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This chapter is from the book

This chapter is from the book

Routing Protocol Operating Fundamentals (3.1.3)

All routing protocols basically perform the same tasks. They all exchange routing updates and converge to build routing tables that are used by the router to make packet forwarding decisions. This section provides an overview of routing protocol fundamentals.

Dynamic Routing Protocol Operation (3.1.3.1)

All routing protocols are designed to learn about remote networks and to quickly adapt whenever there is a change in the topology. The method that a routing protocol uses to accomplish this depends upon the algorithm it uses and the operational characteristics of that protocol.

In general, the operations of a dynamic routing protocol can be described as follows:

  1. The router sends and receives routing messages on its interfaces.
  2. The router shares routing messages and routing information with other routers that are using the same routing protocol.
  3. Routers exchange routing information to learn about remote networks.
  4. When a router detects a topology change, the routing protocol can advertise this change to other routers.

Cold Start (3.1.3.2)

All routing protocols follow the same patterns of operation. When a router powers up, it knows nothing about the network topology. It does not even know that there are devices on the other end of its links. The only information that a router has is from its own saved configuration file stored in NVRAM.

After a router boots successfully, it applies the saved configuration. If the IP addressing is configured correctly, then the router initially discovers its own directly connected networks.

Notice how the routers proceed through the boot process and then discover any directly connected networks and subnet masks. This information is added to their routing tables as follows:

  • R1 adds the 10.1.0.0 network available through interface FastEthernet 0/0 and adds 10.2.0.0 available through interface Serial 0/0/0.
  • R2 adds the 10.2.0.0 network available through interface Serial 0/0/0 and adds 10.3.0.0 available through interface Serial 0/0/1.
  • R3 adds the 10.3.0.0 network available through interface Serial 0/0/1 and adds 10.4.0.0 available through interface FastEthernet 0/0.

With this initial information, the routers then proceed to find additional route sources for their routing tables.

Network Discovery (3.1.3.3)

After initial boot up and discovery, the routing table is updated with all directly connected networks and the interfaces those networks reside on.

If a routing protocol is configured, the next step is for the router to begin exchanging routing updates to learn about any remote routes.

The router sends an update packet out all interfaces that are enabled on the router. The update contains the information in the routing table, which currently comprises all directly connected networks.

At the same time, the router also receives and processes similar updates from other connected routers. Upon receiving an update, the router checks it for new network information. Any networks that are not currently listed in the routing table are added.

Figure 3-5 depicts an example topology setup between three routers, R1, R2, and R3. Notice that only the directly connected networks are listed in each router’s respective routing table.

Figure 3-5

Figure 3-5 Initial Routing Table Before Exchange

Based on this topology, a listing of the different updates that R1, R2, and R3 send and receive during initial convergence is provided:

R1:

  • Sends an update about network 10.1.0.0 out the Serial 0/0/0 interface
  • Sends an update about network 10.2.0.0 out the FastEthernet 0/0 interface
  • Receives update from R2 about network 10.3.0.0 and increments the hop count by 1
  • Stores network 10.3.0.0 in the routing table via Serial 0/0/0 with a metric of 1

R2:

  • Sends an update about network 10.3.0.0 out the Serial 0/0/0 interface
  • Sends an update about network 10.2.0.0 out the Serial 0/0/1 interface
  • Receives an update from R1 about network 10.1.0.0 and increments the hop count by 1
  • Stores network 10.1.0.0 in the routing table via Serial 0/0/0 with a metric of 1
  • Receives an update from R3 about network 10.4.0.0 and increments the hop count by 1
  • Stores network 10.4.0.0 in the routing table via Serial 0/0/1 with a metric of 1

R3:

  • Sends an update about network 10.4.0.0 out the Serial 0/0/1 interface
  • Sends an update about network 10.3.0.0 out the FastEthernet 0/0 interface
  • Receives an update from R2 about network 10.2.0.0 and increments the hop count by 1
  • Stores network 10.2.0.0 in the routing table via Serial 0/0/1 with a metric of 1

Figure 3-6 displays the routing tables after the initial exchange.

Figure 3-6

Figure 3-6 Routing Table After Initial Exchange

After this first round of update exchanges, each router knows about the connected networks of its directly connected neighbors. However, did you notice that R1 does not yet know about 10.4.0.0 and that R3 does not yet know about 10.1.0.0? Full knowledge and a converged network do not take place until there is another exchange of routing information.

Exchanging the Routing Information (3.1.3.4)

At this point the routers have knowledge about their own directly connected networks and about the connected networks of their immediate neighbors. Continuing the journey toward convergence, the routers exchange the next round of periodic updates. Each router again checks the updates for new information.

After initial discovery is complete, each router continues the convergence process by sending and receiving the following updates.

R1:

  • Sends an update about network 10.1.0.0 out the Serial 0/0/0 interface
  • Sends an update about networks 10.2.0.0 and 10.3.0.0 out the FastEthernet 0/0 interface
  • Receives an update from R2 about network 10.4.0.0 and increments the hop count by 1
  • Stores network 10.4.0.0 in the routing table via Serial 0/0/0 with a metric of 2
  • Same update from R2 contains information about network 10.3.0.0 with a metric of 1. There is no change; therefore, the routing information remains the same.

R2:

  • Sends an update about networks 10.3.0.0 and 10.4.0.0 out of Serial 0/0/0 interface
  • Sends an update about networks 10.1.0.0 and 10.2.0.0 out of Serial 0/0/1 interface
  • Receives an update from R1 about network 10.1.0.0. There is no change; therefore, the routing information remains the same.
  • Receives an update from R3 about network 10.4.0.0. There is no change; therefore, the routing information remains the same.

R3:

  • Sends an update about network 10.4.0.0 out the Serial 0/0/1 interface
  • Sends an update about networks 10.2.0.0 and 10.3.0.0 out the FastEthernet 0/0 interface
  • Receives an update from R2 about network 10.1.0.0 and increments the hop count by 1
  • Stores network 10.1.0.0 in the routing table via Serial 0/0/1 with a metric of 2
  • Same update from R2 contains information about network 10.2.0.0 with a metric of 1. There is no change; therefore, the routing information remains the same.

Figure 3-7 displays the routing tables after the routers have converged.

Figure 3-7

Figure 3-7 Routing Table After Convergence

Distance vector routing protocols typically implement a routing loop prevention technique known as split horizon. Split horizon prevents information from being sent out the same interface from which it was received. For example, R2 does not send an update containing the network 10.1.0.0 out of Serial 0/0/0, because R2 learned about network 10.1.0.0 through Serial 0/0/0.

After routers within a network have converged, the router can then use the information within the route table to determine the best path to reach a destination. Different routing protocols have different ways of calculating the best path.

Achieving Convergence (3.1.3.5)

The network has converged when all routers have complete and accurate information about the entire network, as shown in Figure 3-7. Convergence is the time it takes routers to share information, calculate best paths, and update their routing tables. A network is not completely operable until the network has converged; therefore, most networks require short convergence times.

Convergence is both collaborative and independent. The routers share information with each other, but must independently calculate the impacts of the topology change on their own routes. Because they develop an agreement with the new topology independently, they are said to converge on this consensus.

Convergence properties include the speed of propagation of routing information and the calculation of optimal paths. The speed of propagation refers to the amount of time it takes for routers within the network to forward routing information.

As shown in Figure 3-8, routing protocols can be rated based on the speed to convergence; the faster the convergence, the better the routing protocol. Generally, older protocols, such as RIP, are slow to converge, whereas modern protocols, such as EIGRP and OSPF, converge more quickly.

Figure 3-8

Figure 3-8 Converging

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