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Open Shortest Path First – OSPF Fundamentals – Scenario

Published

on June 29, 2009

in BSCI, BSCI Questions, Certification, Cisco Systems, Concepts and Constructs, DR and OSPF

. 0 Comments

GIVEN: For a Given Network, all routers share a common multi-access segment. One router is elected the Designated Router (DR) and another is elected the Backup Designated Router (BDR).

1. Which Parameter determines the router that will be selected as the DR in an OSPF network?

The router with the Highest OSPF Priority on a particular network segment will become the designated router (more info). The DR default OSPF priority for a Cisco Router is 1 but can be manually changed. If there are multiple routers with default OSPF Priority then the Routers elect a Designated Router using the Router ID (more info) (RID) which is basically the highest IP Address with loopbacks being preferred (if configured)

2. Could a router with a priority value of zero assume the role of a DR or a BDR in the OSPF?

An OSPF Priority of 0 (zero) means that the Router is not eligible to be elected for either DR or BDR. The zero interface state is DROTHER.

3. How is the OSPF router ID determined on a Cisco Router?

The OSPF Router ID (RID) on a Cisco Router is determined by the highest IP address configured on an interface (if not manually configured), or IP Address on a loopback interface.

4. What is the role of the DR and BDR in the OSPF network?

The role of the DR and BDR in an OSPF Network is to handle and monitor adjacencies with other routers in the network. This is because in an OSPF network adjacencies grow at a quadratic rate. This means that instead of exchanging routing information with all other routers the routers exchange information with the DR and BDR. Then in turn the DR and BDR relay the information to other routers.

Resources:

Stewart, B,D., Gough, C (2008). CCNP BSCI Official Exam Certification Guide, Fourth Edition. Indianapolis: Cisco Press.

Notes and Notices: This is a part of my personal BSCI notes and research to assist myself in learning and understanding the concepts and theory for the BSCI exam. I learn by making notes reading and writing things down and wish to file them where I cannot lose them. These notes are not to be seen, judged or mistaken for replacements to Cisco recognized and authorized training which I personally support and attend and suggest you undertake if you are going for the BSCI Certification.

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 13

Published

Deon Botha

on June 24, 2009

in BSCI, BSCI Questions, Certification, Cisco Systems, Concepts and Constructs, OSPF and Show

. 0 Comments

Working from the my last couple of OSPF posts I am going to try and crystallize some of the material found by working through questions found in Stewart, B,D., Gough, C (2008). CCNP BSCI Official Exam Certification Guide, Fourth Edition. Indianapolis: Cisco Press book.

13. What is the advantage of the command show ip ospf interface in troubleshooting?

The advantage to use the show ip ospf interface in troubleshooting is that it allows you to verify that the interface is operating in OSPF (operational and configured as required) and see information like the, Area, DR, and BDR, list neighbours and see the network type. This command also allows drill down per interface for more informaiton.

Resources:

Stewart, B,D., Gough, C (2008). CCNP BSCI Official Exam Certification Guide, Fourth Edition. Indianapolis: Cisco Press.

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 6

Published

Deon Botha

on June 18, 2009

in BSCI, BSCI Questions, Certification, Cisco Systems, OSPF and Show

. 0 Comments

6. What command shows which router on a Local Area Network (LAN) is the Backup Designated Router (BDR)?

Displays neighbour information. Used to verify that all neighbours are present.

Router_2#show ip ospf neighbor

Gives information about how OSPF has been configured on each interface.

Router_2#show ip ospf interface

Resources:

Stewart, B,D., Gough, C (2008). CCNP BSCI Official Exam Certification Guide, Fourth Edition. Indianapolis: Cisco Press.

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 5

Published

Deon Botha

on June 17, 2009

in BSCI, BSCI Questions, Certification, Cisco Systems, Concepts and Constructs, Cost and OSPF

. 0 Comments

5. The Metric used by OSPF is cost. How would you change the metric on an interface?

OSPF cost is a 16-bit variable (0 – 65,535) where lower variables are preferred. There are two methods one preferred (at least by me) and one easier.

