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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 6

Published

Deon Botha

on June 18, 2009

in BSCI, BSCI Questions, Certification, Cisco Systems, 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.

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 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 – DR and BDR

Published

Deon Botha

on February 18, 2009

in BDR, BSCI, BSCI Notes, Certification, Cisco Systems, Concepts and Constructs, DR, OSPF and VLAN

. 7 Comments

When routers are connected to the same broadcast segment (I.O.W. several routers are in the same VLAN, on the same switch you getting the idea). One router is assigned the duty to maintain adjacencies with all other routers on the segment. This is the designated router (DR) and the DR router is selected using information in the Hello messages. For redundancy purposes a backup designated router (BDR) is also elected (There is a reason for this, read on).

DRs are created on multi-access links because the number of adjacencies grows at a quadratic rate. For a network of n routers, the number of adjacencies required would be:

Two (2) routers require the following adjacencies:

Four (4) routers require the following adjacencies:

Ten (10) Routers require the following adjacencies:

Maintaining a OSPF segment consumes more bandwidth and requires more processing resources (CPU and memory) as more routers are added onto a OSPF network (Due to keeping the tables updated and probability of changes occuring more frequently etc).

The DR and maintaining relationships

The purpose of a DR is to be the “one router” (sounds like the matrix) to which all other routers are adjacent (the router that has all the routes on the network). Using a DR reduces the number of adjacencies that consume bandwidth and processing to n – 1 (Larger networks will however still require more processing even if you are using a DR). With a DR the adjacencies scale more effectively and efficiently with the network (as one can see in the below figure and table).

To show this in a graphic way one can see how this “adjacency” relationship works without a DR, with a DR, and with a DR and BDR with a small example network using 5 routers.

Taking this a step further and plotting out the exponential growth requirements of OSPF adjacencies the table below shows the number of adjacencies needed for 1 – 10 routers (imagine the CPU and Memory requirements, not to mention the bandwidth consumption). Plan accordingly when implementing OSPF (at this point you generally use OSPF because you have a non-homogenous network environment and need the open standard because of this fact, I dont really see a point otherwise cause its such a resource hog and mission to setup).

The job of the DR

The role of the DR is to receive updates and distribute these updates to each segment router, making sure that each router acknowledges receipt and has a synchronized copy of the Link-State Database (LSDB).

Routers advertise changes to the “AllDRs” multicast address of 224.0.0.6 where the DR then advertise the Link-State advertisements (LSAs) using the “AllSPF” multicast address 224.0.0.5 where each router then ack receipt.

The BDR listens passively to this exchange and maintains a relationship with all the routers.

If the DR stops producing hellos, the BDR promotes itself and assumes the role of DR.

NB. DRs and BDRs are only useful on multi-access links because they reduce adjacencies. The concept of a DR is not used nor usefull on point-to-point links because there can only be one adjacency.

DRs are still however elected on Point-to-Point Ethernet links (most common type of links in networking these days) which is a rather pointless and resource waste/hog (as a DR is not really needed) which is why you will find that many design guides recommend changing Ethernet links to Point-to-Point mode to stop this from happening.

If a DR fails, the BDR is pomoted. The BDR is elected on the basis of highest OSPF priority, ties in OSPF priority are broken in favour of the highest IP ADDRESS.

The default priority is 1 and a priority of 0 (zero) prevents a router from being elected to the DR or BDR role.

Priority can be set from 0-255 (manually) to change the priority from default from the interface,

Router(config-if)#ip ospf priority number

DRs are inherently seen as stable entities once elected into the position, even if a Router joins a network with a “greater” priority the DR will not change.

To give an example of this an OSPF Segment with 5 Routers ( A – E, with different priorities 0 – 3). Taking what has been discussed previously A would be the DR, B the BDR, and E would never be elected. However this neglects the following set of circumstances:

Imagine the following sequence of events in this small segment,

Router C starts first.
1. Router C sends out Hellos and waits the dead time for a response from other routers.
2. Receiving no Response, Router C conducts an Election and becomes the BDR.
3. As there is no DR on this network, Router C then promotes itself to DR.
Router E starts (priority= 0)
1. Router E will not become the BDR due to its priority setting
Router B starts and becomes the BDR.
Router A starts
Router D starts

In the above scenario the startup sequence of the routers caused the election of the DR and BDR (namely Router C is DR and Router B is BDR) which is not what would have been expected. This is because designated routers do not preempt, the elected DR/BDR serves in its role until reboot/failure (DR and BDR are stable entities on the network once elected).

