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Module 10
DHCP (Dynamic Host Configuration Protocol)
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# 📘 CCNA 200-301 - Video 10: DHCP (Dynamic Host Configuration Protocol) ## Deep Study Notes --- ## 📌 Learning Objectives By the end of this video, you should understand: - What DHCP is and why it's used - DHCP DORA process (Discover, Offer, Request, Acknowledge) - DHCP message types and formats - DHCP server configuration on routers and switches - DHCP relay (IP Helper) configuration - DHCP options and scope management - Troubleshooting DHCP issues --- ## 🧠 Core Concepts ### 1. What is DHCP? **Definition:** Dynamic Host Configuration Protocol (DHCP) is a network management protocol that automatically assigns IP addresses and other network configuration parameters to devices on a network. **Analogy:** Think of DHCP like a hotel front desk. When a guest arrives (device connects to network), they check in and are assigned a room number (IP address) for their stay (lease time). When they check out (disconnect), the room becomes available for the next guest. ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ WITHOUT DHCP (Manual Configuration) │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ ┌─────────┐ ┌─────────┐ ┌─────────┐ ┌─────────┐ │ │ │ PC1 │ │ PC2 │ │ PC3 │ │ PC4 │ │ │ │10.10.10.2│ │10.10.10.3│ │10.10.10.4│ │10.10.10.5│ │ │ │/24 │ │/24 │ │/24 │ │/24 │ │ │ │GW:10.10.10.1│GW:10.10.10.1│GW:10.10.10.1│GW:10.10.10.1│ │ │ │DNS:8.8.8.8│ │DNS:8.8.8.8│ │DNS:8.8.8.8│ │DNS:8.8.8.8│ │ │ └─────────┘ └─────────┘ └─────────┘ └─────────┘ │ │ │ │ PROBLEMS: │ │ • Manual configuration on every device (time-consuming) │ │ • IP address conflicts if duplicate entries │ │ • Difficult to manage changes (DNS server changed to 8.8.4.4) │ │ • No centralized management │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ┌─────────────────────────────────────────────────────────────────────────────┐ │ WITH DHCP (Automatic Configuration) │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ ┌─────────┐ ┌─────────┐ ┌─────────┐ ┌─────────┐ │ │ │ PC1 │ │ PC2 │ │ PC3 │ │ PC4 │ │ │ │ DHCP │ │ DHCP │ │ DHCP │ │ DHCP │ │ │ └────┬────┘ └────┬────┘ └────┬────┘ └────┬────┘ │ │ │ │ │ │ │ │ └────────────┴────────────┴────────────┘ │ │ │ │ │ ┌─────────▼─────────┐ │ │ │ DHCP Server │ │ │ │ 10.10.10.1 │ │ │ │ Pool: 10.10.10.2- │ │ │ │ 10.10.10.254│ │ │ │ Gateway: 10.10.10.1│ │ │ │ DNS: 8.8.8.8 │ │ │ └───────────────────┘ │ │ │ │ BENEFITS: │ │ • Automatic IP assignment │ │ • Centralized management │ │ • Prevents IP conflicts │ │ • Easy to change network parameters │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 2. DHCP Features | Feature | Description | |---------|-------------| | **IP Address Assignment** | Automatic, dynamic, or manual (reservation) | | **Lease Time** | Duration IP address is assigned (renewal required) | | **Address Pool** | Range of IP addresses available for assignment | | **Exclusions** | IP addresses reserved for static devices (servers, printers) | | **Options** | Additional parameters (DNS, domain, WINS, etc.) | **DHCP Benefits:** - **Reduced Administration:** No manual IP configuration - **Centralized Management:** Change DNS servers from one location - **No IP Conflicts:** Server tracks all assignments - **Mobility:** Devices get valid IP wherever they connect - **Scalability:** Easily add new devices to network --- ### 3. DHCP DORA Process The four-step process of obtaining an IP address: ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ DHCP DORA PROCESS │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Client DHCP Server │ │ │ │ │ │ │ 1. DISCOVER (Broadcast) │ │ │ │ "Is there a DHCP server out there?" │ │ │ │ ───────────────────────────────────────► │ │ │ │ │ │ │ │ │ │ │ │ 2. OFFER (Unicast or Broadcast) │ │ │ │ "Yes, here's an available IP address" │ │ │ │ ◄─────────────────────────────────────── │ │ │ │ │ │ │ │ │ │ │ │ 3. REQUEST (Broadcast) │ │ │ │ "I'd like to use that IP address" │ │ │ │ ───────────────────────────────────────► │ │ │ │ │ │ │ │ │ │ │ │ 4. ACKNOWLEDGE (Unicast) │ │ │ │ "OK, it's yours. Here are the options" │ │ │ │ ◄─────────────────────────────────────── │ │ │ │ │ │ │ │ │ D = Discover - Client finds DHCP servers │ │ O = Offer - Server offers IP address │ │ R = Request - Client requests offered IP │ │ A = Acknowledge - Server confirms assignment │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` **DHCP Message Types:** | Message | Direction | Description | |---------|-----------|-------------| | **DHCPDISCOVER** | Client → Server | Broadcast to locate DHCP servers | | **DHCPOFFER** | Server → Client | Server offers IP address configuration | | **DHCPREQUEST** | Client → Server | Client requests offered IP | | **DHCPACK** | Server → Client | Server confirms IP assignment | | **DHCPNAK** | Server → Client | Server denies request (bad address) | | **DHCPDECLINE** | Client → Server | Client declines offer (address in use) | | **DHCPRELEASE** | Client → Server | Client releases IP address | | **DHCPINFORM** | Client → Server | Client requests configuration (already has IP) | --- ### 4. DHCP Port Numbers | Protocol | Port | Purpose | |----------|------|---------| | **DHCP Server** | UDP 67 | Listens for client requests | | **DHCP Client** | UDP 68 | Sends requests, receives offers | **Important:** DHCP uses UDP (not TCP) because clients don't have IP addresses yet to establish a TCP connection. --- ### 5. DHCP Lease Process **Lease States:** ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ DHCP LEASE STATES │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ INIT (Initial) │ │ │ │ │ │ Send DISCOVER │ │ ▼ │ │ SELECTING │ │ │ │ │ │ Receive OFFER → Send REQUEST │ │ ▼ │ │ REQUESTING │ │ │ │ │ │ Receive ACK │ │ ▼ │ │ BOUND (IP Address Active) │ │ │ │ │ │ 50% of lease time → Send REQUEST (Renewal) │ │ ▼ │ │ RENEWING (Unicast to original server) │ │ │ │ │ │ 87.5% of lease time → Send REQUEST (Rebinding) │ │ ▼ │ │ REBINDING (Broadcast to any server) │ │ │ │ │ │ Lease expires → Return to INIT │ │ ▼ │ │ INIT │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` **Lease Timers:** | Timer | Default | Description | |-------|---------|-------------| | **Lease Time** | 24 hours (86400 seconds) | How long IP is valid | | **T1 (Renewal)** | 50% of lease (12 hours) | Client attempts to renew with original server | | **T2 (Rebinding)** | 87.5% of lease (21 hours) | Client broadcasts to any server | --- ### 6. DHCP Server Configuration (Router) **Basic DHCP Server Configuration on Router:** ```cisco ! Configure router as DHCP server Router> enable Router# configure terminal ! Create DHCP pool Router(config)# ip dhcp pool LAN_POOL Router(dhcp-config)# network 192.168.1.0 255.255.255.0 Router(dhcp-config)# default-router 192.168.1.1 Router(dhcp-config)# dns-server 8.8.8.8 8.8.4.4 Router(dhcp-config)# domain-name example.com Router(dhcp-config)# lease 7 12 30 ! 