Close-up view of Cisco router ports and connectors with the text "Explained in Simple Terms: Understanding Cisco Router Switching" on an orange background.

Router Switching Mastery: Unlock Efficient Packet Delivery (Updated 2025)

The router is a device that receives a packet from the source on any interface and forwards it to its destination on another interface. This is possible with the router switching function, which encapsulates packets in the data link frame type for the outgoing data link. Before going further, learn the key networking terms for router switching:


Key Networking Terms for Router Switching

To ensure clarity, here are definitions of key terms used in this article:

  • Encapsulation: The process of wrapping a packet (e.g., an IP packet at Layer 3) with a data link layer (Layer 2) header and trailer to prepare it for transmission over a specific medium.
  • De-encapsulation: The process of removing the Layer 2 header and trailer to access the Layer 3 packet for routing decisions.
  • Layer 2 Frame: A data unit at the Data Link Layer (Layer 2) that includes a header (e.g., MAC addresses), payload (e.g., IP packet), and trailer (e.g., CRC).
  • ICMP (Internet Control Message Protocol): A protocol used for diagnostic and error messaging in IP networks, such as ping or traceroute.
  • Routing Table: A database in a router that stores the best paths to various network destinations, used to forward packets.
  • MAC Address: A unique 48-bit address assigned to network interfaces at the Data Link Layer, used for communication within a single network segment.

The router’s routing function selects the best path for the packet destination, and the router’s switching function encapsulates the packet into the data link frame of the outgoing interface. The router switching function performs the following: receiving a packet from one network and sending it to another.

1. The router receives the Layer 2 encapsulated frame and then de-encapsulates the Layer 2 frame header and trailer.

2. After De-encapsulation, the router reads Layer 3 information and the destination IP address of the IP packet to select the best path in the routing table.

3. When selecting a path for the packet destination, the router encapsulates the Layer 3 packet into a new Layer 2 frame and forwards the frame out of the exit interface.

Figures 1 to 6 illustrate packet switching over a routed network. As shown in Figure 1, the laptop generates an ICMP message for the server in the topology. The packet contains layer 3 information on the source and destination layer 3 addresses. The source layer 3 address is the address of the laptop, and the destination layer 3 address is the server’s IP address. As a packet travels from the source to the destination, the Layer 3 IP addresses do not change because the Layer 3 PDU does not change.

However, the Layer 2 data link addresses change at every hop as each router de-encapsulates and re-encapsulates the packet in a new Layer 2 frame. In Figure 1, the Layer 3 packet is encapsulated for the wireless access point and forwarded to Layer 2 (the laptop’s wireless card) and then to Layer 1 for transmitting on port 1. The wireless port is virtual, not a physical port.

Encapsulation into a different type of Layer 2 frame than the one commonly used for receiving packets is common. For example, a home router receives a frame from the wireless port and then sends it to router 3 over an Ethernet interface. So, the encapsulation for wireless and Ethernet is different. Also, encapsulation for Fast Ethernet, Gigabit Ethernet, and serial interfaces is different.

A detailed network diagram is displayed on a laptop, showing the connection between a server, three routers, a wireless router, and a laptop, with specific IP addresses and PDU (Protocol Data Unit) information highlighting the communication path and network troubleshooting. #Router Switching Function
Router Switching Mastery: Unlock Efficient Packet Delivery (Updated 2025) 8

Figure 2 illustrates that the home router receives layer 2 packets in the shape of bits from layer 1 and then de-encapsulates the packet to read layer 3 information. When reading the source and destination, the router selects the proper outgoing interface, again encapsulates the packet, and sends it to Layer 2 and then to Layer 1.

Network diagram showing a connection from a server (IP 192.168.3.2) through three routers to a wireless router, ending at a laptop (IP 192.168.0.1). PDU information displayed for the wireless router, detailing data packet flow.
Router Switching Mastery: Unlock Efficient Packet Delivery (Updated 2025) 9

Notice the source and destination MAC addresses and IP addresses, first on the home router and then on all the routers. The source and destination MAC addresses change at each router, but the IP address does not.