I prefer the “automatic” method because it is not hard coded into IOS (SET) that means when dynamic changes occur on the network a change is needed inside IOS to change the costs again.

This method is the one where you change the automatic cost calculation equation to take into account faster links by fiddling with the numerator (add a couple of zeros).

Router_2(config-if)#up ospf auto-cost reference-bandwidth [reference-bandwidth]

Second method is more straightforward and directly onto interfaces (hard code a value)

Router_2(config-if)#ip ospf cost [cost]

Resources:

Stewart, B,D., Gough, C (2008). CCNP BSCI Official Exam Certification Guide, Fourth Edition. Indianapolis: Cisco Press.

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 4

Published

Deon Botha

on June 17, 2009

in BSCI, BSCI Questions, Certification, Cisco Systems and OSPF

. 2 Comments

4. The address 192.100.56.10/21 has been allocated to an interface on the router. This interface alone is to be included in the OSPF process. State the command that would start the process on this interface.

To include this interface alone into the OSPF process

Router_2(config-network)#network 192.168.56.10 0.0.0.0 area number

According to the book (I don’t understand)

Router_2(config-network)#network 192.168.56.8 0.0.0.7 area number

I would have done it like below if I was going to include the entire range, because you are using the /21 which has 2048 nodes/hosts per network ( I hate it when I start doubting myself?!?!)

Router_2(config-network)#network 192.168.56.0 0.0.7.255 area number

Which coincidentally is also in the book (I’m totally confused but moving swiftly along).

The last method is allowing the /24 mask??? why I don’t know (in the book).

Router_2(config-network)#network 192.168.56.0 0.0.0.255 area number

I suppose all of the answers including the first one allows the address which was the goal. Not that allowing the /24 mask or the weird small portion of hosts make sense to me. (why? and where the logic comes from is making me scratch my head).

Resources:

Stewart, B,D., Gough, C (2008). CCNP BSCI Official Exam Certification Guide, Fourth Edition. Indianapolis: Cisco Press.

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 3

Published

Deon Botha

on June 17, 2009

in BSCI, BSCI Questions, Certification, Cisco Systems, Concepts and Constructs and OSPF

. 0 Comments

3. Is it possible to have more than one OSPF process on a router. How would you do this?

Yes it is possible to have more than one OSPF process on a router.

You may want to do this to create two different OSPF domains / areas with separate routing policies.

To accomplish this simply specify two (or more) OSPF processes in the following way:

Router_2(config)#router ospf 1 Router_2(config-router)#network [ip address] [mask] area [number] Router_2(config-router)#exit Router_2(config)#router ospf 2 Router_2(config-router)#network [ip address] [mask] area [number]

Resources:

Stewart, B,D., Gough, C (2008). CCNP BSCI Official Exam Certification Guide, Fourth Edition. Indianapolis: Cisco Press.

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 1

Published

Deon Botha

on June 15, 2009

in BSCI, BSCI Questions, Certification, Cisco Systems, Concepts and Constructs, OSPF and Priority

. 0 Comments

1. What command is used to manually determine which router on a Local Area Network (LAN) will become the Designated Router (DR)?

The hello message includes a priority field which provides a mechanism to elect a Designated router (DR) and Backup Designated Router (BDR). To be eligible for election the value must be a positive integer between 1 and 255. A priority of 0 (zero) means the router cannot participate in the election process.

The highest priority wins the election process. All Cisco routers have a default priority of 1 (one), the highest Router ID is used as the tiebreaker when no manual adjustment is made.

The command to adjust priority on an interface-by-interface method is:

Router_2(config-if)#ip ospf priority number

In summation the designated router can be determined using the priority command.

Resources:

Stewart, B,D., Gough, C (2008). CCNP BSCI Official Exam Certification Guide, Fourth Edition. Indianapolis: Cisco Press.

Open Shortest Path First – OSPF Fundamentals – Checking or Troubleshooting OSPF Troubleshooting

Published

Deon Botha

on June 12, 2009

in BSCI, BSCI Notes, Concepts and Constructs, Debug and Show

. 0 Comments

I’m using a Simulator and sometimes output differs from what it should be (which is happening more and more often). Make the best out of the explanations when things differ (I’ve used verbatim examples where the simulator just came up blank with output).

