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Module 33
PPPoE (Point-to-Point Protocol over Ethernet)
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# 📘 CCNA 200-301 - Video 33: PPPoE (Point-to-Point Protocol over Ethernet) ## Deep Study Notes --- ## 📌 Learning Objectives By the end of this video, you should understand: - What PPPoE is and why it's used - PPPoE architecture and components - PPPoE discovery and session phases - PPPoE client configuration on Cisco routers - PPPoE server concepts - Troubleshooting PPPoE connections --- ## 🧠 Core Concepts ### 1. What is PPPoE? **Definition:** Point-to-Point Protocol over Ethernet (PPPoE) is a network protocol that encapsulates PPP frames inside Ethernet frames, allowing PPP sessions to be established over Ethernet networks, commonly used for DSL broadband connections. **Analogy:** Think of PPPoE like a secure tunnel through a busy city. The Ethernet network is the city streets (shared medium). PPPoE creates a private tunnel (PPP session) that carries your traffic securely from your home to the ISP's central office, even though you're sharing the street with others. ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ WHY PPPoE? │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Traditional Dial-up (PSTN): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ PC ── Modem ── Telephone Line ── ISP (PPP Session) │ │ │ │ │ │ │ │ • Point-to-point physical connection │ │ │ │ • PPP runs directly over physical link │ │ │ │ • Simple, but slow │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ DSL/Ethernet Networks: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ PC ── Ethernet ── DSL Modem ── DSL Line ── ISP (Ethernet) │ │ │ │ │ │ │ │ • Ethernet is shared medium (multiple customers) │ │ │ │ • Need to differentiate customers │ │ │ │ • Need authentication and IP assignment │ │ │ │ • PPPoE solves this by creating PPP sessions over Ethernet │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ PPPoE Solution: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ PPPoE encapsulates PPP frames in Ethernet frames │ │ │ │ • Multiple PPP sessions over single Ethernet │ │ │ │ • Per-session authentication │ │ │ │ • Per-session IP assignment │ │ │ │ • Familiar PPP features (LCP, NCP, authentication) │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 2. PPPoE Architecture ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ PPPoE ARCHITECTURE │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Customer Premises ISP Network │ │ │ │ ┌─────────────────────────────────────┐ ┌─────────────────────────────┐ │ │ │ PC/Router (Client) │ │ BRAS/Access Concentrator│ │ │ │ │ │ (Server) │ │ │ │ ┌───────────────────────────────┐ │ │ ┌─────────────────────────┐ │ │ │ │ │ Application │ │ │ │ Application │ │ │ │ │ │ (Web, Email, etc.) │ │ │ │ (Web, Email) │ │ │ │ │ └───────────────────────────────┘ │ │ └─────────────────────────┘ │ │ │ │ ┌───────────────────────────────┐ │ │ ┌─────────────────────────┐ │ │ │ │ │ IP (TCP/UDP) │ │ │ │ IP │ │ │ │ │ └───────────────────────────────┘ │ │ └─────────────────────────┘ │ │ │ │ ┌───────────────────────────────┐ │ │ ┌─────────────────────────┐ │ │ │ │ │ PPP (LCP, NCP) │ │ │ │ PPP │ │ │ │ │ └───────────────────────────────┘ │ │ └─────────────────────────┘ │ │ │ │ ┌───────────────────────────────┐ │ │ ┌─────────────────────────┐ │ │ │ │ │ PPPoE │ │ │ │ PPPoE │ │ │ │ │ │ (Discovery + Session) │ │ │ │ (Discovery + Session)│ │ │ │ │ └───────────────────────────────┘ │ │ └─────────────────────────┘ │ │ │ │ ┌───────────────────────────────┐ │ │ ┌─────────────────────────┐ │ │ │ │ │ Ethernet (MAC) │ │ │ │ Ethernet │ │ │ │ │ └───────────────────────────────┘ │ │ └─────────────────────────┘ │ │ │ │ │ │ │ │ │ │ │ └─────────────────┼──────────────────┘ └─────────────────┼────────────┘ │ │ │ │ │ │ │ Ethernet │ Ethernet │ │ ▼ ▼ │ │ ┌───────────┐ ┌───────────┐ │ │ │ DSL Modem │ │ DSLAM │ │ │ └─────┬─────┘ └─────┬─────┘ │ │ │ │ │ │ │ DSL Line │ │ │ └──────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 3. PPPoE Components | Component | Description | |-----------|-------------| | **PPPoE Client** | Customer device (PC or router) that initiates the PPPoE session | | **PPPoE Server (Access Concentrator)** | ISP device that terminates PPPoE sessions, often a BRAS (Broadband Remote Access Server) | | **DSLAM (Digital Subscriber Line Access Multiplexer)** | Aggregates multiple DSL connections | | **BRAS (Broadband Remote Access Server)** | Authenticates users, assigns IP addresses, aggregates traffic | | **Session ID** | Unique identifier for each PPPoE session | --- ### 4. PPPoE Frame Structure ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ PPPoE FRAME STRUCTURE │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Ethernet Frame: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ Destination │ Source │ EtherType │ PPPoE Payload │ FCS │ │ │ │ MAC │ MAC │ (0x8863/ │ │ │ │ │ │ │ │ 0x8864) │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ PPPoE Payload (Discovery Phase - EtherType 0x8863): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ VER │ TYPE │ CODE │ SESSION ID │ LENGTH │ PAYLOAD │ │ │ │ │ (4b)│ (4b) │ (8b) │ (16 bits) │ (16b) │ (Variable) │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ PPPoE Payload (Session Phase - EtherType 0x8864): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ VER │ TYPE │ CODE │ SESSION ID │ LENGTH │ PPP Frame │ │ │ │ │ (4b)│ (4b) │ (8b) │ (16 bits) │ (16b) │ (Variable) │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Fields: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • VER (Version): Always 1 │ │ │ │ • TYPE: Always 1 │ │ │ │ • CODE: Message type (Discovery or Session) │ │ │ │ • SESSION ID: Unique identifier for the PPP session │ │ │ │ • LENGTH: Length of payload │ │ │ │ • EtherType: 0x8863 (Discovery), 0x8864 (Session) │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 5. PPPoE Discovery Phase **Purpose:** Discover the access concentrator and establish a session ID. ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ PPPoE DISCOVERY PHASE │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Client (PC/Router) Access Concentrator (ISP) │ │ │ │ │ │ │ │ 1. PADI (PPPoE Active Discovery Initiation) │ │ │ Broadcast: "Is there any PPPoE server?" │ │ │ ─────────────────────────────────────────────────────────────► │ │ │ │ │ │ 2. PADO (PPPoE Active Discovery Offer) │ │ │ Unicast: "Yes, I am available" │ │ │ ◄───────────────────────────────────────────────────────────── │ │ │ │ │ │ 3. PADR (PPPoE Active Discovery Request) │ │ │ Unicast: "I want to establish a session" │ │ │ ─────────────────────────────────────────────────────────────► │ │ │ │ │ │ 4. PADS (PPPoE Active Discovery Session) │ │ │ Unicast: "Session established, Session ID = X" │ │ │ ◄───────────────────────────────────────────────────────────── │ │ │ │ │ │ Discovery Complete - Session Established │ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` **PPPoE Discovery Messages:** | Message | Direction | Description | |---------|-----------|-------------| | **PADI** | Client → Server | Broadcast to find PPPoE servers (DSL Access Concentrators) | | **PADO** | Server → Client | Unicast response to PADI, offers service | | **PADR** | Client → Server | Unicast request to establish session with specific server | | **PADS** | Server → Client | Unicast acknowledgment with Session ID | | **PADT** | Either → Either | Terminate session | --- ### 6. PPPoE Session Phase **After Discovery Phase:** - Session ID is established - PPP session begins - LCP (Link Control Protocol) negotiation - Authentication (PAP or CHAP) - NCP (Network Control Protocol) negotiation (IPCP for IPv4) - Data transfer ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ PPPoE SESSION PHASE │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Session