7 days, 12 hours, 30 minutes Router(dhcp-config)# exit ! Exclude static IP addresses (servers, printers, router) Router(config)# ip dhcp excluded-address 192.168.1.1 192.168.1.50 Router(config)# ip dhcp excluded-address 192.168.1.254 ! Optional: Configure DHCP options Router(config)# ip dhcp pool LAN_POOL Router(dhcp-config)# option 66 ip 192.168.1.100 ! TFTP server (PXE boot) Router(dhcp-config)# option 150 ip 192.168.1.200 ! TFTP server (Cisco phones) ``` **DHCP Pool Configuration Options:** | Command | Purpose | |---------|---------| | `network [ip] [mask]` | IP address range for assignment | | `default-router [ip]` | Default gateway (option 3) | | `dns-server [ip] [ip2]` | DNS servers (option 6) | | `domain-name [name]` | Domain name (option 15) | | `lease [days] [hours] [minutes]` | Lease duration | | `netbios-name-server [ip]` | WINS server (option 44) | | `option [code] [value]` | Custom DHCP options | --- ### 7. DHCP Server Configuration (Switch) **Layer 3 Switch as DHCP Server:** ```cisco ! Enable DHCP services on Layer 3 switch Switch> enable Switch# configure terminal ! Enable IP routing Switch(config)# ip routing ! Create VLANs and SVIs Switch(config)# vlan 10 Switch(config-vlan)# name Engineering Switch(config-vlan)# exit Switch(config)# interface vlan 10 Switch(config-if)# ip address 192.168.10.1 255.255.255.0 Switch(config-if)# no shutdown Switch(config-if)# exit ! Configure DHCP pool for VLAN 10 Switch(config)# ip dhcp pool VLAN10_POOL Switch(dhcp-config)# network 192.168.10.0 255.255.255.0 Switch(dhcp-config)# default-router 192.168.10.1 Switch(dhcp-config)# dns-server 8.8.8.8 Switch(dhcp-config)# lease 7 Switch(dhcp-config)# exit ! Exclude addresses Switch(config)# ip dhcp excluded-address 192.168.10.1 192.168.10.10 ``` --- ### 8. DHCP Relay (IP Helper) **The Problem:** DHCP uses broadcast messages. Routers do not forward broadcasts by default. DHCP clients and servers in different subnets cannot communicate. **Solution:** DHCP Relay (ip helper-address) converts broadcast to unicast and forwards to DHCP server. ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ DHCP RELAY (IP HELPER) │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Client (VLAN 10) DHCP Server (VLAN 20) │ │ 192.168.10.2/24 192.168.20.10/24 │ │ │ │ ┌─────────┐ ┌─────────────┐ │ │ │ PC1 │ │ DHCP Server │ │ │ └────┬────┘ └──────┬──────┘ │ │ │ │ │ │ │ DISCOVER (Broadcast) │ │ │ │─────────────────────────────────────► │ │ │ │ ┌───────────────┼───────────────┐ │ │ │ │ Router │ │ │ │ │ │ ip helper-address │ │ │ │ │ 192.168.20.10│ │ │ │ │ │ │ │ │ │ │ │ Broadcast → │ │ │ │ │ │ Unicast │ │ │ │ │ └───────────────┼───────────────┘ │ │ │ │ │ │ │ │ OFFER (Unicast) │ │ │ ◄─────────────────────────────────────│ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` **IP Helper Configuration:** ```cisco ! Configure router interface facing clients Router> enable Router# configure terminal Router(config)# interface gigabitEthernet 0/0.10 Router(config-subif)# encapsulation dot1Q 10 Router(config-subif)# ip address 192.168.10.1 255.255.255.0 Router(config-subif)# ip helper-address 192.168.20.10 ! DHCP server IP Router(config-subif)# exit ! Multiple helper addresses (backup servers) Router(config)# interface gigabitEthernet 0/0.10 Router(config-subif)# ip helper-address 192.168.20.10 Router(config-subif)# ip helper-address 192.168.20.11 ``` **What IP Helper Forwards:** By default, ip helper-address forwards these UDP broadcasts: | Protocol | Port | Description | |----------|------|-------------| | DHCP | 67,68 | IP address assignment | | DNS | 53 | Domain name resolution | | TFTP | 69 | Trivial File Transfer | | TACACS | 49 | Authentication | | NetBIOS | 137,138 | Name service | | Time | 37 | Time synchronization | **Customize forwarded protocols:** ```cisco ! Remove default forwarding for a protocol Router(config)# no ip forward-protocol udp 137 ! Disable NetBIOS ! Add custom UDP port forwarding Router(config)# ip forward-protocol udp 69 ! TFTP ``` --- ### 9. DHCP Options **Common DHCP Options:** | Option | Code | Description | |--------|------|-------------| | Subnet Mask | 1 | Client subnet mask | | Router (Default Gateway) | 3 | Default gateway IP | | DNS Server | 6 | Domain Name Server | | Domain Name | 15 | Domain name (e.g., example.com) | | NetBIOS