CCNA Training

 Common ports: Protocol Port HTTP 80 HTTPS 443 DNS 53 DHCP 67/68 SSH 22 Telnet 23 FTP 21

 As a Wireless NOC L1 Engineer or CCNA candidate , troubleshooting LAN/WAN issues means finding the root cause of problems affecting connectivity, performance, or availability. A structured approach helps you resolve issues quickly. 1. LAN (Local Area Network) Troubleshooting LAN issues occur within a building, office, or campus. Common Problems Problem Possible Cause Troubleshooting PC cannot ping default gateway Wrong IP, subnet mask, gateway Check ipconfig , verify IP settings Switch port down Cable unplugged, interface shutdown Check LEDs, show interfaces status Slow LAN Duplex mismatch, congestion Check speed/duplex settings VLAN communication failure Wrong VLAN assignment show vlan brief STP blocking traffic Incorrect topology show spanning-tree DHCP not assigning IP DHCP server unreachable ipconfig /renew , check DHCP server MAC address learning issue CAM table problems show mac address-table 2. WAN Troubleshooting WAN connects branch offices through leased lines, M...

 TCP flags are control bits in the TCP header that manage how TCP connections are established, maintained, and terminated. There are 9 TCP flags in modern TCP: Flag Full Name Purpose SYN Synchronize Initiates a TCP connection and synchronizes sequence numbers. ACK Acknowledgment Indicates the acknowledgment field is valid. Used in most TCP packets after connection setup. FIN Finish Gracefully terminates a TCP connection. RST Reset Abruptly terminates or rejects a connection. PSH Push Requests immediate delivery of data to the receiving application. URG Urgent Indicates urgent data is present; the Urgent Pointer is valid. ECE ECN Echo Used with Explicit Congestion Notification (ECN) to indicate network congestion. CWR Congestion Window Reduced Indicates the sender has reduced its congestion window after receiving an ECE. NS Nonce Sum Rarely used; supports ECN protection (experimental/obsolete in practice). Common TCP Flag Combinations Flags Meaning SYN Connection request SYN, AC...

  Switch1(config)#int fa0/1 Switch1(config-if)#duplex full Switch2(config)#int fa0/1 Switch2(config-if)#half duplex 🧠 Simple Explanation 🟡 Half Duplex Like a walkie-talkie Only one person talks at a time → collisions possible 🟢 Full Duplex Like a telephone call Both people talk at the same time → no collisions 🔍 Verification Commands (Switch) show interfaces fa0/1 You will see: Full-duplex, 100Mb/s or Half-duplex, 100Mb/s

  ❌ “When TTL is 0 then routing loops” This is not the cause of routing loops . It is the result of a routing loop in Layer 3 . ✅ Correct Concept Routing loops cause packets to circulate between routers repeatedly, and as a result the TTL (Time To Live) decreases until it reaches 0. When TTL becomes 0, the packet is dropped. 🧠 How TTL Actually Works Every IP packet starts with a TTL value (e.g., 64, 128, 255) Each router that forwards the packet: Decreases TTL by 1 When TTL = 0: ❌ Packet is dropped Router sends ICMP Time Exceeded 🔁 Routing Loop Example R1 ↙ ↘ R2 ←→ R3 If routing is misconfigured: Packet goes R1 → R2 R2 sends it to R3 R3 sends it back to R2 Loop continues… 📉 TTL Decreases in Loop Hop Router TTL 1 R1 64 2 R2 63 3 R3 62 4 R2 61 ... ... ... final any router 0 ❌ Packet dropped 🚨 Key Point ✔ Routing loop = problem ✔ TTL = protection mechanism TTL is NOT the loop. TTL is the lifesaver that stops infin...

  show interfaces fa0/1 . 🧠 Interface Status (Most Important First) ✅ FastEthernet0/1 is up, line protocol is up (connected) Physical layer = up Data link layer = up ✔ This means: Cable is connected Interface is working No L1/L2 issue 👉 If you see: up/down → cable or duplex issue down/down → cable unplugged or shutdown ⚙️ Hardware & MAC Address Hardware is Lance Interface type chipset (not important for troubleshooting) address is 0000.0c69.8701 MAC address of the switch port Used for switching decisions 📶 Bandwidth & Delay BW 100000 Kbit Interface speed = 100 Mbps Used for routing metrics (EIGRP/OSPF cost) DLY 1000 usec Delay of interface Used in routing calculations 🔧 Duplex & Speed (VERY IMPORTANT) Half-duplex, 100Mb/s Interface is: Half duplex (can send OR receive, not both) Speed = 100 Mbps 👉 Troubleshooting importance: Half duplex can cause: collisions performance issues late ...

AAA ( Authentication Authorization Accounting ) Server RADIUS Server is commonly used to authenticate users connecting to Wi-Fi, VPNs, or wired 802.1X networks. When you connect to a corporate Wi-Fi and enter: Username: rohith Password: Cisco123 Username + Password │ ▼ Access Point (AP) │ RADIUS Request UDP Port 1812 │ ▼ RADIUS Server (Cisco ISE / FreeRADIUS / Microsoft NPS) │ Accept / Reject │ ▼ Access Point │ ▼ Laptop Connected TACACS+ Server is typically used to authenticate and authorize network administrators logging into routers, switches, and firewalls. It also supports command-by-command authorization, making it well suited for managing administrative access. Download putty tool ssh - secured login ...

Wireless Engineer main responsibility is to identify whether the issue is on the client, Access Point (AP), controller, switch, ISP, or authentication server, perform basic troubleshooting, and escalate when necessary. Wireless NOC L1 issues and their solutions. Issue Possible Cause L1 Troubleshooting Resolution User cannot see Wi-Fi SSID AP down, SSID disabled, weak signal Check if other users can see SSID, verify AP status in monitoring tool Restart AP if allowed, escalate if AP offline Connected to Wi-Fi but no Internet DHCP, DNS, gateway, ISP issue Check IP address, ping gateway, ping DNS (8.8.8.8), verify multiple users affected Renew IP, restart client, escalate if gateway unreachable Unable to obtain IP address DHCP server issue, VLAN problem Check if client received APIPA (169.254.x.x), verify DHCP scope Release/renew IP, escalate DHCP issue Authentication failed Wrong password, expired credentials, RADIUS issue Verify username/password, check if others affected Reset password...