All the below is to test if OSPF is functioning properly and has been configured correctly.

Base the output (for examples that worked) on the show commands on the below OSPF Configuration.

Working from a functional configuration on a single router, Router_1 will be the DR on Fe1/0 unless another device on that segment has a priority greater than 100. The link attached to Fe1/0 has a cost of 1. The cost on Fe2/0 has been changed to 10.

The Config

Router_1(config)#router ospf 100 Router_1(config-router)#network 192.168.0.0 0.0.255.255 area 3 Router_1(config-router)#exit Router_1(config)#interface Ethernet 0/0 Router_1(config-if)#ip address 192.168.16.1 255.255.255.240 Router_1(config-if)#ip ospf priority 100 Router_1(config-if)#exit Router_1(config)#interface Ethernet 0/1 Router_1(config-if)#ip address 192.168.16.15 255.255.255.240 Router_1(config-if)#ip ospf priority 20 Router_1(config-if)#exit Router_1(config)#interface Ethernet 0/2 Router_1(config-if)#ip address 192.168.16.30 255.255.255.240 Router_1(config-if)#ip ospf priority 15 Router_1(config-if)#exit Router_1(config)#interface Ethernet 0/3 Router_1(config-if)#ip address 192.168.16.17 255.255.255.240 Router_1(config-if)#ip ospf cost 10

Checking the Configuration

As we’ve been through before in previous posts, the show commands are detailed and comprehensive views on the health and status of the network and hardware (there are a gazillion of them and the detail can be overwhelming same with debug commands). To understand the output from the show commands read-on.

The show-ip ospf Command

The show-ip ospf command shows how OSPF is running on a given router. Output includes the number of times that the SPF routing algorithm has run (indicates the stability of the network). From the previous posts the SPF routing Algorithm runs when there is “instability” on the network (the higher the number of “recalculations” the less stable the network).

Router_2#show ip ospf [process-id]

Output

Explanation

The show ip ospf Database Command

The show ip ospf database command when issued will display the contents of the routers topological database and the different Link State Advertisements (LSAs) that have populated the database (Internal Routers will only display router and network LSAs).

Router_2#show ip ospf database

Output

Explanation

Show ip ospf interface Command

The show ip ospf interface command shows how OSPF has been configured and how it is working on an interface. This level of detail is excellent to troubleshoot config errors.

Router_2#show ip ospf interface [type number]

The command shows information such as the Designated Router (DR) and Backup Designated Router (BDR), a list of neighbours, and the network type.

Output

Explanation

There are some things that don’t come up once again on my output. This would be because I am using a simulator when studying and not “real” kit. You can’t really expect the lab to do “Everything” you want but it does a good job of giving you the basic ideas. Use your imagination.

From here on in things are verbatim from the book. The deviations on the Simulator is so GREAT from what it should be (a.k.a a blank output) that I really can’t study from that).

Show ip ospf neighbour Command

The show ip ospf neighbour command shows OSPF neighbours (known neighbours can be viewed using this command).

Router_2#show ip ospf neighbor

Output

The command can be made more granular and the neighbours can be viewed in a per-interface method

The command can once again be expanded further to show a deep-dive per interface view in as much detail as possible. Use the command displayed below.

Router_2#show ip ospf neighbor {type number} {neighbour id} [detail]

Output

Explanation

Show ip protocols Command

The show ip protocols command shows the configuration of IP routing protocols configured on the router. The command brings up how protocols were configured and how they interact with one another (updates, interactions, etc). Great for troubleshooting configuration errors and understanding how the network is communicating about routes

Router_2#show ip protocols

Output

Explanation

Show ip route Command

The show ip route command shows the IP routing table on the router. This particular command shows how the network is known to the router and how the router discovered routes. Most of us know about this one and would have used it many times before. I know I have.

Router_2#show ip route

Debug Commands

A rather dangerous command is debug (this is because it can make a router totally freak out). This is due to the fact that the debug command has the highest process priority and can consume all resources on the router causing the router to freeze up and need a power cycle.