Name Server | 44 | WINS server | | NetBIOS Node Type | 46 | NetBIOS node type (1=b, 2=p, 4=m, 8=h) | | TFTP Server | 66 | TFTP server IP (PXE boot) | | Bootfile Name | 67 | Boot file name (PXE boot) | | VoIP Configuration | 150 | TFTP server for Cisco phones | **Configuring DHCP Options:** ```cisco ! Option 66 - TFTP Server (PXE boot) Router(config)# ip dhcp pool PXE_POOL Router(dhcp-config)# network 192.168.1.0 255.255.255.0 Router(dhcp-config)# option 66 ip 192.168.1.100 ! Option 150 - Cisco Phone TFTP Router(config)# ip dhcp pool VOICE_POOL Router(dhcp-config)# network 192.168.100.0 255.255.255.0 Router(dhcp-config)# option 150 ip 192.168.100.10 ! Option 67 - Boot File Name Router(dhcp-config)# option 67 ascii "pxelinux.0" ``` --- ### 10. DHCP Reservations (Static Mapping) **Purpose:** Assign the same IP address to a specific device based on MAC address (like static IP but managed by DHCP). ```cisco ! Configure DHCP reservation Router(config)# ip dhcp pool PRINTER_RESERVATION Router(dhcp-config)# host 192.168.1.50 255.255.255.0 Router(dhcp-config)# hardware-address 00:1a:2b:3c:4d:5e Router(dhcp-config)# client-name LaserJet_4300 Router(dhcp-config)# default-router 192.168.1.1 Router(dhcp-config)# dns-server 8.8.8.8 Router(dhcp-config)# exit ! Alternative: Use address reservation in existing pool Router(config)# ip dhcp pool LAN_POOL Router(dhcp-config)# address 192.168.1.50 255.255.255.0 client-id 01:00:1a:2b:3c:4d:5e ``` --- ### 11. DHCP Verification Commands | Command | Purpose | |---------|---------| | `show ip dhcp pool` | Display DHCP pool information | | `show ip dhcp binding` | Display active IP address leases | | `show ip dhcp conflict` | Display IP address conflicts | | `show ip dhcp server statistics` | Display server statistics | | `debug ip dhcp server events` | Debug DHCP server events | | `debug ip dhcp server packets` | Debug DHCP packets | | `show ip interface [interface]` | Verify IP helper configuration | **Example Outputs:** ```cisco Router# show ip dhcp pool Pool LAN_POOL : Utilization mark (high/low) : 100 / 0 Subnet size (first/next) : 0 / 0 Total addresses : 254 Leased addresses : 45 Pending event : none 1 subnet is currently in the pool : Current index IP address range Leased addresses 192.168.1.100 192.168.1.1 - 192.168.1.254 45 Router# show ip dhcp binding Bindings from all pools not associated with VRF: IP address Client-ID/ Lease expiration Type Hardware address/ User name 192.168.1.2 0100.1a2b.3c4d.5e Jul 20 2024 08:30 AM Automatic 192.168.1.3 0100.1a2b.3c4d.5f Jul 20 2024 09:15 AM Automatic 192.168.1.50 0100.1a2b.3c4d.60 Jul 25 2024 12:00 PM Automatic (Reserved) Router# show ip dhcp conflict IP address Detection method Detection time VRF 192.168.1.100 Gratuitous ARP Jul 19 2024 10:30 AM 192.168.1.150 Ping Jul 19 2024 11:00 AM ``` --- ### 12. DHCP Client Configuration **Router as DHCP Client:** ```cisco ! Configure router interface to obtain IP via DHCP Router(config)# interface gigabitEthernet 0/0 Router(config-if)# ip address dhcp Router(config-if)# no shutdown ``` **Switch as DHCP Client (Management Interface):** ```cisco ! Configure management VLAN to obtain IP via DHCP Switch(config)# interface vlan 1 Switch(config-if)# ip address dhcp Switch(config-if)# no shutdown ``` --- ### 13. Troubleshooting DHCP | Problem | Symptom | Solution | |---------|---------|----------| | **No IP Address** | Client gets 169.254.x.x (APIPA) | Check DHCP server reachability, scope availability | | **DHCP Server Unreachable** | No OFFER received | Verify ip helper-address on router interfaces | | **IP Conflict** | Device cannot communicate | Check for static IP in DHCP range, use `show ip dhcp conflict` | | **Scope Exhausted** | New clients can't get IP | Increase scope