Phase (PPP over Ethernet): │ │ │ │ Client (PPPoE Session) Access Concentrator │ │ │ │ │ │ │ │ 1. LCP (Link Control Protocol) │ │ │ Configure link options (MRU, authentication, magic number) │ │ │ ◄─────────────────────────────────────────────────────────────► │ │ │ │ │ │ 2. Authentication (PAP or CHAP) │ │ │ User authentication │ │ │ ◄─────────────────────────────────────────────────────────────► │ │ │ │ │ │ 3. IPCP (Internet Protocol Control Protocol) │ │ │ IP address assignment │ │ │ ◄─────────────────────────────────────────────────────────────► │ │ │ │ │ │ 4. Data Transfer │ │ │ IP packets encapsulated in PPP, then PPPoE, then Ethernet │ │ │ ◄─────────────────────────────────────────────────────────────► │ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 7. PPPoE MTU Considerations **MTU Issues:** - Ethernet MTU: 1500 bytes - PPPoE adds 8-byte header - PPP adds additional overhead (up to 6 bytes) - Effective MTU for IP: 1492 bytes ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ PPPoE MTU CALCULATION │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Original IP Packet: 1500 bytes │ │ │ │ After PPP encapsulation: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ PPP Header (2-6 bytes) + IP Packet (1500 bytes) │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ Total: 1502-1506 bytes │ │ │ │ After PPPoE encapsulation: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ PPPoE Header (8 bytes) + PPP Frame │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ Total: 1510-1514 bytes │ │ │ │ PROBLEM: Exceeds Ethernet MTU of 1500! │ │ │ │ SOLUTION: Reduce IP MTU to 1492 │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ IP Packet: 1492 bytes │ │ │ │ + PPP Header: 1492-1496 bytes │ │ │ │ + PPPoE Header: 1500-1504 bytes (fits within Ethernet MTU) │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Common MTU Settings: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Ethernet: 1500 │ │ │ │ • PPPoE: 1492 (recommended) │ │ │ │ • PPPoE with VLAN: 1488 (1492 - 4 for VLAN tag) │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 8. PPPoE Client Configuration (Cisco Router) **Method 1: Dialer Interface (Recommended)** ```cisco ! Configure VLAN (if required) vlan 10 name PPPoE ! ! Configure physical interface interface GigabitEthernet0/0 no ip address pppoe enable pppoe-client dial-pool-number 1 ! ! Configure dialer interface interface Dialer1 mtu 1492 ip address negotiated encapsulation ppp ppp authentication chap pap ppp chap hostname user@isp.com ppp chap password isppassword ppp pap sent-username user@isp.com password isppassword dialer pool 1 dialer idle-timeout 0 ! Never disconnect ! ! Default route ip route 0.0.0.0 0.0.0.0 Dialer1 ! ! Optional: NAT for internal network ip nat inside source list 10 interface Dialer1 overload access-list 10 permit 192.168.1.0 0.0.0.255 ``` **Method 2: VLAN Interface (Alternative)** ```cisco ! Configure VLAN interface interface Vlan10 no ip address pppoe enable pppoe-client dial-pool-number 1 ! ! Dialer interface (same as above) interface Dialer1 mtu 1492 ip address negotiated encapsulation ppp ppp authentication chap ppp chap hostname user@isp.com ppp chap password isppassword dialer pool 1 ! ! Default route ip route 0.0.0.0 0.0.0.0 Dialer1 ``` --- ### 9. PPPoE Server Configuration (Cisco Router) ```cisco ! Enable PPPoE globally bba-group pppoe ISP-GROUP virtual-template 1 ! ! Configure virtual-template interface Virtual-Template1 mtu 1492 ip unnumbered Loopback0 peer default ip address pool PPPoE-POOL ppp authentication chap pap ! ! Configure IP pool ip local pool PPPoE-POOL 10.1.1.10 10.1.1.100 ! ! Configure physical interface for PPPoE interface GigabitEthernet0/0 no ip address pppoe enable group ISP-GROUP ! ! Loopback for IP unnumbered interface Loopback0 ip address 10.255.255.1 255.255.255.255 ! ! Authentication username user@isp.com password isppassword ``` --- ### 10. PPPoE Verification Commands | Command | Purpose | |---------|---------| | `show ip interface brief` | Display interface status (Dialer1) | | `show interface dialer [num]` | Display dialer interface status | | `show pppoe session` | Display PPPoE sessions | | `show pppoe interface` | Display PPPoE-enabled interfaces | | `debug pppoe events` | Debug PPPoE discovery events | | `debug pppoe packets` | Debug PPPoE packets | | `debug ppp authentication` | Debug PPP authentication | **Example Outputs:** ```cisco Router# show pppoe session 1 client session Uniq ID PPPoE RemMAC LocalMAC SID State 0 Gi0/0 aaaa.bbbb.cccc dddd.eeee.ffff 1 UP Router# show interface dialer 1 Dialer1 is up, line protocol is up Hardware is Unknown Internet address is 203.0.113.25/32 MTU 1492 bytes, BW 56 Kbit/sec, DLY 20000 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation PPP, LCP Open, Open: IPCP Keepalive set (10 sec) Dialer interface is bound to pool 1 Bound to: GigabitEthernet0/0 is up, line protocol is up Router# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route Gateway of last resort is 0.0.0.0 to network 0.0.0.0 C 203.0.113.25 is directly connected, Dialer1 C 192.168.1.0/24 is directly connected, GigabitEthernet0/1 S* 0.0.0.0/0 is directly connected, Dialer1 ``` --- ### 11. PPPoE Troubleshooting | Problem | Symptom | Solution | |---------|---------|----------| | **No Session** | `show pppoe session` empty | Check physical connectivity, verify pppoe enable | | **Discovery Fails** | PADI sent, no PADO | Check server availability, VLAN configuration | | **Authentication Fails** | LCP UP, authentication fails | Verify username/password, authentication protocol | | **No IP Address** | IP unnumbered or negotiated fails | Check IPCP negotiation, pool availability | | **MTU Issues** | Large packets fail | Reduce MTU to 1492, configure MSS adjust | | **Session Drops** | Intermittent connectivity | Check keepalive, physical link stability | **Troubleshooting Commands:** ```cisco ! Check PPPoE session status Router# show pppoe session Router# show pppoe session summary ! Debug discovery phase Router# debug pppoe events Router# debug pppoe packets ! Debug PPP authentication Router# debug ppp authentication ! Debug IPCP negotiation Router# debug ppp negotiation ``` --- ### 12. PPPoE with NAT (Common Home/Office Setup) ```cisco ! Standard PPPoE + NAT configuration ! ! Define internal network interface GigabitEthernet0/1 ip address 192.168.1.1 255.255.255.0 ip nat inside ! ! PPPoE interface interface Dialer1 mtu 1492 ip address negotiated ip nat outside encapsulation ppp ppp authentication chap ppp chap hostname user@isp.com ppp chap password isppassword dialer pool 1 ! ! Physical interface for PPPoE interface GigabitEthernet0/0 no ip address pppoe enable pppoe-client dial-pool-number 1 ! ! NAT configuration ip nat inside source list 10 interface Dialer1 overload access-list 10 permit 192.168.1.0 0.0.0.255 ! ! Default route ip route 0.0.0.0 0.0.0.0 Dialer1 ``` --- ### 13. PPPoE MSS Adjustment **Problem:** PMTUD (Path MTU Discovery) failures can cause connectivity issues. **Solution:** Adjust TCP Maximum Segment Size (MSS) to prevent fragmentation. ```cisco ! Adjust TCP MSS on dialer interface interface Dialer1 ip tcp adjust-mss 1452 ! (1452 = 1492 - 20 IP header - 20 TCP header) ``` --- ## 🔧 Complete Configuration Examples ### Lab 1: PPPoE Client Configuration **Topology:** ``` Router (CPE) ──────── DSL Modem ──────── ISP Network Gi0/0 ``` **Complete Router Configuration:** ```cisco hostname CPE ! ! Configure physical interface for PPPoE interface GigabitEthernet0/0 no ip address pppoe enable pppoe-client dial-pool-number 1 ! ! Configure dialer interface interface Dialer1 mtu 1492 ip address negotiated encapsulation ppp ppp authentication chap pap ppp chap hostname user@isp.com ppp chap password isppassword ppp pap sent-username user@isp.com password isppassword dialer pool 1 dialer idle-timeout 0 ip