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Routing Process

Routers forward packets by stripping incoming Layer 2 frames, consulting the routing table for the best next hop, decrementing the TTL, recalculating the IP header checksum, and building a new Layer 2 frame before transmitting the packet to the next hop.

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About this video

The routing process begins the moment a frame arrives at a router interface. The router processes the incoming Layer 2 header exactly as any network device would, checking whether the destination MAC address is its own. Once that match is confirmed, the Layer 2 information has served its purpose, and the router shifts its attention to the Layer 3 IP header to determine where the packet ultimately needs to go. With the destination IP address in hand, the router consults its routing table to identify the best next hop — the most efficient path available toward the destination network. Before the packet moves forward, the router decrements the TTL field in the IP header by one. This decrement is a critical safeguard: if a routing loop exists anywhere in the network, the TTL will eventually reach zero, causing the packet to be discarded rather than circulating indefinitely and consuming bandwidth. Because modifying the TTL changes the IP header, the router must also recalculate and update the header checksum to maintain data integrity. With those adjustments complete, the router constructs a brand-new Layer 2 header suited to the outgoing interface and uses the physical layer to place the packet onto the transmission medium. This sequence — receive, inspect, consult the routing table, decrement TTL, update checksum, reframe, and transmit — repeats at every router along the path until the packet reaches its final destination.

What you'll learn

What's covered

IP Routing Process

Aligned to

Cisco CCNA
3.2 Determine how a router makes a forwarding decision by default
3.1 Interpret the components of routing table
CompTIA Network+
2.1 Explain characteristics of routing technologies
Cisco CCST Networking
4.4 Explain basic routing concepts

Key terms

Router
A network device that forwards data packets between networks based on IP addresses.
Data Link Layer
Layer 2 of the OSI model responsible for node-to-node data transfer and error detection on a single network segment.
Media Access Control
MAC
A sublayer of the Data Link layer that controls how devices on a network gain access to a medium and transmit data.
Packet
A unit of data formatted for transmission over a network, containing a header, payload, and sometimes a trailer.
Time to Live
TTL
A value in a packet that limits its lifespan on a network, preventing it from circulating indefinitely.
Encapsulation
The process of wrapping data with protocol headers as it passes down the layers of the OSI model.
Routing Table
A database stored in a router that lists known network destinations, the metric (cost) to reach them, and the next-hop address or outgoing interface to forward packets toward those destinations. Routers consult this table for every packet they forward.
Checksum
A value calculated from packet header data and used to detect errors; recalculated by a router after modifying the IP header fields such as TTL.

Transcript

So what that routing process looks like is that a frame will actually come into this router, and that router will process that frame just like any other machine would process that frame. That would be the layer 2 information that's in there, coming into that router.

And so then, once that router determines, well, this layer 2 is destined for me, the MAC address is destined for me, now what I need to do is figure out what network this is going to be sending to. So it takes a look at the layer 3 information, and it matches it up against its routing table to determine what is the next best hop.

Once it does that, it's going to take that time to live and it's going to decrement it by one. So that way, whatever that time to live is, it's going to subtract one from that. And that means that if there is a routing loop, at some point in time that packet would die and would not continue to live forever.

And because it decrements that time to live on there, it also has to adjust the checksum that was within the IP header. So it adjusts the checksum, and then once it's done that, then it can forward that information on to the next router, that next hop. But of course, to do that, to put it onto this physical line, it's going to need to reform a new layer 2 header and then use layer 1 to physically put it on the line to get it on its way.

So that is the routing process. That's what routers do to process the packets that are coming into it, so that way it can send on to the next hop to get it pushed along its way.

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