Network PC World

  VLSM (Variable Length Subnet Mask) and CIDR (Classless Inter-Domain Routing) are both techniques for efficient IP address allocation, but they serve different purposes: VLSM (Variable Length Subnet Mask) : VLSM allows different subnets within the same network to use different subnet masks, making it possible to allocate IP addresses more efficiently based on need (i.e., smaller subnets for smaller networks, larger subnets for larger ones). Used mainly within internal networks (intra-domain) to maximize the usage of available IP space. Requires routers that support classless routing protocols (e.g., OSPF, EIGRP, or RIPv2). CIDR (Classless Inter-Domain Routing) : CIDR is a method of assigning IP addresses without adhering to the traditional class-based system (A, B, C), allowing for more flexible and hierarchical IP address allocation. CIDR is primarily used for routing between networks (inter-domain), particularly on the Internet, to reduce routing table sizes and prevent IP exh...

 Classful Routing: Classful routing refers to a method where routing decisions are made based on the fixed subnet mask of IP address classes (A, B, C). It doesn’t transmit subnet mask information in routing updates, assuming default subnet masks based on IP address class. This approach was commonly used in older protocols like RIPv1 and IGRP . Key Characteristics : No subnet information is shared between routers. IP addresses are divided strictly into classes (A, B, C, etc.). It doesn’t support Variable Length Subnet Masking (VLSM). Less efficient use of IP address space due to fixed class boundaries. Example : If a router sees an IP address in the range 192.168.1.0 , it assumes the default subnet mask of /24 (255.255.255.0), as per Class C rules. Classless Routing: Classless routing allows for the use of Variable Length Subnet Masking (VLSM) and sends routing updates with subnet mask information. This allows for more flexible and efficient use of IP address space. Classless rou...

In this post, we will configure Multiple Spanning Tree (MST) , a protocol designed to optimize spanning tree instances by mapping multiple VLANs to fewer instances. This reduces overhead on network devices, enhances scalability, and speeds up convergence. We'll configure MST on both core/root switches and access switches , ensuring that only the required VLANs are active on each switch. The configuration will focus on assigning VLANs to specific MST instances, defining root priorities, and controlling VLAN availability on trunk links between switches. This setup ensures efficient traffic flow, minimizes network downtime, and improves overall stability. We'll also define MST regions and revision numbers to maintain consistency across the network. By following this guide, you'll optimize spanning tree operations while maintaining flexibility in VLAN creation and deployment across your infrastructure. Configuration for Core-SW1 (Primary Root for Instance 1) ! Define MST re...

 Here's a simple guide on when to use BPDU Guard and BPDU Filter : BPDU Guard : Purpose : To protect the network from unauthorized devices or switches that could participate in the spanning tree process and potentially cause loops. When to Use : On access ports where end devices (like PCs, printers, or servers) are connected. When you want to automatically shut down a port if a BPDU is received, indicating that another switch or device with STP capabilities is connected. Ensures the network remains loop-free by disabling the port when an unexpected BPDU is detected. BPDU Filter : Purpose : To suppress the sending and receiving of BPDUs on a port, effectively preventing STP participation. When to Use : On edge ports (access ports) where you want to prevent STP interactions but don't want to shut the port down upon BPDU reception. In specific scenarios like when you are sure that no switch will be connected, but you don’t want to disrupt the port's operation if a BPDU is...

  Route Filtering with Any Routing Protocol Route filtering is used to selectively control which routes are advertised or received from neighboring routers, helping manage traffic flows, reduce memory utilization, or enhance security. In vector-based routing protocols, filtering occurs during the advertisement of routes. In link-state protocols like OSPF, filtering typically happens at the Area Border Routers (ABRs) when routes enter or leave an area. Example of Route Filtering in OSPF In this example, ip prefix-list is used to create filtering rules that deny or permit specific routes. These rules are applied using the area command to control which routes enter or exit an area. Technical Tip : When configuring OSPF route filtering with prefix lists, always keep in mind the hierarchical structure of areas and the flow of LSAs. Filtering usually applies at ABRs to prevent specific LSAs from being propagated across areas. R2 Configuration : ip pref...

