Home > Articles > Certification > Cisco Certification > CCNA

Network Routing Protocols and Methods, Part 2

In the conclusion of this series, Sean Wilkins, co-author of CCNA Routing and Switching 200-120 Network Simulator, explains how to untangle the detailed network information provided by two popular network routing protocols.
Like this article? We recommend

Overview

This article takes the theory of how a dynamic routing protocol should work, bringing it into reality with two walkthroughs. The first walkthrough focuses on how Routing Information Protocol (RIP) works as a dynamic vector protocol. The second walkthrough shows how the Open Shortest Path First (OSPF) protocol works as a link-state protocol.

For these walkthroughs, we'll use the simple three-router topology shown in Figure 1.

Figure 1 Topology for the walkthroughs.

The following table shows the IP addresses assigned for this topology. This information is common to the walkthroughs for both RIP and OSPF.

Router

Interface

IP Address/Network

R1

Loopback0

10.0.0.1/24

GigabitEthernet0/1

192.168.1.1/24

R2

GigabitEthernet0/1

192.168.2.1/24

GigabitEthernet0/2

192.168.1.2/24

R3

GigabitEthernet0/2

192.168.2.2/24

Loopback0

20.0.0.1/24

RIP Walkthrough

Since RIP is a distance vector protocol, it learns about routes when its neighbors advertise a copy of their routing tables. After receiving the routing tables, RIP combines this information with its own interfaces to form its own routing table. This information for RIP is initially stored inside the RIP database that can be viewed on each respective device. Listing 1 shows an abbreviated version of this database for R1:

Listing 1—R1 RIP Database.

10.0.0.0/24    directly connected, Loopback0
20.0.0.0/24
    [2] via 192.168.1.2, 00:00:25, GigabitEthernet0/1
192.168.1.0/24    directly connected, GigabitEthernet0/1
192.168.2.0/24
    [1] via 192.168.1.2, 00:00:25, GigabitEthernet0/1

Here, R1 is showing that the 10.0.0.0/24 (Loopback0) and 192.168.1.0/24 (GigabitEthernet0/1) networks are directly connected, and two additional networks are located out the GigabitEthernet0/1 interface (toward R2). The routing table of R1 reflects this same information, as shown in Listing 2:

Listing 2—R1 Routing Table.

      10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        10.0.0.0/24 is directly connected, Loopback0
L        10.0.0.1/32 is directly connected, Loopback0
      20.0.0.0/24 is subnetted, 1 subnets
R        20.0.0.0 [120/2] via 192.168.1.2, 00:00:17, GigabitEthernet0/1
      192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.1.0/24 is directly connected, GigabitEthernet0/1
L        192.168.1.1/32 is directly connected, GigabitEthernet0/1
R     192.168.2.0/24 [120/1] via 192.168.1.2, 00:00:17, GigabitEthernet0/1

Keep in mind that neither of these tables tells you about the devices connected to the 192.168.2.0/24 or 20.0.0.0/24 networks. The same details are shown on R2 and R3 when viewing their RIP databases and routing tables in Listings 3–6.

Listing 3—R2 RIP Database.

10.0.0.0/24
    [1] via 192.168.1.1, 00:00:21, GigabitEthernet0/2
20.0.0.0/24
    [1] via 192.168.2.2, 00:00:22, GigabitEthernet0/1
192.168.1.0/24    directly connected, GigabitEthernet0/2
192.168.2.0/24    directly connected, GigabitEthernet0/1

Listing 4—R2 Routing Table.

      10.0.0.0/24 is subnetted, 1 subnets
R        10.0.0.0 [120/1] via 192.168.1.1, 00:00:09, GigabitEthernet0/2
      20.0.0.0/24 is subnetted, 1 subnets
R        20.0.0.0 [120/1] via 192.168.2.2, 00:00:11, GigabitEthernet0/1
      192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.1.0/24 is directly connected, GigabitEthernet0/2
L        192.168.1.2/32 is directly connected, GigabitEthernet0/2
      192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.2.0/24 is directly connected, GigabitEthernet0/1
L        192.168.2.1/32 is directly connected, GigabitEthernet0/1

For R2, the only networks that are not directly connected are the Loopback0 interfaces of R1 and R3.

Listing 5—R3 RIP Database.

10.0.0.0/24
    [2] via 192.168.2.1, 00:00:23, GigabitEthernet0/2
20.0.0.0/24    directly connected, Loopback0
192.168.1.0/24
    [1] via 192.168.2.1, 00:00:23, GigabitEthernet0/2
192.168.2.0/24    directly connected, GigabitEthernet0/2

Listing 6—R3 Routing Table.

      10.0.0.0/24 is subnetted, 1 subnets
R        10.0.0.0 [120/2] via 192.168.2.1, 00:00:10, GigabitEthernet0/2
      20.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        20.0.0.0/24 is directly connected, Loopback0
L        20.0.0.1/32 is directly connected, Loopback0
R     192.168.1.0/24 [120/1] via 192.168.2.1, 00:00:10, GigabitEthernet0/2
      192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.2.0/24 is directly connected, GigabitEthernet0/2
L        192.168.2.2/32 is directly connected, GigabitEthernet0/2

The R3 tables look like a mirror version of R1, with the R1 Loopback0 interface and the link between R1 and R2 being advertised by RIP. R3 is not aware of which device(s) are connected to these other networks—only that they can be located out its GigabitEthernet0/2 interface.