A network diagram showing a server (IP 192.168.3.2) connected to three routers, and then to a wireless router (IP 192.168.0.1) which is connected to a laptop. The PDU information at Router0 displays inbound and outbound packets, detailing the IP and Ethernet headers as well as the ports used for transmission and reception.
Router Switching Mastery: Unlock Efficient Packet Delivery (Updated 2025) 10

Also, notice in Figure 3 and Figure 4 that the ports between Router3 and Router2 have no MAC addresses in the frame. This is a serial link, and MAC addresses are only required on multi-access networks, such as Ethernet. A serial link is a point-to-point connection and uses a different Layer 2 frame that does not require the MAC address.

Network diagram showing the connection between a server, three routers, a wireless router, and a laptop. The PDU information at Router1 displays details about inbound and outbound packets, including IP headers, Ethernet II headers, and HDLC frames, emphasizing data packet routing and processing through the network.
Router Switching Mastery: Unlock Efficient Packet Delivery (Updated 2025) 11

For example, when Ethernet frames destined for Server0 are received on Router3 from the Fa0/0 interface, they are de-encapsulated and then re-encapsulated for the serial interface. When Router2 receives the frame, it is de-encapsulated again and then re-encapsulated into an Ethernet frame with a destination MAC address.

Network diagram showing the connection between a server, three routers, a wireless router, and a laptop. PDU (Protocol Data Unit) details are displayed at Router2, illustrating inbound and outbound data packet information, including IP addresses, Ethernet headers, and ports used.
Router Switching Mastery: Unlock Efficient Packet Delivery (Updated 2025) 12
Network diagram showing the connection between Server0, Router2, Router1, Wireless Router0, and Laptop1. The PDU information at Server0 shows OSI model details for inbound and outbound packets, including IP addresses and Ethernet headers.
Router Switching Mastery: Unlock Efficient Packet Delivery (Updated 2025) 13

The table below better summarizes the process of sending a packet from Laptop 1 to Server 0. You can see the packet source IP, MAC address, Destination IP, and MAC address.

Notice that the source and destination IP addresses do not change until the packet reaches the final destination. However, the source and destination MAC addresses change for each hop. At stage 11th, when the server responds to Laptop 1, the source and destination addresses change accordingly because now Server 0 sends a reply message to Laptop 1. So, this time, the source is server 0.

FAQs

  • What is the difference between encapsulation and de-encapsulation in router switching?

    Encapsulation wraps a Layer 3 packet with a Layer 2 frame for transmission, while de-encapsulation removes the Layer 2 frame to access the Layer 3 packet for routing decisions. This process occurs at each router hop.

  • Why do MAC addresses change at each router hop?

    MAC addresses are specific to Layer 2 and change at each hop because they identify devices within a single network segment. Each router re-encapsulates the packet into a new Layer 2 frame for the next segment.

  • How does a routing table help in router switching?

    The routing table stores the best paths to network destinations. The router uses it to determine the outgoing interface for a packet based on its destination IP address, ensuring efficient forwarding.

  • Why don’t IP addresses change during packet forwarding?

    IP addresses (Layer 3) identify the source and destination across the entire network and remain constant to ensure the packet reaches its intended recipient, regardless of the number of router hops.

  • What is the role of ICMP in router switching?

    ICMP is used for diagnostic and error messaging, like ping or traceroute. In router switching, it helps test connectivity by sending packets that routers process and forward to the destination.

Avatar of Asad Ijaz

Asad Ijaz

NetworkUstad's lead networking architect with CCIE certification. Specializes in CCNA exam preparation and enterprise network design. Authored 2,800+ technical guides on Cisco systems, BGP routing, and network security protocols since 2018. Picture this: I'm not just someone who writes about tech; I'm a certified expert in the field. I proudly hold the titles of Cisco Certified Network Professional (CCNP) and Cisco Certified Network Associate (CCNA). So, when I talk about networking, I'm not just whistling in the dark; I know my stuff! My website is like a treasure trove of knowledge. You'll find a plethora of articles and tutorials covering a wide range of topics related to networking and cybersecurity. It's not just a website; it's a learning hub for anyone who's eager to dive into the world of bits, bytes, and secure connections. And here's a fun fact: I'm not a lone wolf in this journey. I'm a proud member and Editor of Team NetworkUstad. Together, we're on a mission to empower people with the knowledge they need to navigate the digital landscape safely and effectively. So, if you're ready to embark on a tech-savvy adventure, stick around with me, Asad Ijaz Khattak. We're going to unravel the mysteries of technology, one article at a time!"