OSPF requires two routers to become neighbors before they can exchange OSPF LSAs and then calculate new routes. Chapter 21 of the CCNA 200-301 Volume 1 focuses on neighbor relationships. The chapter examines why some links require a Designated Router (DR) and Backup Designated Router (BDR), and while others do not. The chapter then goes on to work through the range of settings that might prevent routers from becoming neighbors.
As with all the posts in this series, this post helps you re-create some of the examples from that chapter using Cisco Packet Tracer.
The big idea is pretty simple: Repeat the Examples in the Official Cert Guide as part of your lab practice for CCNA using Packet Tracer.
The detailed ideas require some reading. To get your head around what kind of content is here in the blog for these labs, read:
Chapter Intro: A brief description of the topics in that chapter of the book.
Download Link: Links to a ZIP; the ZIP holds all the .PKT files for this chapter.
Table of PKT files, by Example: A table that lists each example in the chapter, with the files supplied for each. Also lists a note about whether the PKT topology matches the book example exactly or not.
Tips: When we build the files, we come across items that we think might confuse you when trying the examples with PT. We write those notes in this section!
OSPF includes a large number of interlocking pieces with a large number of configuration settings. One great way to separate the parts of OSPF to help think through what OSPF is doing, and to troubleshoot problems, is to ask a simple question: Do routers that should become OSPF neighbors become neighbors? If not, you have set of issues to consider, but if the routers become neighbors, you have another set of concerns.
Chapter 21 of the CCNA 200-301 Official Cert Guide, Vol 1 focuses on features of OSPF neighbors. The first half of the Chapter examines what happens with working OSPF neighbors depending on the OSPF network type. One type – broadcast – requires the routers to elect a Designated Router (DR) and Backup Designated Router (BDR), but other types like point-to-point do not. The first half of the chapter examines those settings, the DR/BDR election, and how to change the OSPF network type.
The second half of the chapter examines the issues that can prevent two routers from becoming OSPF neighbors. Many misconfigured settings can prevent two routers from becoming neighbors, and you should know those settings and what happens when you misconfigure those settings.
When building the content for this post, we review the examples in the book and decide whether it makes sense to supply a Packet Tracer (.pkt) file to match the example. If we choose to support an example by supplying a matching .pkt file, the .pkt file includes a topology that matches the example as much as possible. It also includes the device configurations as they should exist at the beginning of the example.
In some cases, the .pkt file shows two instances of the lab topology – one above and one below. We include two such topologies when the book example includes configuration commands, for these purposes:
|
Example |
.PKT Includes Initial State of Example? | .PKT Also Includes Ending State of Example? | Exact Match of Interface IDs? |
| 21-1 | Yes | Yes | Yes |
| 21-2 | Yes | No | Yes |
| 21-3 | Use 21-2 | No | Yes |
| 21-4 | Use 21-2 | No | Yes |
| 21-5 | Yes | Yes | Yes |
| 21-6 | Yes | No | Yes |
| 21-7 | Yes | Yes | Yes |
| 21-8 | Yes | No | N/A |
| 21-9 | Yes | Yes | Yes |
| 21-10 | Yes | Yes | Yes |
| 21-11 | Yes | No | Yes |
| 21-12 | Not Supplied | Not Supplied | N/A |
| 21-13 | Not Supplied | Not Supplied | N/A |
The example shows the configuration for router R1. Note that the PT file, for the “initial” state devices, pre-configures similar OSPF settings for routers R2, R3, and R4.
Note: You should use the PT file for Example 21-2 for Examples 21-3 and 21-4 as well.
Note that PT does not support the show ip ospf interface brief command. Use the show ip ospf interface command (without the brief keyword) instead.
Note that PT does not support the show ip ospf interface brief command. Use the show ip ospf interface command (without the brief keyword) instead.
Note that PT does not support the show ip ospf interface brief command. Use the show ip ospf interface command (without the brief keyword) instead.
PT does not except the shutdown command in ospf config mode, so we did not supply PT files for these examples.
“this files .pkt are not compatible whith this version of packet tracer” I am using PT 7.2.2
Yep, you figured it out. We built the files with PT 7.3, and PT does not allow for backwards compatibility of later PT files with earlier PT versions. You’ll need to upgrade PT to open the files.
Hi Wendell, the command show ip ospf interface brief is unrecognized command in Packet Tracer 7.3. Is this a limitation of PT? Thanks.
Hi Jerry,
You’r exactly right. You can find that specific facts, and others like them, in the Tips accordion item for an example. EG, if PT has an unsupported command that’s shown in the book example, we try and list it in the accordion item under Tips. FYI
Thank you Wendell for all the help you have rendered. Can you please share the password to the routers in the packet tracer file for OSPF.
cisco
(See link to series intro, under “Confused? New to…” heading at top of the posts for these kinds of answers.)
Wendell
PS you’re very welcome! Glad you’re enjoying the content.