Good practice would be to turn on debug commands for a specific function and then turn off that debug command as soon as the needed information has been gathered.

To turn of all debug commands that could be active on the router:

Router_2#no debug all

The particular commands with relevance to OSPF:

Router_2#debug ip ospf events

This command displays information about OSPF-related events, such as adjacency, flooding information, designated router selection, and SPF calculation.

Router_2#debug ip packet

This command is IP debugging and includes packets received, generated, and forwarded. Fast-Switched packets do not generate messages.

If I added some value to your Cisco Experience with this post please add some value to my studies and leave a comment, question, suggestion, note of thanks or encouragement for me to hurry up and complete my certifications. My reasoning for wanting some interaction is that the last Recruiter said I need CCNP, Juniper and a Specialization track. The LOOOOONG Road to Cisco Indeed. Thanks Deon

Open Shortest Path First – OSPF Fundamentals – Configuring Options On an Internal Router

Published

Deon Botha

on June 5, 2009

in BSCI, BSCI Notes, Certification, Cisco Systems, Concepts and Constructs, Cost and Priority

. 4 Comments

To allow you to tune OSPF on an Internal Router you have the following options available at your disposal:

Router ID
Loopback interface
cost command
priority command

Defining the Router ID and Loopback interface

The Router ID (when thinking about OSPF Domains) is used to identify any given Router in the Link State Advertisements (LSAs) in an OSPF Database. A given Router on a network requires an ID (Router ID) to participate in an OSPF Domain. The Router ID can be assigned/set in one of two fashions:

By the administrator (manually assigned)
Left to the discretion of the router (automatically assigned by the automatic election process)

In most configurations the Router ID is set by the administrator as this makes it easier to track events, internal documentation, and system-administration remotely or even on the router itself.

Setting the Router ID

The Router ID as discussed previously must be present for a Router to participate in an OSPF Domain. This Router ID can be set in one of two methods:

Setting the Router ID – Method One (Less Preferred)

The OSPF Router ID can be defined using the router-id (background link) command found in IOS.

Using the Router-ID command isn’t always the best idea when combining BGP and OSPF as both technologies employ the same method in electing a Router ID. So manually setting a Router ID (using the Router-id command) for OSPF could cause BGPs Router ID to differ from BGP which causes administration problems as well as could cause other problems at a later stage when combining the two technologies.

Should you still want to change the Router ID using this method anyway the command is:

Router(config)#router ospf Router(config-if)#router-id ip-address

Dissecting the above when the Router ID has been chosen/assigned using the router-id command, the Router ID is kind of stable and may possibly change (barring a power cycle or a OSPF process reset). This is an important factor because changing the Router ID post (after) configuration could possibly break some OSPF configurations, such as virtual links (which as the name implies don’t exist, and could take some thinking to logically get working again).

Setting the Router ID – Method Two (Preferred)

Should the router-id command not be present/available on the router IOS you are using, and you want a more reliable method (sticky) that spans, the Router ID is automatically elected via:

The highest IP Address of a manually created loopback interface.
If there is no configured Loopback interface then the Router ID will be the highest IP Address of the first active (on boot-up) physical interface.

I inherently look for the easier easier solution to a problem that will work long term, more work once off doesn’t matter (first off config) the solution must stick and continue working through power outages, reboots and the CEO trying his hand at “setting up” his own hardware, voice being a good example i.e. putting each end point into it’s own vlan thus ensuring QoS while the organization grows (there are limitations in the number of vlans, but I have yet to reach them).

Looking at the top the better method to control the Router ID (using automatic means) is through the use of the Loopback interface IP address. A loopback interface is a virtual interface (duh) that will always be active (cannot flap) and will be the first active interface (*shrug* logic dictates as it becomes the Router-ID, comments as I haven’t tested this out?).

The command to use a loopback is as follows (CCNA topic):

Router(config)#interface loopback inerface-number Router(config-if)#ip address ip-address subnet-mask

Consider assigning loopbacks a /32 mask 255.255.255.255 (1 end point) to minimize the ip space usage of the virtual interface(s) on the network (use it dont use it). I have come across some places that say that loopbacks wont work with the /32 mask (cant find source again), I will verify once I setup my kit again. In that case use the smallest (/31 or /30).

