Stream of Knowledge

 When learning about networking, one of the most critical layers to understand in the OSI Model is the Transport Layer (Layer 4) . This layer acts as a bridge between the applications you use daily (Layer 7) and the network infrastructure that actually delivers data (Layers 1–3). Two major protocols dominate this layer: TCP (Transmission Control Protocol) – reliable, connection-oriented. UDP (User Datagram Protocol) – fast, connectionless. The Transport Layer. In this guide, we’ll explore the Transport Layer in the OSI model , break down the TCP and UDP headers , and explain why this layer is so important for both network engineers (CCNA, CCNP, CCIE prep) and everyday internet users. 🔹 What is the Transport Layer (Layer 4 of the OSI Model)? The Transport Layer is responsible for end-to-end communication between applications running on different devices across a network. It ensures that: Data gets delivered to the correct application using port numbers . Data ...

  In this lesson, we explain   what the OSI model is   in an easy and understandable language. It is one of the most important concepts in networking, so we break it down into pieces to help you understand exactly what its purpose is. What is data encapsulation? To understand the OSI model, you must first understand what data encapsulation is. Let's explore the following example. Imagine you want  to send a letter to a friend  who lives in another city to invite him to your wedding. What if you send the letter without an envelope, with any information, such as the sender's and recipient's names, addresses, and postcodes? What if you simply write the letter and drop it in the mailbox at the post office?  Most readers of this CCNA course are so young that  they've never sent a physical letter in their lives . They live in the digital age and have grown up with emails and instant text messages. However, surprisingly, everyone understands the concept of th...

 How ARP Works: Understanding ARP Requests, Replies, and ARP Cache Address Resolution Protocol (ARP) is an essential protocol in the world of networking. It's responsible for mapping IP addresses to MAC addresses within a local area network (LAN). 🔄 How ARP Works When a host wants to communicate with another device, it needs the MAC address associated with the destination IP. If the MAC address is unknown, the host sends out a broadcast ARP Request asking: 🗨️ “Who has IP address X.X.X.X? Tell me your MAC address.” The device with the matching IP sends an ARP Reply with its MAC address, allowing communication to begin. 📨 ARP Messages ARP uses two packet types: ARP Request Destination MAC: FF-FF-FF-FF-FF-FF (broadcast) Target MAC: 00-00-00-00-00-00 (unknown) ARP Reply Uses unicast MAC addresses for both source and destination Header Fields Include: Source MAC and IP Target MAC and IP 🧪 Real-World Examples Host-to-Host on Same Network PC2 want...

 Configuring and Verifying VTP v2 Configuring VTP Verifying the topology Before you start configuring VLAN Trunking Protocol on Cisco switches, it is very important to first verify that all inter-switch links are  trunks . Especially in lab/test environments, engineers often spent time troubleshooting VTP issues and in the end, it turns out that the problem is not with the VTP but with the Interswitch links. IMPORTANT TO REMEMBER  VTP messages are sent and received on trunk links only. In this configuration example, we will use the topology shown in Figure 1. Before we start configuring the VTP, let's verify the trunks and how many VLANs are configured. The easiest way to verify this by checking Switch 2, because it has links to all other switches. SW2# sh interfaces trunk Port Mode Encapsulation Status Native vlan Fa0/1 desirable n-802.1q trunking 1 Fa0/2 desirable n-802.1q trunking 1 Fa0/3 desirable ...

 Understanding VTP Versions, Revision Numbers, and VTP Pruning in Cisco Networks VLAN Trunking Protocol (VTP) is a vital tool for simplifying VLAN management in large Layer 2 networks. But not all VTP versions are created equal. Understanding the evolution from VTP v1 to VTP v3 , along with the role of the VTP Revision Number and VTP Pruning , is essential for every network engineer. 🔁 VTP Version Comparison ➡ VTP Version 1 Default on older Cisco switches Supports VLANs 1–1005 Transparent mode relays only matching domain/version messages Drops unknown TLVs ➡ VTP Version 2 Default on newer switches Adds support for extended VLANs (1006–4094) in transparent mode Forwards unknown TLVs Relays VTP messages regardless of domain/version in transparent mode Skips consistency checks if MD5 digest is valid ➡ VTP Version 3 Major upgrade with extended VLAN support in advertisements Supports Private VLANs and MST (Multiple Spanning Tree) Introduces Prim...

 What is VTP (VLAN Trunking Protocol) and Why It Matters in Large Networks In modern enterprise networks, scalability and consistency are critical—especially when managing VLANs across dozens or even hundreds of switches. Traditionally, VLANs are configured locally on each switch, which makes the process slow, repetitive, and error-prone. This is where VTP (VLAN Trunking Protocol) comes into play. ✅ What is VTP? VTP is a Layer 2 messaging protocol developed by Cisco to centralize the management of VLAN configurations . Instead of logging into each switch to manually add or delete a VLAN, a network admin can do it once on a VTP Server switch, and the changes automatically propagate to all VTP Clients within the same domain. 🧠 Why is VTP Useful? Imagine managing 100+ switches. Without VTP, every VLAN change would require manual updates on every device. With VTP, a single change can be distributed network-wide. This reduces: Configuration time Human error Inconsiste...

 Fixing Cisco SD-WAN Blackholes with OMP Send-Backup-Paths The Problem: SD-WAN Blackholes During Link Failures In Cisco SD-WAN , the Overlay Management Protocol (OMP) is responsible for advertising routes between vEdges. However, by default, vSmart only advertises the best routes based on the OMP best-path selection algorithm. Let’s take a real-world example: 📍 Scenario: vEdge-1 has two Transport Locators (TLOCs): MPLS (T11) and Biz-Internet (T12). When the MPLS TLOC on vEdge-1 fails , it stops advertising the OMP route to 10.1.1.0/24 . vSmart selects the best available route , which still goes via vEdge-1 but over Biz-Internet. Problem? vEdge-3 doesn’t have an overlay tunnel to the Biz-Internet TLOC of vEdge-1. 🔻 Result: The route to 10.1.1.0/24 becomes invalid and unresolved . vEdge-3 completely loses connectivity to the data center despite having a valid tunnel to vEdge-2. Traceroute and ping fail to reach 10.1.1.1. The Solution: Enabling OMP Send-Backup-Paths By defau...