size, reduce lease time | | **VLAN Mismatch** | DHCP requests not reaching server | Verify VLAN configuration on switch trunk | **APIPA (Automatic Private IP Addressing):** - Windows assigns 169.254.0.0/16 address when DHCP fails - Indicates client cannot reach DHCP server **Troubleshooting Commands:** ```cisco ! Verify DHCP server is reachable Router# ping 192.168.20.10 ! Check if DHCP pool has available addresses Router# show ip dhcp pool ! Verify IP helper configuration Router# show ip interface gigabitEthernet 0/0.10 ! Look for "Helper address: 192.168.20.10" ! Check for DHCP conflicts Router# show ip dhcp conflict ! Clear DHCP binding (if needed) Router# clear ip dhcp binding 192.168.1.2 ! Debug DHCP (use cautiously in production) Router# debug ip dhcp server events Router# debug ip dhcp server packets ``` --- ## 🔧 Complete Configuration Examples ### Lab 1: Router as DHCP Server **Topology:** ``` ┌─────────────────┐ │ Router │ │ 192.168.1.1 │ │ DHCP Server │ └────────┬────────┘ │ ┌────────▼────────┐ │ Switch │ └────────┬────────┘ │ ┌────────────────────┼────────────────────┐ │ │ │ ┌────▼────┐ ┌────▼────┐ ┌─────▼─────┐ │ PC1 │ │ PC2 │ │ PC3 │ │ DHCP │ │ DHCP │ │ DHCP │ └─────────┘ └─────────┘ └───────────┘ ``` **Router Configuration:** ```cisco hostname Router ! ! Configure interface interface GigabitEthernet0/0 ip address 192.168.1.1 255.255.255.0 no shutdown ! ! DHCP configuration ip dhcp excluded-address 192.168.1.1 192.168.1.50 ! ip dhcp pool LAN_POOL network 192.168.1.0 255.255.255.0 default-router 192.168.1.1 dns-server 8.8.8.8 8.8.4.4 domain-name local.lab lease 7 ! end ``` **Switch Configuration (Access VLAN 1):** ```cisco hostname Switch ! interface FastEthernet0/1 switchport mode access switchport access vlan 1 ! interface FastEthernet0/2 switchport mode access switchport access vlan 1 ! interface FastEthernet0/3 switchport mode access switchport access vlan 1 ! end ``` --- ### Lab 2: DHCP Relay (Multiple VLANs) **Topology:** ``` ┌─────────────────┐ │ DHCP Server │ │ 192.168.100.10 │ └────────┬────────┘ │ ┌────────▼────────┐ │ Router │ │ (DHCP Relay) │ └────────┬────────┘ │ ┌──────────────┼──────────────┐ │ │ │ VLAN 10 VLAN 20 VLAN 30 192.168.10.0 192.168.20.0 192.168.30.0 ``` **Router Configuration:** ```cisco hostname Router ! ! Interface to DHCP Server interface GigabitEthernet0/0 ip address 192.168.100.1 255.255.255.0 no shutdown ! ! Subinterfaces for VLANs with DHCP Relay interface GigabitEthernet0/1.10 encapsulation dot1Q 10 ip address 192.168.10.1 255.255.255.0 ip helper-address 192.168.100.10 ! interface GigabitEthernet0/1.20 encapsulation dot1Q 20 ip address 192.168.20.1 255.255.255.0 ip helper-address 192.168.100.10 ! interface GigabitEthernet0/1.30 encapsulation dot1Q 30 ip address 192.168.30.1 255.255.255.0 ip helper-address 192.168.100.10 ! end ``` **DHCP Server Configuration (Multiple Pools):** ```cisco hostname DHCP_Server ! ! DHCP Pools for each VLAN ip dhcp excluded-address 192.168.10.1 192.168.10.10 ip dhcp excluded-address 192.168.20.1 192.168.20.10 ip dhcp excluded-address 192.168.30.1 192.168.30.10 ! ip dhcp pool VLAN10_POOL network 192.168.10.0 255.255.255.0 default-router 192.168.10.1 dns-server 8.8.8.8 ! ip dhcp pool VLAN20_POOL network 192.168.20.0 255.255.255.0 default-router 192.168.20.1 dns-server 8.8.8.8 ! ip dhcp pool VLAN30_POOL network 192.168.30.0 255.255.255.0 default-router 192.168.30.1 dns-server 8.8.8.8 ! ! Interface to Router interface GigabitEthernet0/0 ip address 192.168.100.10 255.255.255.0 no shutdown ! end ``` --- ## 📊 Text-Based Diagrams ### DHCP Message Flow with Relay ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ DHCP WITH RELAY DETAILED FLOW │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Client (VLAN 10) Router (Relay) DHCP