Routed, Routable, and Routing Protocols

The entire communication process between two devices on a computer network is mainly divided into two parts. The source and destination devices control and manage the first part, while the intermediate devices that connect them control and manage the second part. This tutorial explains both in detail.

To manage communication, source and destination devices utilize a routed protocol, while intermediate devices employ a routing protocol. For instance, if devices A and B are connected via devices C and D, A and B use a routed protocol, while C and D use a routing protocol.

The source device uses a routed protocol to pack data packets for transportation. The destination device uses the same routed protocol to unpack the received data packets from the source device.

In computer networks, routers function as intermediate devices that connect different networks. Routers use routing protocols to discover available routes and determine the most efficient path between source and destination devices.

Consider another example. In a network, two PCs (PC0 and PC1) are connected through three routes: route-a, route-b, and route-c. When PC0 transmits a large data file to PC1, the routed protocol on PC0 executes all required tasks, such as segmenting the file into smaller pieces for transmission across available routes and appending the source address, destination address, sequence number, and other relevant parameters to each segment.

Breaking a large file into smaller pieces is called segmentation, while adding essential information to each segment is called encapsulation. The routed protocol on the source device performs both segmentation and encapsulation. After completing these tasks, the routed protocol determines the appropriate method for transmitting the packet to the destination device. The device can send a packet to the destination device by using one of two methods. These methods send the packet directly to the destination device and via the default gateway router. If the destination device is available in the local network, the source device uses the first method. If the destination device is available in the remote network, the source device uses the second method.

If the destination device is located on a remote network, the source device forwards the data packet to the default gateway router. The default gateway router examines the destination address and forwards the packet either directly to the destination or to the next router along the path. If it has multiple routes to the same destination, it uses the shortest, fastest route to forward the packet. To learn all available routes and calculate the shortest and fastest route to the destination, the default gateway router and other intermediate routers use a routing protocol.

The destination device employs the same routed protocol to reconstruct the original file from the received data packets. In the example, the routed protocol on PC1 performs this reconstruction. Source and destination devices use the same routed protocol to encapsulate and decapsulate data packets. Routers connecting these devices employ a common routing protocol to discover available routes and select the optimal path.

TCP/IP Networks

In TCP/IP networks, source and destination devices use the IP protocol for encapsulation and de-encapsulation of data packets. Thus, IP serves as the sole routed protocol in TCP/IP environments.

Multiple routing protocols are available in TCP/IP networks, including RIP, EIGRP, and OSPF. Each protocol employs distinct techniques and algorithms to discover available routes and determine the optimal path. Network administrators select routing protocols based on specific network requirements.

Differences Between Routed Protocols and Routing Protocols

The primary differences between routed protocols and routing protocols are as follows.

Routed protocols

• End devices use routed protocols to send and receive data packets.
• Routed protocols provide addressing to end devices.
• Routed protocols encapsulate and de-encapsulate data packets.
• If the destination address is unreachable on the local network, the routing protocol forwards the packet to the default gateway.
• A routed protocol does not determine the specific route a packet takes to reach a remote destination from the default gateway.
• The IP protocol is an example of a routed protocol.

Routing Protocols

• Intermediate routers use routing protocols to discover routes and calculate the best route between a source and a destination.
• Routing protocols store discovered routes in routing tables.
• Routing protocols continuously update and manage routing tables.
• Routing protocols exchange information between routers.
• Routing protocols do not care what is inside a packet. They only care about delivering the packet to the correct destination.
• RIP, IGRP, EIGRP, OSPF, and IS-IS are some examples of routing protocols.

Example

The following image shows a network. In this network, PC0 and PC1 are connected to PC2 via four routers; R1, R2, R3, and R4.

Suppose an application running on PC0 wants to send data to PC1. The application calls the IP protocol and hands that data over to it. The IP protocol breaks data into packets and adds the source and destination addresses to each packet. If the destination address is located in the same IP subnet, the IP protocol sends packets directly to the destination host. The entire routing process is controlled by the routed (IP) protocols of PC0 and PC1.

Now, suppose the same application wants to send data to PC2. The same process is repeated until the packet forwarding decision is made by the IP protocol. This time, since the destination host (PC2) is located in a different IP subnet, the IP protocol sends packets to the default gateway router.

The default gateway router not only maintains records of all remote networks but also of all available paths for each remote network. It maintains these records in the routing table. A typical routing table entry consists of two pieces: the network address and the interface on which that network is available.

When a router receives a packet on any of its interfaces, it reads the packet's destination network and looks it up in the routing table. If the routing table contains a record for the destination network, the router uses the record to forward the packet. If the routing table doesn’t contain a record for the destination network, the router discards the packet. If multiple paths to a remote network exist, the router chooses the fastest path.

In this example, the default gateway router R1 has two paths to reach PC2's network. When it receives packets from PC0, it compares both paths and forwards them over the fastest path. For this, R1 uses the routing protocol. Routing protocols help routers find all paths and select the best path for each destination.

PC2 receives packets from its default gateway router (R4). Routed (IP) protocol running on PC2 processes the received packets.

Conclusion

Routed protocols and routing protocols serve complementary functions in computer networking. Routed protocols, such as IP, manage end-to-end data transfer and encapsulation between devices. Routing protocols, including RIP, OSPF, and EIGRP, enable routers to discover, maintain, and select optimal paths for packet delivery across the network. Recognizing the distinction and interaction between these protocols is essential for designing and maintaining efficient and reliable networks.