CCNA 640-802 Lab - Configure, verify, and troubleshoot OSPF

Exam: 640-802

Exam Objective: Configure, verify, and troubleshoot OSPF

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Contents

• Introduction
• Technology Background
• Lab Scenario
• Lab Objectives
• Lab Solution

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Introduction

In this lab you will have to perform configuration tasks in relation to OSPF areas, as well as some redistribution of routes between RIPv2 and OSPF. To complete this lab you will need access to either lab consisting of four Cisco routers or a router simulation program. There are a number of free router simulators available for download from the Internet. As with any other program you download from the Internet make sure you scan it for viruses

Technology Background

When OSPF special areas are implemented the result the ability to support more scalability in networks and increased network stability. The memory of the routers within these areas is not used as much because the LSA messages that are sent are decreased. How much LSA traffic is decreased depends on the area that is implemented. The OSPF areas are stand area, backbone area, stub area, totally area, and NSSA.

OSPF standard areas are the default OPSF area type which accept the following LSA message types: route summary, link updates, and route summaries.

OSPF backbone areas are the area type that all other areas connect to. It also accepts the following LSA message types: route summary, link updates, and route summaries.

Stub areas do not accept any external routes into the area (LSA type 5). These areas cannot contain Autonomous System Boundary Routers (ASBRs) unless the ASBR is also an Area Border Router (ABR). To send packets outside the area a default route is used.

Totally stubby areas do not accept external routes or summary routes from external areas. These areas cannot contain Autonomous System Boundary Routers (ASBRs) unless the ASBR is also an Area Border Router (ABR). To send packets outside the area a default route is used.

NSSA has the same benefits of stub and totally stubby areas, plus also accepts type 7 LSAs and ASBRs
. In order for an area to be a stub or a totally stub area, there are a number of criteria that must be met:

All routers within the area must be configured as stub router prior to forming a neighbor relationship.

There must be only one exit (ABR) from the area. If it is acceptable for packets to not take the optimal route to a destination, then this rule can be avoided if the ABRs both interject default routes into the area.

The area cannot be a backbone area.

The area cannot have virtual links traveling through it.

Routers configured as just ASBR are not permitted within the area.

The remainder of this tutorial and lab will focus on stub areas, totally stub areas, and NSSA.

Stub Areas:

After OSPF is configured, if an OSPF area is to be made a stub area this must be complete. For an area to be considered a stub area all routers need to be defined as stub routers. Stub areas are typically used in a hub and spoke topology. A common example would be a head office and remote office. The head office network would be the hub and the routers in the remote office would be considered the spoke routers. An example of this can be found in the diagram below:

In the diagram above RouterA is in the branch office stub area and RouterB's S0/0/0 interface is also in this area. RouterB's other interface is within the company's head office backbone (transit) area as is one of the interfaces of RouterC.

Once OSPF is properly enabled on RouterA and RouterB then these routers must be configured as stub routers. After this is done, the cost of the default router can be changed. The following commands are required to configure the router as a stub and to change the default cost:

areaarea-id stub [no summary]

The area-id parameter is used to identify the area and can either be a decimal number or a dotted decimal number.

The optional [no summary] parameter is what is used to ensure the ABR does not send summary LSAs into the area. This optional parameter will be discussed more in the next section of this tutorial.

area area-id default-cost cost

The area-id parameter is used to identify the area and can either be a decimal number or a dotted decimal number.

The cost parameter is used to change the default cost (1) of the summary route. The cost can be in the range of 0 to 16777215.

In the above figure RouterB is the ABR. To properly configure RouterA and RouterB the following commands will be required: (Based on the assumption that the interfaces have been properly configured.)

RouterA

router ospf 23
network 172.17.0.0 0.0.255.255 area 5
area 5 stub

RouterB

router ospf 23
network 172.18.0.0 0.0.255.255 area 0
network 172.17.0.0 0.0.255.255 area 5
area 5 stub

Totally Stubby Areas:

Totally stubby areas are a Cisco proprietary implementation. These areas block external router (LSA type 5), summary router (LSA type 3), and interarea routes (LSA type 4). The end result is even more memory saving.

To configure a totally stubby area, after OSPF has been configured, all routers within the area must be configured as stub routers with the area stub command. Then on the ABR the area stub command the no summary parameter must be issued. In the previous example, RouterB's configuration would be as follows:

router ospf 55
network 172.18.0.0 0.0.255.255 area 0
network 172.17.0.0 0.0.255.255 area 5
area 5 stub no-summary

NSSA:

NSSA (not so stubby area) was first introduced in RFC 3101 (supported by Cisco IOS 11.2) to allow the some external routes into the stubby area. This is achieved with a special LSA type (7). The NSSA ASBR creates this LSA and the ABR takes this LSA and make it into a type 5 LSA (default route) and passes this into the rest of the area. The steps to configuring NSSA is the same as stub area except instead of issuing the stub area command on all routers the following command needs to be issued on all routers:

area area-id nsaa [no-resdistribution] [default-information-originate [metric metric-value] [metric-type type-value]] [no-summary]

The area-id parameter is used to identify the NSSA and can be either a decimal number or a dotted decimal number.

The optional [no-resdistribution] parameter is an NSSA ABR and the redistribution routes are only to go into the standard area and not the NSSA area.

The optional default-information-originate parameter is what is used to generate type 7 LSAs.

The optional metric parameter sets the metric for default area. This value can be in the range of 0 to 16777214.

The optional metric-type parameter sets the metric type of default routes. Type 1 external routes and Type 2 external routes.

The no-summary parameter sets the area as an NSSA but without summary routes can be interjected into it.

Stub Area Verification:

To ascertain LSA details the show ip ospf database command is used.
To ascertain all routes the show ip route command is used.
To ascertain the OSPF area types the show ip ospf command is used.
To ascertain details of all type 7 LSA the show ip ospf database nssa-external command.

Lab Scenario

For this OSPF lab consider the following network:

Connect you lab as shown using the labeled IP addresses.

Lab Objectives

You are tasked to configure OSPF on Routers Beta and Charlie. The criterion that is to be met is the following:

• Configure RIPv2 on Alspha using your own IP addressing sceme. Area 49 is to only accept inter-area routes and a default route from RIP. RIP is to use a metric of 10. There are to be no external routes from the backbone. The OSPF process ID for router Beta is 13.

• Area 49 is to be configured as a Not so Stubby Area (NSSA). Charlie is to have a process-id of 25.

Lab Solution

Beta:

router ospf 13
redistribution rip metric 10
network 172.20.19.0 0.0.0.255 area 49
area 49 nssa


Charlie:

router ospf 25
network 172.21.0.0 0.0.255.255 area 0
network 172.20.19.0 0.0.0.255 area 49
area 49 nssa no-summary