What I want to take away from this is should the Router ID be chosen/assigned using the loopback interface, the Router ID is stable and wont change. What’s important here is that a Loopback interfaces is NOT a physical interface thus cannot go up and down (flap) and therefore is not as unstable element in the network (i.e. loopback is stable) and thus is more desirable. Even in the event of a power cycle the loopback will once again be the Router ID.

Think about including the Loopback interface in the general network commands even if you aren’t configuring OSPF. This gives you an easy point to ping to should there be a need to troubleshoot (can I reach point A from point B) this should tell you alot about Layer 1 if the interface is un-shut and it has an ip-address.

Changing the COST

The COST metric on a Cisco is calculated as 100,000,000 bps divided by the bandwidth of the interface in bits per seconds. Sometimes when using a fast interface type (FE and GE) or when dealing with inter-vendor situations (Cisco / 3COM / HP) changing the default cost metric becomes a requirement (this is due to (1) faster link speeds not calculating correctly (2) or the metric equations being different between inter-vendor kit).

The command to change default cost:

Router(config-if)#ip ospf cost cost

The cost variable is a 16-bit value (0 to 65,535). The lower values being the more preferred costs while higher being less preferred (shown below).

As you can see in the above table Fast Ethernet is the “drop off point” (where all things being 1) for the “fast” links (Fast Ethernet and Gigabit Ethernet both equal 1). In this case it would be better to manipulate the default cost so that the Gigabit Ethernet link is preferred over the Fast Ethernet Link. This would mean changing the cost per interface.

Another way to deal with high-bandwidth paths is to change the way a Cisco calculates cost (mess with the equation). We adjust the numerator in the automatic calculation (the Cisco automatic formula) to make some things happen in this case. To do this use the ospf auto-cost reference-bandwidth command on IOS, the default is 100 (Fast Ethernet) adjust it to 1000 (Gigabit Ethernet) and you will “fix” the equation.

Router(config-router)#ospf auto-cost reference-bandwidth reference bandwidth

Two very important NOTES (1) ospf auto-cost reference-bandwidth should be applied to all routers in an area if it is applied at all and the command (2) ip ospf cost overrides the calculated cost calculated by auto-cost reference-bandwidth

Determining the DR with the Priority Command

The hello field includes a priority field (if you can still remember) thus providing a mechanism by which designated router (DR) and backup designated Router (BDR) gets elected.

To be eligible for election, the priority must be a positive integer between 1 and 255 (if the priority is 0 (zero) the router cannot participate in the election process).

The highest priority wins (Cisco Router Default is 1) the election process.

Because the default is 1, to break all ties the Cisco Router ID is used as the deciding factor in the election process (ergo why hard coding the Router ID is a bad idea). The command to adjust priority (interface-by-interface mind you):

Router(config-if)#ip ospf priority number

Open Shortest Path First – OSPF Fundamentals – Configuring OSPF in a Single Area

Published

Deon Botha

on April 9, 2009

in BSCI, BSCI Notes, Certification, Cisco Systems, Concepts and Constructs and OSPF

. 5 Comments

The command requirements for configuring OSPF in a single area is relatively (compared to say other routing protocols) few in number and simple; the implications of the commands are somewhat complicated but need to be understood.

Required Commands for Configuring OSPF WITHIN a Single Area

We are going to configure an OSPF internal router. An Internal router being one with all interfaces that lie within a single area. The sole OSPF function on an internal router is to route within an area.

The Router needs to understand how to participate in the OSPF network:

OSPF Process – Declare an OSPF process.
Participating interfaces – Identify the interfaces to be used by OSPF.
Area – Definitions are done per interface. This discussion assumes that all active interfaces are in the same area.
Router ID – A unique 32-bit ID, usually drawn from an interface IP Address.

Enabling the OSPF Routing Protocol

Router(config)#router ospf process-number

In the above the process-number is not globally significant. It is possible to have more than one process running on a router (although that would be an unusual configuration, but not unheard of) and two OSPF processes could route for different parts of the network. The process number does not have to be the same on every router in the area.

