instead of network and tie a specific IP to a MAC address via hardware-address — useful for printers and servers that need a consistent IP without manual static configuration.
DHCP lease time should match the environment — long leases (7 days) for stable wired workstations, short leases (1–4 hours) for guest Wi-Fi where devices frequently join and leave.
On the CCNA exam: know the four DORA steps, why broadcasts do not cross routers, what ip helper-address does, the difference between network (dynamic pool) and host (static binding), and which commands verify active leases vs pool utilisation. Also review VLANs and DNS as closely related topics.

Next Steps: With DHCP distributing IP addressing automatically, continue to OSPF Single-Area Configuration to establish dynamic routing between the subnets. For distributing the HSRP virtual IP as the default gateway via DHCP, combine this lab with HSRP — First Hop Redundancy. To secure DHCP against rogue servers and starvation attacks, see DHCP Snooping & Dynamic ARP Inspection. For the dedicated relay agent lab, see DHCP Relay Agent — ip helper-address. For managing access to the router itself after DHCP is running, revisit SSH Configuration.

TEST WHAT YOU LEARNED

1. An engineer configures a DHCP pool for 192.168.10.0/24 on R1 but forgets to exclude 192.168.10.1 (R1's own interface IP). PC1 connects and receives 192.168.10.1 via DHCP. What happens to network traffic?

A. Nothing — R1 detects the conflict and automatically moves the lease to the next available address B. An IP address conflict occurs — both R1's interface and PC1 have 192.168.10.1. Traffic destined for the router's gateway may reach PC1 instead, causing intermittent connectivity failures and ARP instability for all hosts on the subnet C. R1 automatically excludes its own interface addresses from all DHCP pools D. The DHCP ACK is rejected by PC1 because the address is already in use on the network

Correct answer is B. Cisco IOS does not automatically exclude the router's own interface addresses from DHCP pools — this is a manual step using ip dhcp excluded-address. If the router's interface IP is included in the pool range, DHCP may assign it to a client. Both devices now claim 192.168.10.1, causing an IP conflict. ARP responses become unpredictable — sometimes the router answers ARP requests for .1, sometimes PC1 does. Other hosts trying to reach the default gateway experience intermittent failures. Always exclude static addresses before defining pools.

2. PC3 is on the 192.168.30.0/24 subnet connected to R2. R1 is the DHCP server at 10.0.12.1. R2 has `ip helper-address 10.0.12.1` configured on Gi0/0 (the uplink to R1) instead of Gi0/1 (the LAN interface). What happens when PC3 sends a DHCP Discover?

A. DHCP works normally — the helper-address forwards the request regardless of which interface it is configured on B. DHCP works but uses broadcast instead of unicast to reach R1 C. R2 forwards the Discover to R1 then discards the reply D. PC3's DHCP Discover arrives on Gi0/1 but ip helper-address is only applied to Gi0/0 — R2 does not intercept the broadcast on Gi0/1 and drops it. PC3 receives no DHCP offer and falls back to APIPA (169.254.x.x)

Correct answer is D. The ip helper-address command is processed only when a broadcast is received on the interface where it is configured. PC3's DHCP Discover arrives on R2's Gi0/1 (the LAN-facing interface). Since helper-address is on Gi0/0 (the uplink), R2 has no relay instruction for broadcasts arriving on Gi0/1 — it simply drops the broadcast. PC3 never receives a DHCP Offer and uses APIPA. The fix is to move ip helper-address 10.0.12.1 to Gi0/1 — the interface closest to the clients.

3. `show ip dhcp pool` on R1 shows the STAFF-VLAN10 pool has 254 total addresses and 253 leased addresses. PC5 tries to connect but gets an APIPA address. What is the cause?

