Routing Vs Routed Protocols

Routing protocols apply a set of rules to a network topology to determine the best path to a destination from a given reference point. They also communicate network topology information to other routers in their networks. Routing protocols build routing tables from the gathered information. Examples of routing proto cols are Open Short Path First (OSPF) and IPX's Routing Information Protocol (IPX RIP).

In contrast, a routed protocol is a protocol that contains layer 3 information that allows it to be moved from one destination to another. Examples of a routed protocol include IP and IPX.

Routing protocols can be divided into three types—distance vector, link state, and hybrid. These three routing protocol classifications are discussed in the following sections.

Distance Vector Protocols

Distance vector protocols, such as RIP, determine a path to a network using hop count as the metric. A hop count is a number that increments each time a packet traverses a router.

Convergence—the process that ensures all routers in a network have the same network information as quickly as possible—of distance vector protocols is considerably slower, and periodic updates are sent at set intervals. Figure 2.3 shows how networks are discovered when using a distance vector protocol.

Each router in Figure 2.3 will have the same IP routing table and will send and receive periodic updates. Not every routing protocol sends out periodic updates at the same interval. The distance vector protocol IP RIP sends a periodic update every 30 seconds.

Link State Protocols

Link state protocols, such as IS-IS and OSPF, create a topology of the network with each router running that protocol as the root of the tree. Link state protocols implement the shortest path first (SPF) algorithm to determine the path to a network. The metric used by these protocols is cost, which is determined by an administrator or calculated by the routing protocol based on a mathematical formula. A network with the lowest cost is chosen as the preferred path to a remote network. Link state protocols have no concept of hop count. The speed of convergence with link state protocols is much faster when a network change occurs. This is because a faster algorithm is used and the CPU is heavily utilized to compute changes rapidly. When using link state protocols, updates are only sent when a topological change occurs or at an interval set by an administrator.

Link state protocols use hello packets to discover neighbors. A hello packet is an IP packet sent at regular intervals. When a topology change occurs, a link state packet is sent to all neighbors with information regarding any new neighbors, metric changes, or down networks. When a router receives a link state packet, it records the information in its local database and reconstructs a path to the new network. If a remote network goes down, the routing table entry will be removed.

Routing Table show ip route is subnetted, 1 subnets C is directly connected, Loopback0 D [90/2681856] via, 1w1d, S0

and received /

/ —

Updates sent and received

Updates sent and received

Routing Table

Figure 2.3 Learning networks using distance vector protocols.

Link state packets are used to notify remote neighbors of available networks. The aim is to form a link state database that contains all the available networks. The steps needed to form the database are as follows (Figure 2.4 depicts these steps):

1. Send link state packets to describe the links in a network.

2. Combine link state packets to form a link state database.

3. Run the shortest path first (SPF) algorithm.

4. Create a link tree with the router running the SPF algorithm as the root.

5. Insert networks into the routing table.

Hybrid Routing Protocols

Cisco has created a routing protocol called Enhanced Interior Gateway Routing Protocol (EIGRP). EIGRP combines the characteristics of both link state and distance vector routing protocols. This protocol is called a hybrid protocol because of this combination. A hybrid routing protocol uses distance vector characteristics for choosing a routing path and link state characteristics for changes. EIGRP maintains neighbor and topology tables instead of a link state database.

1. Router R2 sends a link state packet advertising the new Ethernet network.

2. Router R1 receives the link state packet and installs the new netork into the link state database.

3. Router R1 runs shortest path algorithm to determine shortest cost path to the new network.

4. R1 forms a tree, with itself as the root of the tree.

5. The new network is inserted into the IP routing table.

Router R2 activates new Ethernet network

Figure 2.4 Steps taken by link state protocols to form the link state database.

Now that you have a general appreciation for routing protocols, let's talk about some common routing protocol characteristics.

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