Routers are specialized network devices that operate primarily at the Network Layer (IP Layer). Their fundamental role is to interconnect different networks (subnets or autonomous systems) and forward data packets (IP datagrams) between them, making intelligent forwarding decisions based on network addresses.

Main Functional Components of a Router:

1. Input Ports (Line Cards/Modules): These are the physical interfaces where incoming data packets arrive from network links. Each input port performs several critical functions:
2. Line Termination: Handles the physical layer details (signal conversion, framing).
3. Link-Layer Processing: Processes the data link layer protocol (e.g., Ethernet framing, error checking, MAC address handling).
4. Lookup/Forwarding Engine: This is a crucial component that performs the destination IP address lookup. It examines the destination IP address in the packet's IP header and consults the router's forwarding table (also known as the Forwarding Information Base or FIB) to determine the appropriate output port through which the packet should be sent. This often involves longest prefix matching (detailed later).
5. Input Queues (Buffers): If the switching fabric (the internal pathway) or the desired output port is currently busy, incoming packets are temporarily stored in these buffers. This is where queueing begins to play a role in router performance.
6. Switching Fabric (or Switching System): This is the internal component that provides the high-speed pathway for moving packets from the input ports to the correct output ports. Its design is critical for the router's overall throughput. Common types include:
7. Switching via Memory: Packets are copied from an input port's memory to the router's central memory, and then copied from central memory to the output port's memory. This is simple but slow, bottlenecked by memory bandwidth. Suitable for small, older routers.
8. Switching via a Bus: Input ports place packets onto a shared high-speed bus, and output ports pick them off. Faster than memory-based switching but limited by the bus's bandwidth, which can become a bottleneck as router speeds increase.
9. Switching via an Interconnection Network (e.g., Crossbar Switch, Banyan Network): A highly parallel and scalable architecture where multiple input-output transfers can occur simultaneously. This is used in high-performance, enterprise, and core Internet routers to achieve very high throughput.
10. Output Ports (Line Cards/Modules): These are the physical interfaces from which data packets are transmitted onto outgoing network links. Each output port performs:
11. Output Queues (Buffers): If the switching fabric delivers packets to an output port faster than the outgoing link can transmit them, packets are temporarily stored in these buffers. These buffers are essential for smoothing out traffic bursts and preventing packet loss during periods of congestion.
12. Link-Layer Processing: Prepares the packets for transmission over the specific outgoing link (e.g., adds Ethernet trailers, performs CRC).
13. Line Termination: Handles the physical layer transmission details.
14. Routing Processor (Control Plane): This is the 'brain' of the router, responsible for managing the router's non-forwarding functions. It typically operates on a slower timescale than the data plane (the input/output ports and switching fabric). Its key responsibilities include:
15. Running Routing Protocols: Communicating with other routers to exchange routing information (e.g., using OSPF, BGP, RIP) and constructing the global routing tables.
16. Maintaining Routing Tables: Building and updating the router's forwarding table (FIB) based on the information learned from routing protocols.
17. Network Management: Handling configuration, monitoring, and diagnostic tasks (e.g., SNMP, CLI access).
18. Processing Exceptional Packets: Handling IP datagrams that require special processing (e.g., packets with IP options, or packets destined for the router itself like ICMP messages).

Each input port performs several critical functions:
- Line Termination: Handles the physical layer details (signal conversion, framing).
- Link-Layer Processing: Processes the data link layer protocol (e.g., Ethernet framing, error checking, MAC address handling).
- Lookup/Forwarding Engine: This is a crucial component that performs the destination IP address lookup. It examines the destination IP address in the packet's IP header and consults the router's forwarding table (also known as the Forwarding Information Base or FIB) to determine the appropriate output port through which the packet should be sent. This often involves longest prefix matching (detailed later).
- Input Queues (Buffers): If the switching fabric (the internal pathway) or the desired output port is currently busy, incoming packets are temporarily stored in these buffers. This is where queueing begins to play a role in router performance.

Switching Fabric (or Switching System): This is the internal component that provides the high-speed pathway for moving packets from the input ports to the correct output ports. Its design is critical for the router's overall throughput. Common types include:
- Switching via Memory: Packets are copied from an input port's memory to the router's central memory, and then copied from central memory to the output port's memory. This is simple but slow, bottlenecked by memory bandwidth. Suitable for small, older routers.
- Switching via a Bus: Input ports place packets onto a shared high-speed bus, and output ports pick them off. Faster than memory-based switching but limited by the bus's bandwidth, which can become a bottleneck as router speeds increase.
- Switching via an Interconnection Network (e.g., Crossbar Switch, Banyan Network): A highly parallel and scalable architecture where multiple input-output transfers can occur simultaneously. This is used in high-performance, enterprise, and core Internet routers to achieve very high throughput.

Output Ports (Line Cards/Modules): These are the physical interfaces from which data packets are transmitted onto outgoing network links. Each output port performs:
- Output Queues (Buffers): If the switching fabric delivers packets to an output port faster than the outgoing link can transmit them, packets are temporarily stored in these buffers. These buffers are essential for smoothing out traffic bursts and preventing packet loss during periods of congestion.
- Link-Layer Processing: Prepares the packets for transmission over the specific outgoing link (e.g., adds Ethernet trailers, performs CRC).
- Line Termination: Handles the physical layer transmission details.

Routing Processor (Control Plane): This is the 'brain' of the router, responsible for managing the router's non-forwarding functions. It typically operates on a slower timescale than the data plane (the input/output ports and switching fabric). Its key responsibilities include:
- Running Routing Protocols: Communicating with other routers to exchange routing information (e.g., using OSPF, BGP, RIP) and constructing the global routing tables.
- Maintaining Routing Tables: Building and updating the router's forwarding table (FIB) based on the information learned from routing protocols.
- Network Management: Handling configuration, monitoring, and diagnostic tasks (e.g., SNMP, CLI access).
- Processing Exceptional Packets: Handling IP datagrams that require special processing (e.g., packets with IP options, or packets destined for the router itself like ICMP messages).

Queueing in Routers and Head-of-Line (HOL) Blocking:
- Importance of Buffers (Queues): Buffers at input and output ports are crucial. They allow a router to absorb temporary bursts of traffic where the arrival rate exceeds the forwarding or transmission rate. Without adequate buffers, packets would be immediately dropped during congestion, leading to severe packet loss.
- Input Queueing and HOL Blocking: If packets are primarily queued at the input ports, a phenomenon called Head-of-Line (HOL) blocking can occur. This happens when a packet at the front of an input queue is blocked from moving to the switching fabric because its desired output port is busy, even if other packets further down the same input queue could be forwarded to available output ports. HOL blocking significantly limits the overall throughput of the router.
- Output Queueing: Modern high-performance routers are designed to minimize or eliminate input queueing and instead rely heavily on output queueing. Packets are moved through the switching fabric to the appropriate output port's buffer as quickly as possible. Congestion management then primarily occurs at the output queues. This design avoids HOL blocking at input queues but requires a very fast switching fabric that can deliver packets to output queues at a rate higher than the sum of all input line rates.