Server (VLAN 100) │ │ │ │ │ │ │ │ │ │ 1. DHCPDISCOVER │ │ │ │ │ (Broadcast) │ │ │ │ │─────────────────────►│ │ │ │ │ │ │ │ │ │ │ 2. DHCPDISCOVER │ │ │ │ │ (Unicast to server) │ │ │ │ │────────────────────────►│ │ │ │ │ │ │ │ │ │ 3. DHCPOFFER │ │ │ │ │ (Unicast to relay) │ │ │ │ │◄────────────────────────│ │ │ │ │ │ │ │ │ 4. DHCPOFFER │ │ │ │ │ (Unicast to client)│ │ │ │ │◄─────────────────────│ │ │ │ │ │ │ │ │ │ 5. DHCPREQUEST │ │ │ │ │ (Broadcast) │ │ │ │ │─────────────────────►│ │ │ │ │ │ │ │ │ │ │ 6. DHCPREQUEST │ │ │ │ │ (Unicast to server) │ │ │ │ │────────────────────────►│ │ │ │ │ │ │ │ │ │ 7. DHCPACK │ │ │ │ │ (Unicast to relay) │ │ │ │ │◄────────────────────────│ │ │ │ │ │ │ │ │ 8. DHCPACK │ │ │ │ │ (Unicast to client)│ │ │ │ │◄─────────────────────│ │ │ │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ## ✅ Exam Tips (For CCNA 200-301) | Topic | What Cisco Tests | |-------|------------------| | **DORA Process** | Memorize order: Discover, Offer, Request, Acknowledge | | **Port Numbers** | DHCP Server UDP 67, Client UDP 68 | | **IP Helper** | `ip helper-address` forwards DHCP broadcasts | | **DHCP Options** | Option 3 (gateway), 6 (DNS), 66 (TFTP), 150 (VoIP) | | **Lease Timers** | T1 = 50%, T2 = 87.5% of lease time | | **Excluded Addresses** | Prevent static IP conflicts | | **APIPA** | 169.254.0.0/16 - Windows default when DHCP fails | ### Common Exam Scenarios: **Scenario 1:** "A client receives IP address 169.254.1.100. What does this indicate?" - **Answer:** DHCP failed; client assigned APIPA address **Scenario 2:** "DHCP server is in a different subnet than clients. What must be configured?" - **Answer:** IP helper-address on the router interface facing clients **Scenario 3:** "A DHCP server assigns IP addresses, but clients cannot access the internet. What DHCP option is missing?" - **Answer:** Default gateway (option 3) not configured ### Mnemonics: **DORA Process:** **"DORA the Explorer"** - **D**iscover - **O**ffer - **R**equest - **A**cknowledge **DHCP Ports:** **"Server 67, Client 68"** **Lease Renewal:** **"Half to Renew, 7/8 to Rebroadcast"** - 50% = Renew with original server - 87.5% = Rebroadcast to any server --- ## 📝 Summary (1-Minute Revision) ``` DHCP (Dynamic Host Configuration Protocol): PURPOSE: ├── Automatically assign IP addresses ├── Distribute network configuration (gateway, DNS) └── Centralized IP management DORA PROCESS: ├── D = DISCOVER (Broadcast, UDP 68→67) ├── O = OFFER (Unicast/Broadcast) ├── R = REQUEST (Broadcast) └── A = ACK (Unicast) PORTS: ├── Server: UDP 67 └── Client: UDP 68 CONFIGURATION (Router): ├── ip dhcp pool [name] ├── network [ip] [mask] ├── default-router [ip] ├── dns-server [ip] └── ip dhcp excluded-address [range] DHCP RELAY: ├── ip helper-address [dhcp-server-ip] ├── Forwards DHCP broadcasts across subnets └── Configure on router interface facing clients VERIFICATION: ├── show ip dhcp pool ├── show ip dhcp binding └── show ip dhcp conflict TROUBLESHOOTING: ├── APIPA (169.254.x.x) = DHCP failure ├── Check ip helper-address for cross-subnet └── Verify scope has available addresses ``` --- ## 🧪 Practice Questions **1. What does DORA stand for in DHCP?** - A) Discover, Offer, Request, Acknowledge - B) Discover, Offer, Respond, Accept - C) Device, Operation, Request, Answer - D) Dynamic, Option, Request, Acknowledge <details> <summary>Answer</summary> <b>A) Discover, Offer, Request, Acknowledge</b> - These are the four steps in DHCP IP address assignment. </details> **2. Which UDP ports are used by DHCP?** - A) Server: 67, Client: 68 - B) Server: 68, Client: 67 - C) Server: 53, Client: 53 - D) Server: 69, Client: 69 <details> <summary>Answer</summary> <b>A) Server: 67, Client: 68</b> - DHCP server listens on UDP 67, client on UDP 68. </details> **3. A client receives IP address 169.254.10.5. What does this indicate?