Routing Protocol Operating Fundamentals

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.

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.

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

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 are 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.

After this first round of update exchanges, each router knows about the connected networks of their 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.

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

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

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.

The network has converged when all routers have complete and accurate information about the entire network, as shown in Figure 1. Convergence time 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.

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.

Routing protocols can be classified into different groups according to their characteristics. Specifically, routing protocols can be classified by their:

? Purpose- Interior Gateway Protocol (IGP) or Exterior Gateway Protocol (EGP)

? Operation- Distance vector, link-state protocol, or path-vector protocol

? Behavior - Classful (legacy) or classless protocol

For example, IPv4 routing protocols are classified as follows:

? RIPv1 (legacy)- IGP, distance vector, classful protocol

? IGRP (legacy)- IGP, distance vector, classful protocol developed by Cisco (deprecated from 12.2 IOS and later)

? RIPv2- IGP, distance vector, classless protocol

? EIGRP- IGP, distance vector, classless protocol developed by Cisco

? OSPF- IGP, link-state, classless protocol

? IS-IS- IGP, link-state, classless protocol

? BGP- EGP, path-vector, classless protocol

The classful routing protocols, RIPv1 and IGRP, are legacy protocols and are only used in older networks. These routing protocols have evolved into the classless routing protocols, RIPv2 and EIGRP, respectively. Link-state routing protocols are classless by nature.

An autonomous system (AS) is a collection of routers under a common administration such as a company or an organization. An AS is also known as a routing domain. Typical examples of an AS are a company?s internal network and an ISP?s network.

The Internet is based on the AS concept; therefore, two types of routing protocols are required:

? Interior Gateway Protocols (IGP)- Used for routing within an AS. It is also referred to as intra-AS routing. Companies, organizations, and even service providers use an IGP on their internal networks. IGPs include RIP, EIGRP, OSPF, and IS-IS.

? Exterior Gateway Protocols (EGP)- Used for routing between AS. It is also referred to as inter-AS routing. Service providers and large companies may interconnect using an EGP. The Border Gateway Protocol (BGP) is the only currently-viable EGP and is the official routing protocol used by the Internet.

Note: Because BGP is the only EGP available, the term EGP is rarely used; instead, most engineers simply refer to BGP.

Date: 2016-06-12; view: 193