A. The pool is nearly exhausted — only 1 address remains. But with 253 leased and exclusions covering .1–.20, many leases may be from disconnected devices. Clear stale bindings with clear ip dhcp binding * and reduce the lease time to reclaim addresses faster B. The DHCP server process needs to be restarted — pools over 90% full stop responding C. PC5's NIC is incompatible with the DHCP server version on Cisco IOS D. The excluded-address range is too large — .1 through .20 wasted too many addresses

Correct answer is A. With 253 out of 254 addresses leased, the pool is effectively full — only 1 address remains. In an active network, this last address may already be in use. DHCP pools do not stop at 90% — they exhaust completely. The most common cause is stale bindings from devices that disconnected without releasing their lease (powered off, unplugged). Use clear ip dhcp binding * to reset all dynamic leases (valid clients will re-request immediately). Long-term, reduce the lease duration and consider expanding the subnet.

4. What does the giaddr field in a DHCP packet contain when a relay agent forwards a Discover to the server, and why is it critical?

A. The client's requested IP address — tells the server what the client prefers B. The DHCP server's IP address — ensures the reply goes to the correct server C. The relay agent's interface IP address on the client's subnet — the DHCP server uses this to identify which pool to serve. Without giaddr, the server cannot determine which subnet the client is on and cannot select the correct pool D. The gateway configured in the DHCP pool — used to pre-populate the default gateway option in the Offer

Correct answer is C. The giaddr (Gateway IP Address) field is set by the relay agent to its own IP address on the client-facing interface (e.g., 192.168.30.1). The DHCP server receives this relayed Discover and looks at the giaddr to determine which subnet pool to use — it finds the pool whose network statement contains the giaddr IP. Without giaddr, the server would have no way to know the client is on the 192.168.30.0/24 subnet and could not select the REMOTE-VLAN30 pool. This is why pool matching for relay clients is based on the relay interface IP, not the client's source IP (which is still 0.0.0.0).

5. An engineer wants a network printer at MAC address 00A0.C9B4.3210 to always receive 192.168.10.15 via DHCP. The address .15 is within the excluded range (.1–.20). Which configuration achieves this?

A. Remove 192.168.10.15 from the excluded range and rely on DHCP to assign it first B. Add a second ip dhcp excluded-address line that specifically unexcludes .15 C. Configure a static IP on the printer — DHCP static bindings cannot use excluded addresses D. Create a separate pool using host 192.168.10.15 255.255.255.0 and hardware-address 00A0.C9B4.3210. Static bindings bypass the excluded-address check — the address is reserved specifically for this MAC and is not offered to any other client

Correct answer is D. DHCP static manual bindings use the host command instead of network and tie the address to a specific MAC via hardware-address. Static bindings operate independently of the excluded-address list — the excluded range prevents dynamic assignment but does not block static bindings. The printer always receives .15 when it sends a DHCP Discover with its MAC. This is the correct approach for devices that need consistency without full static IP configuration on the device itself.

6. `show ip dhcp conflict` shows 192.168.10.30 as a conflict. What caused this entry and what are the two steps to fix it?

A. Before offering .30, IOS pinged it and received a reply — a device already has .30 statically assigned. Fix: exclude the address (ip dhcp excluded-address 192.168.10.30) and clear the conflict log (clear ip dhcp conflict *) B. Two DHCP clients simultaneously requested the same address — fix by reducing the lease time C. The address was previously assigned and the lease expired without the client releasing it — clear the binding with clear ip dhcp binding 192.168.10.30 D. The DHCP server tried to assign the address but the client declined it — DHCP records all declines as conflicts

Correct answer is A. Cisco IOS DHCP performs a ping check before offering an address. If any device responds to the ping, the address is logged as a conflict and removed from the available pool — preventing a duplicate assignment. This means a device has .30 configured statically (outside of DHCP). The two-step fix: (1) Add ip dhcp excluded-address 192.168.10.30 so DHCP permanently skips this address, and (2) run clear ip dhcp conflict * to clear the conflict log. Without clearing the log, the address remains flagged even after exclusion.

7. Why do DHCP Discover and Request messages use broadcast (255.255.255.255) instead of unicast?

A. Broadcasts are faster — DHCP uses them to reduce server response time B. The DHCP standard mandates broadcasts for security — unicast DHCP is not permitted C. The client does not yet have an IP address and does not know the server's IP — it cannot form a valid unicast packet. Broadcasting reaches all devices on the local segment including any DHCP servers. A relay agent is required to cross router boundaries since routers do not forward broadcasts D. DHCP uses broadcasts to allow multiple servers to respond simultaneously — the client picks the best offer

Correct answer is C. At the start of the DORA process, the client has no IP address (source IP = 0.0.0.0) and does not know any DHCP server's IP address. Since it cannot form a valid unicast IP packet without a source address, and it does not know the destination, it broadcasts. The broadcast reaches all devices on the local subnet — including any DHCP servers. This broadcast-dependence is exactly why DHCP does not work across routers by default and requires ip helper-address to convert the broadcast to a directed unicast at the router boundary.