  Routing Concepts Summary Classical IOS vs. IOS XE : Classical IOS is monolithic, meaning all features are in one file, and failures in one function can cause the entire system to fail, requiring a reboot for upgrades. IOS XE is used on ASR1K and operates more like Linux with an IOS interface, allowing access to the Linux shell and daemon-based operations. Prefix Lists : Prefix lists match both the prefix and prefix length for routing decisions. EIGRP can advertise directly connected networks without extra configuration. The "ge" option helps define specific prefix ranges. IP Routing Overview : Routers forward packets by comparing the destination IP address to the routing table, always choosing the most specific match (longest match rule). Static routes can remain even if the next-hop becomes unreachable using the "permanent" keyword. IP Packet Fields : Version : Identifies the IP version (e.g., IPv4). Header Length : Defines the length of t...

  UplinkFast and BackboneFast are two optimizations used in Legacy STP (802.1D) to improve convergence times during certain network failures, particularly in large topologies. Here’s a breakdown of both: UplinkFast : Purpose : UplinkFast was designed to improve recovery time when a direct uplink to the root bridge fails, especially in access layer switches with redundant uplinks to the distribution layer. How it Works : If the primary uplink (Root Port) fails, UplinkFast immediately switches to a backup uplink (an Alternate Port) without waiting for the normal spanning tree convergence process. This allows the switch to rapidly restore connectivity by immediately transitioning the Alternate Port to the Forwarding state. Use Case : Primarily used in access switches with redundant uplinks to quickly restore connectivity when the main uplink fails. Convergence Time : UplinkFast can achieve sub-second recovery times for uplink failures. BackboneFast : Purpose : BackboneFast speeds u...

 In RSTP (Rapid Spanning Tree Protocol), the Proposal and Agreement process is key to optimizing convergence times when a topology change occurs. Here’s how it works: Proposal Stage : When a switch detects a link coming up (such as when a new switch is connected or a port goes from blocking to forwarding), it assumes it could become the designated bridge on that port. The switch sends a Proposal BPDU on that port, which signals to the downstream switch that it intends to make the port a designated port and immediately start forwarding traffic. Agreement Stage : The downstream switch (receiving the Proposal BPDU) checks whether the port on its side should be in blocking or forwarding state. If the downstream switch agrees with the proposal and can allow the new topology without creating loops, it sends an Agreement BPDU back to the original switch. Upon receiving the Agreement, the original switch places the port into the Forwarding state immediately without waiting for the lon...

Why Legacy STP's Listening and Learning States Were Essential for Half-Duplex Networks and How RSTP Optimizes for Modern Ethernet The listening and learning states in legacy STP (802.1D) were largely a product of network design at the time, which included many half-duplex connections and shared network segments (e.g., hubs and collision domains). These states were necessary to ensure stability and prevent loops in such environments. Here's why these states were important: Half-Duplex Networks and Collision Domains : In half-duplex environments (e.g., when hubs were common), collisions could occur because multiple devices shared the same medium. This meant that careful management of forwarding decisions was crucial to avoid packet loss and network loops. The listening and learning states gave STP time to make sure there were no loops or improper configurations in the network, allowing BPDUs to propagate across the network and determine the best path for forwarding. Listeni...

Asynchronous transmission uses start and stop bits to signify the beginning bit ASCII character would actually be transmitted using 10 bits e.g.: A "0100 0001" would become "1 0100 0001 0". The extra one (or zero depending on parity bit) at the start and end of the transmission tells the receiver first that a character is coming and secondly that the character has ended. This method of transmission is used when data is sent intermittently as opposed to in a solid stream. In the previous example the start and stop bits are in bold. The start and stop bits must be of opposite polarity. This allows the receiver to recognize when the second packet of information is being sent. Synchronous transmission uses no start and stop bits but instead synchronizes transmission speeds at both the receiving and sending end of the transmission using clock signal(s) built into each component. A continual stream of data is then sent between the two nodes. Due to there being no st...