tcp adjust-mss 1452 ! ! Configure inside interface interface GigabitEthernet0/1 ip address 192.168.1.1 255.255.255.0 ip nat inside ! ! NAT configuration ip nat inside source list 10 interface Dialer1 overload access-list 10 permit 192.168.1.0 0.0.0.255 ! ! Default route ip route 0.0.0.0 0.0.0.0 Dialer1 ! end ``` --- ### Lab 2: PPPoE Server Configuration (Simulated) **Topology:** ``` ISP Router (PPPoE Server) ──────── Customer Router (PPPoE Client) Gi0/0 Gi0/0 ``` **ISP Router Configuration:** ```cisco hostname ISP ! ! Create local user username user@isp.com password isppassword ! ! Create BBA group bba-group pppoe ISP-PPPOE virtual-template 1 ! ! Configure IP pool ip local pool PPPoE-POOL 10.1.1.10 10.1.1.100 ! ! Configure virtual-template interface Virtual-Template1 mtu 1492 ip unnumbered Loopback0 peer default ip address pool PPPoE-POOL ppp authentication chap pap ! ! Configure physical interface interface GigabitEthernet0/0 no ip address pppoe enable group ISP-PPPOE ! ! Loopback for IP unnumbered interface Loopback0 ip address 10.255.255.1 255.255.255.255 ! end ``` --- ### Lab 3: PPPoE with VLAN Tagging **Topology:** ``` Router (CPE) ──────── DSL Modem (VLAN 10) ──────── ISP Network Gi0/0.10 ``` **Router Configuration:** ```cisco hostname CPE ! ! Create VLAN 10 (if required by ISP) interface GigabitEthernet0/0.10 encapsulation dot1Q 10 pppoe enable pppoe-client dial-pool-number 1 ! ! Dialer interface (same as previous) interface Dialer1 mtu 1488 ! 1492 - 4 for VLAN tag ip address negotiated encapsulation ppp ppp authentication chap ppp chap hostname user@isp.com ppp chap password isppassword dialer pool 1 ! end ``` --- ## ✅ Exam Tips (For CCNA 200-301) | Topic | What Cisco Tests | |-------|------------------| | **PPPoE Purpose** | PPP over Ethernet for DSL broadband | | **Discovery Phase** | PADI, PADO, PADR, PADS messages | | **Session Phase** | PPP over Ethernet, LCP, authentication, NCP | | **EtherType** | 0x8863 (Discovery), 0x8864 (Session) | | **MTU** | 1492 (1500 - 8) | | **MSS Adjust** | `ip tcp adjust-mss 1452` | | **Dialer Interface** | Virtual interface for PPPoE | ### Common Exam Scenarios: **Scenario 1:** "A customer cannot connect to the internet via DSL. The router shows 'pppoe session not established'. What is the first thing to check?" - **Answer:** Physical connectivity and `pppoe enable` on the interface **Scenario 2:** "What is the recommended MTU for PPPoE?" - **Answer:** 1492 bytes (1500 - 8 for PPPoE header) **Scenario 3:** "What are the four discovery phase messages in order?" - **Answer:** PADI, PADO, PADR, PADS ### Mnemonics: **PPPoE Discovery Messages:** **"I O R S" - Initiate, Offer, Request, Session** - **I**: PADI (Initiation) - **O**: PADO (Offer) - **R**: PADR (Request) - **S**: PADS (Session) **PPPoE EtherTypes:** **"D for Discovery, S for Session"** - Discovery: 0x8863 - Session: 0x8864 **PPPoE MTU:** **"1500 minus 8 = 1492"** --- ## 📝 Summary (1-Minute Revision) ``` PPPoE (Point-to-Point Protocol over Ethernet): PURPOSE: ├── PPP over Ethernet for DSL broadband ├── Authentication and IP assignment ├── Multiple PPP sessions over shared Ethernet ARCHITECTURE: ├── Client: PC or Router (CPE) ├── Server: Access Concentrator (BRAS) └── Transport: Ethernet, DSL DISCOVERY PHASE (EtherType 0x8863): ├── PADI: Client broadcasts "I need service" ├── PADO: Server responds "I can serve you" ├── PADR: Client requests "Start session" └── PADS: Server grants "Session ID = X" SESSION PHASE (EtherType 0x8864): ├── LCP: Link configuration ├── Authentication: PAP or CHAP ├── NCP (IPCP): IP address assignment └── Data transfer MTU CONSIDERATIONS: ├── Ethernet MTU: 1500 ├── PPPoE header: 8 bytes ├── Effective MTU: 1492 └── MSS adjustment: 1452 (TCP) CONFIGURATION (Client): ├── interface Dialer1 ├── mtu 1492 ├── ip address negotiated ├── encapsulation ppp ├── ppp authentication chap ├── ppp chap hostname [user] ├── ppp chap password [pass] ├── dialer pool 1 ├── interface Gi0/0 ├── pppoe enable ├── pppoe-client dial-pool-number 1 └── ip route 0.0.0.0 0.0.0.0 Dialer1 VERIFICATION: ├── show pppoe session ├── show interface dialer1 ├── show ip route └── debug pppoe events ``` --- ## 🧪 Practice Questions **1. What is the EtherType for PPPoE Discovery packets?