8. What is the difference between `show ip dhcp binding` and `show ip dhcp pool`?

A. They show the same information — binding is the detailed view and pool is the summary B. show ip dhcp binding shows individual active leases — which IP was assigned to which MAC with expiry time. show ip dhcp pool shows pool-level statistics — total addresses, leased count, current index, and utilisation without individual lease details C. show ip dhcp pool shows dynamic leases and show ip dhcp binding shows only static bindings D. show ip dhcp binding only works on the relay router — show ip dhcp pool works on the DHCP server

Correct answer is B. These two commands complement each other. show ip dhcp binding is host-level detail — it shows every individual lease (IP address, client MAC, expiry time, type automatic/manual, interface). Use it when you need to find which device has a specific IP or verify a specific client received a lease. show ip dhcp pool is pool-level summary — it shows the subnet range, how many total addresses the pool has, how many are currently leased, and the next address index. Use it for capacity planning and to detect pool exhaustion.

9. R1 has a DHCP pool defined for 192.168.30.0/24 with `default-router 192.168.30.1`. R2 is the relay agent with `ip helper-address 10.0.12.1` on Gi0/1 (192.168.30.1). PC3 receives a DHCP address of 192.168.30.25 from R1. What default gateway does PC3 receive?

A. 10.0.12.1 — the relay agent's uplink IP toward R1 B. 10.0.12.2 — R2's IP on the uplink interface since that is the path to the internet C. 192.168.30.25 — the client uses its own IP as gateway when no other is specified D. 192.168.30.1 — the default-router value from the REMOTE-VLAN30 pool. R1 includes this in the DHCP Offer's Option 3 field. PC3 uses R2's LAN interface (192.168.30.1) as its gateway, which is correct for reaching other networks

Correct answer is D. The default-router command in the DHCP pool sets DHCP Option 3 — the default gateway that the server includes in its Offer and Acknowledge messages. R1 sends 192.168.30.1 as the gateway in its Offer. PC3 receives the Acknowledge and configures its default gateway as 192.168.30.1 — which is R2's Gi0/1 interface, the physically correct next hop for PC3. This is why the default-router in a relayed pool must match the relay router's LAN interface IP, not any of R1's interfaces.

10. A host on the 192.168.20.0/24 (Guest) VLAN is showing an IP address of 169.254.45.12. The DHCP pool is defined on R1 and hosts on the 192.168.10.0/24 (Staff) VLAN are getting DHCP addresses correctly. What is the most likely cause for Guest VLAN hosts failing DHCP?

A. The DHCP pool for VLAN 20 has the wrong lease time configured B. The GUEST-VLAN20 pool is exhausted — all addresses are leased C. The router sub-interface for VLAN 20 (Gi0/0.20) is either missing, shutdown, or has the wrong IP/encapsulation — DHCP Discover broadcasts from VLAN 20 never reach R1's DHCP process because the Layer 3 interface for that VLAN is not up D. The dns-server in the GUEST-VLAN20 pool is unreachable — DHCP fails when DNS is unavailable

Correct answer is C. Since Staff VLAN hosts (VLAN 10) are getting DHCP addresses correctly but Guest VLAN hosts (VLAN 20) are not, the DHCP server process itself is working — the problem is specific to VLAN 20. In a Router-on-a-Stick or Layer 3 switch setup, each VLAN needs its own sub-interface or SVI with the correct IP and encapsulation. If Gi0/0.20 is shut down, missing, or has wrong 802.1Q encapsulation, DHCP Discovers from VLAN 20 never reach R1's routing process — they are dropped at the interface layer. Check with show ip interface brief and show interfaces Gi0/0.20.

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