OSPF Walkthrough

Unlike RIP, which is a distance vector protocol, , OSPF is a link-state protocol. OSPF makes its routing decisions based on a database that provides a picture of which networks are located off of every router in the same link-state domain. As with RIP, this information is stored initially in a local OSPF database, which is used to determine the best path for each specific destination, based on Dijkstra's shortest path first (SPF) algorithm.

The first part of the OSPF database we'll examine is the overview. Listing 7 shows a high-level view of the entries in the database; it doesn't show the specific advertised networks.

Listing 7—OSPF Database Overview.

            OSPF Router with ID (1.1.1.1) (Process ID 10)

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         95          0x80000002 0x0076C0 2
2.2.2.2         2.2.2.2         96          0x80000003 0x00025C 2
3.3.3.3         3.3.3.3         102         0x80000002 0x0076A2 2

                Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.1.2     2.2.2.2         96          0x80000001 0x0009B0
192.168.2.2     3.3.3.3         102         0x80000001 0x003478

Listing 7 shows that three different OSPF routers are in the network, advertised as router link states using the router-ID (RID): 1.1.1.1 (R1), 2.2.2.2 (R2), and 3.3.3.3 (R3). The entry also shows two network link entries that advertise two transit networks. Transit networks are links where multiple OSPF devices exist; in this case, the link between R1 and R2 and the link between R2 and R3 are transit networks.

Listing 8 shows the router link-state entry for R1:

Listing 8—R1 Router Link-State Entry.

          OSPF Router with ID (1.1.1.1) (Process ID 10)

              Router Link States (Area 0)

LS age: 165
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 1.1.1.1
Advertising Router: 1.1.1.1
LS Seq Number: 80000002
Checksum: 0x76C0
Length: 48
Number of Links: 2

  Link connected to: a Stub Network
   (Link ID) Network/subnet number: 10.0.0.0
   (Link Data) Network Mask: 255.255.255.0
    Number of MTID metrics: 0
     TOS 0 Metrics: 1

  Link connected to: a Transit Network
   (Link ID) Designated Router address: 192.168.1.2
   (Link Data) Router Interface address: 192.168.1.1
    Number of MTID metrics: 0
     TOS 0 Metrics: 1

This entry contains some useful information. It shows that R1 is connected to a stub network (not a link with another OSPF router), which is connected to the 10.0.0.0/24 network. It's also connected to a transit network. The information for the network connected to this transit network is shown in a separate linked network link-state entry shown in Listing 9 (To find the correct link, look at the advertising router and link ID.)

Listing 9—R1 Linked Network Entry.

LS age: 248
Options: (No TOS-capability, DC)
LS Type: Network Links
Link State ID: 192.168.1.2 (address of Designated Router)
Advertising Router: 2.2.2.2
LS Seq Number: 80000001
Checksum: 0x9B0
Length: 32
Network Mask: /24
      Attached Router: 2.2.2.2
      Attached Router: 1.1.1.1

This entry shows that R1 is connected to a transit network that's connected to the 192.168.1.0/24 network.

Now take a look at the R2 router link entry in Listing 10:

Listing 10—R2 Router Link Entry.

          OSPF Router with ID (1.1.1.1) (Process ID 10)

              Router Link States (Area 0)

LS age: 482
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 2.2.2.2
Advertising Router: 2.2.2.2
LS Seq Number: 80000003
Checksum: 0x25C
Length: 48
Number of Links: 2

  Link connected to: a Transit Network
   (Link ID) Designated Router address: 192.168.1.2
   (Link Data) Router Interface address: 192.168.1.2
    Number of MTID metrics: 0
     TOS 0 Metrics: 1

  Link connected to: a Transit Network
   (Link ID) Designated Router address: 192.168.2.2
   (Link Data) Router Interface address: 192.168.2.1
    Number of MTID metrics: 0
     TOS 0 Metrics: 1

This entry shows that R2 is connected to two different transit networks. As with the R1 entry, these transit networks can be linked to their respective entries, as shown in Listing 11:

Listing 11—R2 Linked Network Entries.

          OSPF Router with ID (2.2.2.2) (Process ID 10)

              Net Link States (Area 0)

Routing Bit Set on this LSA in topology Base with MTID 0
LS age: 551
Options: (No TOS-capability, DC)
LS Type: Network Links
Link State ID: 192.168.1.2 (address of Designated Router)
Advertising Router: 2.2.2.2
LS Seq Number: 80000001
Checksum: 0x9B0
Length: 32
Network Mask: /24
      Attached Router: 2.2.2.2
      Attached Router: 1.1.1.1

Routing Bit Set on this LSA in topology Base with MTID 0
LS age: 558
Options: (No TOS-capability, DC)
LS Type: Network Links
Link State ID: 192.168.2.2 (address of Designated Router)
Advertising Router: 3.3.3.3
LS Seq Number: 80000001
Checksum: 0x3478
Length: 32
Network Mask: /24
      Attached Router: 3.3.3.3
      Attached Router: 2.2.2.2

These entries show that the two R2 connected transit networks are connected to the 192.168.1.0/24 and 192.168.2.0/24 networks, respectively.