The OSPF Network Command

Once OSPF is turned on (the above command), you must define the interfaces that are to participate in OSPF and the area that they reside in:

Router(config-router)#network network-number wilcard-mask area area-number

NB.Take note of the above command. Many errors occur in configuration with this command, normally due to misapplication of the wildcard-mask parameter, either including too many or too few interfaces in a particular OSPF area.

Similar to other routing protocols like RIP, the network command identifies the interface on which the OSPF process is to be active. Unlike RIP however this command has a wilcard mask that allows it to be very specific. All interfaces that match the network wildcard mask will be active within the given area.

One can apply the network command in different ways, each method will yield different yet similar results.

FE 0/0 : 192.168.0.1 / 24
FE 0/1 : 192.168.1.1 / 24
FE 0/2 : 192.168.2.1 / 24
FE 0/3 : 192.168.3.1 / 24
S o/1 : 10.10.1.1 / 30
s 1/1 : 10.10.2.1 / 30

We can enable OSPF area 0 (zero) on all interfaces with the following command:

Router(config-router)#network 0.0.0.0 255.255.255.255 area 0

When using this approach you may include interfaces inadvertently that you may not want to include (as this is a sweeping statement config line).

The second method would be to break the network into the 10 network and the 192 network, as follows:

Router(config-router)#network 10.0.0.0 0.255.255.255 area 0 Router(config-router)#network 192.168.0.0 0.0.3.255 area 0

This approach gives a little more control over the two different networks (192.x.x.x and 10.x.x.x) splitting them into two config lines.

Another method would be to separately enable OSPF on each interface;

Router(config-router)#network 192.168.0.1 0.0.0.0 area 0 Router(config-router)#network 192.168.1.1 0.0.0.0 area 0 Router(config-router)#network 192.168.2.1 0.0.0.0 area 0 Router(config-router)#network 192.168.3.1 0.0.0.0 area 0 Router(config-router)#network 10.10.1.1 0.0.0.0 area 0 Router(config-router)#network 10.10.2.1 0.0.0.0 area 0

This option is more time consuming to deploy but gives much more control over what interface specifically is included and not included in area 0 (zero) which will enable much more control over the routing process.

All the above achieve the same thing (six interfaces places in area 0 (zero) begin to process OSPF traffic).

The technique that is used should be functional, effective and efficient given the topology and application on the network while still maintaining the ability to be documented thoroughly (Keep It Simple Stupid KISS or as simple as possible, because you might not be the one to always maintain the network).

NB.Be intimately familiar (CCNA) with wildcard masks and the network command to enable OSPF on router interfaces

The area parameter puts the designated interface into an area. A router can have different interfaces in different areas (as mentioned earlier thus making the router an Area Border Routers (ABR)). The area-number is a 32-bit field and format can either be a simple decimal (0, 1, 2, 3, 4) or dotted decimal( 0.0.0.1, 0.0.0.2, 0.0.03, 0.0.0.4). Some implementations of OSPF might only understand one of the formats (keep in mind that some vendors throw the dotted decimal around 0.0.0.1 will become 1.0.0.0), Cisco understands both formats.

After identifying the interfaces on the router that are participating in the OSPF domain, hellos are exchanged, LSAs are sent, and the router inserts itself into the network.

NB.If there are stub networks connected to a OSPF router, it is useful to issue the command redistribute connected subnets. This command includes the connected subnets in OSPF advertisements without actually running OSPF on these routers. A route-map is often used with this command to exclude interfaces that are explicitly configured with OSPF

Next up Internal Router Config in more detail….

Network Ninja

Tag Archive for 'Config'

Open Shortest Path First – OSPF Fundamentals – Scenario

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 13

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 6

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 5

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 4

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 3

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 1

Open Shortest Path First – OSPF Fundamentals – Checking or Troubleshooting OSPF Troubleshooting

Open Shortest Path First – OSPF Fundamentals – Configuring Options On an Internal Router

Open Shortest Path First – OSPF Fundamentals – Configuring OSPF in a Single Area

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