** - A) DHCP server is working correctly - B) DHCP server is unreachable; APIPA assigned - C) Static IP address configured - D) Duplicate IP address detected <details> <summary>Answer</summary> <b>B) DHCP server is unreachable; APIPA assigned</b> - 169.254.0.0/16 is APIPA range used when DHCP fails. </details> **4. What command configures DHCP relay on a router interface?** - A) `dhcp relay enable` - B) `ip helper-address [server-ip]` - C) `ip dhcp relay [server-ip]` - D) `dhcp server [server-ip]` <details> <summary>Answer</summary> <b>B) `ip helper-address [server-ip]`</b> - This command forwards DHCP broadcasts to the specified server. </details> **5. What is the default DHCP lease time on Cisco routers?** - A) 1 hour - B) 12 hours - C) 24 hours - D) 7 days <details> <summary>Answer</summary> <b>C) 24 hours</b> - Default lease time is 1 day (86400 seconds). </details> **6. At what percentage of lease time does a client attempt to renew with the original DHCP server?** - A) 25% - B) 50% - C) 75% - D) 87.5% <details> <summary>Answer</summary> <b>B) 50%</b> - T1 timer triggers renewal at 50% of lease time. </details> **7. Which DHCP option specifies the default gateway?** - A) Option 1 - B) Option 3 - C) Option 6 - D) Option 15 <details> <summary>Answer</summary> <b>B) Option 3</b> - Router/Default Gateway option. </details> **8. What is the purpose of `ip dhcp excluded-address`?** - A) Prevent DHCP server from assigning specific IPs - B) Remove IP addresses from the pool - C) Block DHCP clients - D) Disable DHCP service <details> <summary>Answer</summary> <b>A) Prevent DHCP server from assigning specific IPs</b> - Used to reserve IPs for static devices. </details> **9. Which DHCP message is broadcast to locate DHCP servers?** - A) DHCPOFFER - B) DHCPREQUEST - C) DHCPDISCOVER - D) DHCPACK <details> <summary>Answer</summary> <b>C) DHCPDISCOVER</b> - Client broadcasts to find available DHCP servers. </details> **10. What does APIPA stand for?** - A) Automatic Private IP Addressing - B) Advanced Protocol IP Allocation - C) Automatic Public IP Address - D) Address Protocol IP Assignment <details> <summary>Answer</summary> <b>A) Automatic Private IP Addressing</b> - Windows feature that assigns 169.254.x.x when DHCP fails. </details> **11. A DHCP server is in a different subnet from clients. What must be configured for clients to receive IP addresses?** - A) DHCP relay (ip helper-address) - B) Static routes - C) VLAN trunking - D) DHCP snooping <details> <summary>Answer</summary> <b>A) DHCP relay (ip helper-address)</b> - Forwards DHCP broadcasts across subnets. </details> **12. Which command displays active DHCP leases on a router?** - A) `show dhcp lease` - B) `show ip dhcp binding` - C) `show ip dhcp pool` - D) `show dhcp clients` <details> <summary>Answer</summary> <b>B) `show ip dhcp binding`</b> - Displays all active IP address leases. </details> --- ## 📌 Next Steps After completing Video 10, you should be ready for: - **Video 11:** DNS and Network Time Protocol (NTP) - **Video 12:** First Hop Redundancy Protocols (HSRP, VRRP, GLBP) **Lab Practice:** 1. Configure router as DHCP server for single subnet 2. Add DHCP exclusions for static devices 3. Configure DHCP reservations for printers/servers 4. Configure DHCP relay for multiple VLANs 5. Verify DHCP operation with `show ip dhcp binding` 6. Test DHCP failure scenario (disable server, observe APIPA) --- **Ready for Video 11?** Share the link or say "next" and I'll continue with DNS and Network Time Protocol (NTP). I'll create comprehensive deep notes for **Video 11: DNS and Network Time Protocol (NTP)** based on the standard CCNA 200-301 curriculum. ---