Finally, let's take a look at the R3 router link entry in Listing 12:

Listing 12—R3 Router Link Entry.

          OSPF Router with ID (1.1.1.1) (Process ID 10)

              Router Link States (Area 0)

LS age: 720
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 1.1.1.1
Advertising Router: 1.1.1.1
LS Seq Number: 80000002
Checksum: 0x76C0
Length: 48
Number of Links: 2

  Link connected to: a Stub Network
   (Link ID) Network/subnet number: 10.0.0.0
   (Link Data) Network Mask: 255.255.255.0
    Number of MTID metrics: 0
     TOS 0 Metrics: 1

  Link connected to: a Transit Network
   (Link ID) Designated Router address: 192.168.1.2
   (Link Data) Router Interface address: 192.168.1.1
    Number of MTID metrics: 0
     TOS 0 Metrics: 1

Like the R1 router entry, this entry shows that R3 is connected to a stub network and a transit network. The stub network connects to the 20.0.0.0/24 network, and the transit entry can be linked to the entry shown in Listing 13:

Listing 13—R3 Linked Network Entry.

Routing Bit Set on this LSA in topology Base with MTID 0
LS age: 779
Options: (No TOS-capability, DC)
LS Type: Network Links
Link State ID: 192.168.2.2 (address of Designated Router)
Advertising Router: 3.3.3.3
LS Seq Number: 80000001
Checksum: 0x3478
Length: 32
Network Mask: /24
      Attached Router: 3.3.3.3
      Attached Router: 2.2.2.2

This entry shows that R3 is connected to a transit network that is connected to the 192.168.2.0/24 network.

When you combine all of this information, you get a view of how the network is seen by OSPF. The protocol is able to get a complete list of all the networks that are inside the OSPF domain and make forwarding decisions based on this full view. (This view can be restricted as well, with other features not discussed in this article.)

Obviously, walking through the OSPF database is quite a bit more confusing then walking through the RIP database. The way that OSPF organizes information is intended to be efficient for the protocol—not necessarily for the viewing engineer. Although this article picks out the specific linked entries that match with the specific devices in the network, it's important to remember that all of these entries exist in the OSPF database of all routers in the OSPF domain (by default). Differentiating which entry links with which router is a matter of tracing the RIDs and networks, which is well outside the scope of this article.

To bring the OSPF portion of this article together, let's look at the routing tables of the routers in Listings 14–16, to see how this information contained within their databases is translated once OSPF has a chance to calculate the best routes to insert.

Listing 14—R1 Routing Table.

      10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        10.0.0.0/24 is directly connected, Loopback0
L        10.0.0.1/32 is directly connected, Loopback0
      20.0.0.0/24 is subnetted, 1 subnets
O        20.0.0.0 [110/3] via 192.168.1.2, 01:04:32, GigabitEthernet0/1
      192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.1.0/24 is directly connected, GigabitEthernet0/1
L        192.168.1.1/32 is directly connected, GigabitEthernet0/1
O     192.168.2.0/24 [110/2] via 192.168.1.2, 01:08:50, GigabitEthernet0/1

Listing 15—R2 Routing Table.

      10.0.0.0/24 is subnetted, 1 subnets
O        10.0.0.0 [110/2] via 192.168.1.1, 01:04:07, GigabitEthernet0/2
      20.0.0.0/24 is subnetted, 1 subnets
O        20.0.0.0 [110/2] via 192.168.2.2, 01:04:07, GigabitEthernet0/1
      192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.1.0/24 is directly connected, GigabitEthernet0/2
L        192.168.1.2/32 is directly connected, GigabitEthernet0/2
      192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.2.0/24 is directly connected, GigabitEthernet0/1
L        192.168.2.1/32 is directly connected, GigabitEthernet0/1

Listing 16—R3 Routing Table.

      10.0.0.0/24 is subnetted, 1 subnets
O        10.0.0.0 [110/3] via 192.168.2.1, 01:04:39, GigabitEthernet0/2
      20.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        20.0.0.0/24 is directly connected, Loopback0
L        20.0.0.1/32 is directly connected, Loopback0
O     192.168.1.0/24 [110/2] via 192.168.2.1, 01:09:23, GigabitEthernet0/2
      192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.2.0/24 is directly connected, GigabitEthernet0/2
L        192.168.2.2/32 is directly connected, GigabitEthernet0/2

Summary

While this article (especially the OSPF portion) could be a bit confusing, you must remember that this is how the databases look in a very simple network. Imagine what they look like in a larger network! Keep in mind that many network engineers don't completely understand the contents of these tables; they're able to get through their work by using this information combined with network documentation and other observed configuration on the devices.

In reading through this article (possibly multiple times), you should get an idea of how distance vector and link-state protocols differ in their view of the network and how they make their decisions.

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