** - A) 0x0800 - B) 0x0806 - C) 0x8863 - D) 0x8864 <details> <summary>Answer</summary> <b>C) 0x8863</b> - PPPoE Discovery Phase uses EtherType 0x8863. </details> **2. What is the recommended MTU for PPPoE?** - A) 1500 - B) 1492 - C) 1480 - D) 1400 <details> <summary>Answer</summary> <b>B) 1492</b> - 1500 - 8 bytes for PPPoE header = 1492. </details> **3. Which PPPoE discovery message is broadcast by the client?** - A) PADO - B) PADR - C) PADI - D) PADS <details> <summary>Answer</summary> <b>C) PADI</b> - PADI (PPPoE Active Discovery Initiation) is broadcast to find servers. </details> **4. What is the purpose of the dialer interface in PPPoE configuration?** - A) Physical interface for DSL - B) Virtual interface for PPP session - C) VLAN interface - D) Loopback interface <details> <summary>Answer</summary> <b>B) Virtual interface for PPP session</b> - The dialer interface hosts the PPP session. </details> **5. Which command enables PPPoE on a Cisco interface?** - A) `pppoe enable` - B) `pppoe client` - C) `encapsulation pppoe` - D) `pppoe session` <details> <summary>Answer</summary> <b>A) `pppoe enable`</b> - This enables PPPoE on the physical interface. </details> **6. What is the EtherType for PPPoE Session packets?** - A) 0x0800 - B) 0x0806 - C) 0x8863 - D) 0x8864 <details> <summary>Answer</summary> <b>D) 0x8864</b> - PPPoE Session Phase uses EtherType 0x8864. </details> **7. What does BRAS stand for?** - A) Broadband Remote Access Server - B) Basic Rate Access Server - C) Broadband Router Access System - D) Basic Remote Authentication Server <details> <summary>Answer</summary> <b>A) Broadband Remote Access Server</b> - BRAS terminates PPPoE sessions from customers. </details> **8. Which command displays active PPPoE sessions?** - A) `show pppoe` - B) `show pppoe session` - C) `show ppp session` - D) `show pppoe status` <details> <summary>Answer</summary> <b>B) `show pppoe session`</b> - Displays active PPPoE sessions and session IDs. </details> **9. What is the recommended TCP MSS adjustment for PPPoE?** - A) 1400 - B) 1452 - C) 1492 - D) 1500 <details> <summary>Answer</summary> <b>B) 1452</b> - 1492 - 20 (IP header) - 20 (TCP header) = 1452. </details> **10. Which PPPoE message terminates a session?** - A) PADR - B) PADS - C) PADT - D) PADO <details> <summary>Answer</summary> <b>C) PADT</b> - PADT (PPPoE Active Discovery Terminate) ends the session. </details> **11. What is the purpose of the `dialer pool` command?** - A) Specify physical interface for PPPoE - B) Create IP pool for DHCP - C) Configure dialer interface group - D) Set authentication method <details> <summary>Answer</summary> <b>A) Specify physical interface for PPPoE</b> - Links the dialer interface to the physical interface. </details> **12. Which debug command shows PPPoE discovery messages?** - A) `debug pppoe discovery` - B) `debug pppoe events` - C) `debug pppoe packets` - D) `debug ppp authentication` <details> <summary>Answer</summary> <b>B) `debug pppoe events`</b> - Shows PPPoE discovery phase events. </details> --- ## 📌 Next Steps After completing Video 33, you should be ready for: - **Video 34:** GRE Tunnels - **Video 35:** VPN Technologies **Lab Practice:** 1. Configure PPPoE client on a router 2. Verify with `show pppoe session` 3. Configure PPPoE server (if possible) 4. Test MTU issues with ping with DF bit set 5. Configure NAT with PPPoE 6. Troubleshoot authentication failures --- **Ready for Video 34?** Share the link or say "next" and I'll continue with GRE Tunnels. I'll continue with **Video 34: GRE Tunnels** based on the standard CCNA 200-301 curriculum. ---