In this network as it stands now If Router C restarts, Router B promotes itself to DR and Router A is elected BDR while C is down. If Router B goes down, Router A promotes itself and elects Router C or Router D (whichever has the highest IP Address). Finally when the BDR is rebooted, Router B wins the election for BDR.

NOTE: In addition to rebooting, clearing the OSPS process using the the command clear ip ospf process * on the DR will force the DR and BDR election.

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 canâ€™t 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 – Neighbours and Adjacencies

Published

Deon Botha

on October 10, 2008

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

. 0 Comments

OSPF develops neighbour relationships with routers on the same link by exchanging hello messages (a.k.a hellos).

At the initial exchange of hellos, the routers add each other to their respective Neighbour Tables (The Neighbour Table in this case acting as a list of connected OSFP enabled routers).

OSPF Enabled Routers send multicast hellos with a destination address 224.0.0.5 on all OSPF-enabled interfaces.

OSPF sends out hellos every 10 seconds on a broadcast link (a link with more than 2 nodes on the same segment like Ethernet) and 30 seconds on a non-broadcast (a link with only 2 nodes on the same segment; exceptions *shrug* exist for NBMA) link.

The Hello message contains:

After the initial hello exchange between two routers, an exchange of network information begins. After routers have synchronized their information they are adjacent.

Routers must go though various states from the initial relationship “hello” that transitions through a process before forming a “full” adjacency as shown above in the picture.

Once a full adjacency is achieved, tables between routers must be kept updated to prevent loops and routing errors. LSAs are re-sent when a change occurs, or every 30 minutes to keep routing information “fresh”.

Going through the different “states” a neighbour relationship can be in:

Down – This is the first OSPF neighbour state, this state means that no hellos (information) has been received from any neighbour(s).
Attempt - This state is only valid for manually configured neighbours in a Non-broadcast multi access (NBMA) environment. In Attempt state, the router sends unicast hellos every poll interval to the neighbour from which hellos have not been received within the dead interval.
Init - This state indicates that the router has received a hello packet from its neighbour, but the receiving router’s ID was not included in the hello.
2-way – This state indicates that the bi-directional communication has been established between two routers
Exstart – Once the Designated Router (DR) and Backup Designated Router (BDR) are elected, the actual process of exchanging link-state information can start between the routers and their DR and BDR.
Exchange – In this state, OSPF routers exchange database descriptor (DBD) packets.
Loading - In this state, the actual exchange of link-state information occurs.
Full - In this state, routers are fully adjacent with each other. All the routers and networks LSAs are exchanged and the router databases are fully synchronized.

Hellos between routers continue to be sent periodically and the adjacency is maintained as long as hellos are exchanged. Missing hello messages result in a router declaring the adjacency being declared dead.

As soon as OSPF identifies a problem, it modifies its LSAs accordingly and sends the updated LSAs to the remaining neighbours (with full adjacencies).

Being event-driven, this LSA process intrinsically improves convergence time and reduces the amount of information that needs to be sent across the network.

A key piece of information exchange in LSAs is the OSPF metric information. Many OSPF vendors assign each link a cost of 10, Cisco makes cost inversely proportional to a 100 Mbs

An Admin can override the default cost. This would be done for compatibility reasons (with other OSPF speakers or because the link is more than 100 Mbps).

Sometimes the meric is equivalent for multiple paths to a destination. In this case, OSPF will load balance over each of the equivalent interfaces. Cisco routers will automatically perform equal-cost load balancing for up to four paths, but this parameter can be increased by configuration to as many as sixteen paths.

The cost is applied to the outgoing interface. The routing process will select the lowest cumulative cost to a remote network.

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 canâ€™t 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.

Network Ninja

Tag Archive for 'BDR'

Open Shortest Path First – OSPF Fundamentals – Scenario

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

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 – DR and BDR

Open Shortest Path First – OSPF Fundamentals – Neighbours and Adjacencies

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