Home > Articles > Certification > Cisco Certification > CCNA

Network Routing Protocols and Methods, Part 1

In this two-part series, Sean Wilkins, co-author of CCNA Routing and Switching 200-120 Network Simulator, discusses current network routing protocols and methods for implementing them. This article covers the types of static versus dynamic routing protocols and the differences between them.
Like this article? We recommend

Overview

Once an engineer has a basic understanding of how packet forwarding/routing works, the next step is to move on to the method of configuring routing. (If you're still not sure how packet forwarding/routing works, take a look at my article "Basic Layer 3 Routing Concepts.")

This article discusses the main categories of routing configuration and goes over the different types of dynamic routing protocols that are available and commonly used.

Choosing How to Configure Network Routing

There are two primary ways of configuring routing: statically and dynamically. Static routing is typically used on very small networks, where the number of routes to be configured is just as small. However, static routing is sometimes used in larger networks as a small piece of an overall dynamically controlled routing infrastructure. In such cases, static routes typically configure default routes, or direct traffic out of the network to another network controlled by another party.

Dynamic routing is built to deal with network changes automatically as the topology changes within the network, and are typically used on all other network types not previously mentioned.

Let's visualize this with the topology shown in Figure 1. This example shows a number of different paths that a message can take when leaving R1 and destined for R6. For example, it could go R1 > R2 > R4 > R6, or it could go R1 > R3 > R4 > R6. When configuring static routing, typically the engineer has a single path in mind to reach the destination. So, to configure a route from R1 to R6 using the path through R2 and R4, a static route would need to exist on R1, R2, and R4 for the forward trip (R1 > R6), and another route would need to exist on R6, R4, and R2 for the reverse trip (from R6 back to R1). Keep in mind that this route is for a single reachable destination.

Figure 1 Base topology.

To provide reachability to every destination that potentially exists in the network, the engineer would need to calculate the best paths for each destination initially, and then individually configure each route along each best path. Obviously, this process can quickly become very time-consuming. And what happens if a failure exists along the calculated best path?

These reasons indicate why dynamic routing protocols are used on larger networks, and static routing is left for very small networks or other specific use cases. Dynamic routing protocols avoid the manual configuration problem; they also are built to handle many complex topologies, as well as dealing with network outages—without the need for additional administrative attention.

Types of Dynamic Routing

After selecting the dynamic routing method, the engineer must make some additional decisions involving the types of dynamic routing protocols. At the highest level are two main categories of dynamic routing protocol: exterior gateway protocols (EGPs) and interior gateway protocols (IGPs). EGPs connect multiple network domains; they're called exterior because the protocol is exterior to the network domains. (A domain in this case is a specific organization or service provider network.) IGPs handle the routing inside these network domains; these protocols are the most common, as each company with a network handles its own network domain.

Modern networks use only a single EGP: the Border Gateway Protocol (BGP). As you might expect, a number of different IGPs are available, depending on the requirements of each network. Four IGPs are the most popular:

  • Open Shortest Path First (OSPF)
  • Enhanced Interior Gateway Routing Protocol (EIGRP)
  • Intermediate System to Intermediate System (IS-IS)
  • Routing Information Protocol (RIP)

These EGP and IGP protocols are split into three different types of routing protocol: distance vector, link-state, and path vector. There are a number of important differences between these types. The next three sections discuss each type of routing protocol in detail.

Distance Vector Protocols

A few main attributes are associated with traditional distance vector routing protocols:

  • The router's view of the network is only discovered from the known routes of its neighbors ("routing by rumor").
  • The router sends a complete version of its routing table to its neighbors at regular intervals.
  • They take a long time to converge after a failure because the update is only sent at regular intervals.
  • Compared with link-state protocols, distance vector protocols have a higher risk of causing routing loops.

The only modern routing protocol that is traditionally distance vector is RIP. It advertises a complete copy of its routing table every 30 seconds via all enabled interfaces.

Once a router receives an update from a neighbor, it places the update into the routing table (assuming that there are no conflicts). If something happens and the route becomes unreachable, the route remains in the routing table for at least 180 seconds (default invalid timer) before being marked as unreachable. This technique slows down the convergence of a failed route and delays traffic from being routed along an alternate path.

RIP comes in three versions: RIP version 1, RIP version 2, and RIPng. Their operations are very similar; however, there are a few differences: RIP version 1 broadcasts updates, whereas RIP version 2 multicasts updates. This design prevents non-RIP devices from processing extra packets every 30 seconds. RIP version 2 also supports subnetted (classless) networks, while RIP version 1 only supports classful networks. RIPng works similarly to RIP version 2, but for IPv6 addresses.

EIGRP is technically a distance vector protocol in that its view is based on its neighbors' views alone, but it has some features that make it act like a link-state protocol. This is why EIGRP is usually discussed as being a hybrid protocol, using attributes from both traditional distance vector as well as link-state protocols. Crossover features include the use of neighborships and triggered partial updates, to name two.

Loop management for distance vector routing protocols uses two different methods: poison reverse and split horizon. Poison reverse is used every time a new route is learned from a neighbor; when this happens, the router automatically sends a route advertisement back out the learned interface with an infinite metric. This action prevents the neighbor from using that route to the newly learned destination.

Split horizon is also used when a router advertises routes out its interfaces. With split horizon, if a route was learned on interface 1, it will not advertise that same route back out interface 1.

Link-State Protocols

Link-state protocols provide each of the routers in a network with a network-wide view, giving each router the ability to calculate the best path through the network. All modern link-state routing protocols use Dijkstra's shortest path algorithm.

Link-state protocols work by establishing neighborships with connected routers and exchanging a copy of the known network (not just the routing table contents) to these neighbors. All updates that are received are immediately relayed to all other connected neighbors.

Both OSPF and IS-IS are link-state protocols, and both use Dijkstra's algorithm for best-path mapping. However, their configurations are quite different, and the selection of one versus the other is based mainly on the requirements of a specific network. OSPF is more commonly deployed in enterprises, and IS-IS is more popular in service provider networks.

Path Vector Protocols

The only path vector protocol in use today is BGP. When BGP is configured, it will have a copy of all reachable autonomous systems (BGP's term for network domains, discussed earlier) and the path to reach them. This information gives the protocol a clear view of which networks it can reach and the advertised path to each.

A BGP router also uses a number of different path attributes to determine which path it will end up using to reach a specific destination. Since BGP is an EGP and is used to route between routing domains, often multiple paths are available to reach a destination. By using a combination of these path attributes, a router selects the route it uses.

Summary

Whether to use a static or dynamic method of routing is frequently determined by the size of the network; if the network is very small (a few nodes), a static routing solution could work. However, even if the network is small, a dynamic routing method can be used without a lot of configuration, and it will continue to work as the network grows.

The type of dynamic routing protocol to use largely depends on the vendor of the networking equipment in use. OSPF and EIGRP are two of the most popular routing protocols used on enterprise networks. Of these two, EIGRP is typically considered easier to configure, but until recently it has been Cisco proprietary, and therefore typically not supported on other vendors' equipment.

If you're deploying only Cisco equipment, the selection is a bit more dependent on the training of your staff and which protocol they best understand and prefer.

Part 2 of this series will walk through how a network converges for a distance vector network versus a link-state network, along with covering a few common additional dynamic routing protocol features.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020