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Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 11

Published

on June 24, 2009

in BSCI, BSCI Questions, Certification, Cisco Systems, Concepts and Constructs and Link State Advertisements

. 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.

11. what is the sequence number and where is it held?

The sequence number is a field with numbers so that older and/or newer versions of LSA advertisements can be recognized from each other (start 0×8000 0001 end 0xFFFF FFFF before rolling to the start again). The eventual goal is that all routers in the same AS has the same LSDB which is then processed using SPF from which the best routes are selected and a routing table created.

The sequence number is held within the Topology Database.

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 10

Published

Deon Botha

on June 24, 2009

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

. 2 Comments

10. How would you show the OSPF process ID of the router?

The OSPF process ID is the ID of the OSPF process to which the interface belongs. Defined by the below command

Router_1(config)#router ospf [process id]

The process ID is local to the router, and two OSPF neighboring routers can have different OSPF process IDs. (This is not true of Enhanced Interior Gateway Routing Protocol [EIGRP], in which the routers need to be in the same autonomous system). IOS can run multiple OSPF processes on the same router, and the process ID merely distinguishes one process from the another. The process ID should be a positive integer.

To show the OSPF Process ID use the following commands:

Router_1#show ip ospf

In the first line of output Routing Process ospf xxx with ID xxx.xxx.xxx.xxx

Router_1#show ip ospf database

In the first line of output OSPF Router with ID (xxx.xxx.xxx.xxx) (Process ID xxx)

Router_1#show ip ospf interface

In the third (plus or minus) line of output Process ID xxx, Router ID xxx.xxx.xxx.xxx, network type xxxxxxxxx, Cost: x

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 2

Published

Deon Botha

on June 16, 2009

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

. 0 Comments

2. What Parameter is used to calculate the metric of a link in OSPF on a Cisco Router?

The OSPF metric used to calculate link speed is 100,000,000 divided by the bandwidth of the interface in bits per second.

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….

Open Shortest Path First – OSPF Fundamentals – Multiple Areas

Published

Deon Botha

on March 3, 2009

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

. 0 Comments

An OSPF area is a logical grouping of routers that runs OSPF with identical topological databases. An area is a subdivision of the OSPF routing domain. Each area runs SPF separately and summaries are passed between each area.

Problems associated with OSPF in a Single Area

Consider a growing OSPF network with a single area. Several problems come out in relation to capacity capabilities:

The SPF algorithm runs more frequently the larger the network gets, the greater the probability of a network change and a recalculation of the entire area (iow the more resources OSPF chews up). Each of these recalculations in a large network takes longer and involves more “work” with each recalculation for a small area (the expenditure of scarce resources time, cpu, memory, etc).
The larger the OSPF area, the greater the size of the routing table (duh). The routing table is not sent out (like in Distance Vector Routing Protocols). In OSPF this means that the the greater the size of the table the longer the lookup becomes. The memory requirements on the router also increase as the size of the routing table increases.
In a large network, the routers topological database increases in size and eventually becomes unmanageable (the topological database is exchanged between adjacent routers at least every 30 minutes).

As the various databases (Routing Table, Topological Database, Neighbor Table) increase in size and the calculation increase in frequency the CPU utilization increases and memory availability decreases (inverse relationship). This can affect network latency or cause link congestion, resulting in various additional problems (convergence times, loss of connectivity, loss of packets, system hangs) which is bad for networks.

Area Structure

OSPF creates a two-level hierarchy of areas.

Area Zero (Naught) a.k.a the backbone are or transit area. This is always the central area; all the other areas (stub areas that move towards the edge) attach to Area Zero. Area Zero forms the top level in the hierarchy and remaining areas form the bottom level of the hierarchy. This hierarchical design supports summarization and minimizes routing table entries.

Routers within Area Zero are called backbone routers. Routers that link to Area Zero and another area are called Area Border Routers (ABR). OSPF routers that redistribute routing information from another protocol are called Autonomous System Boundary Routers (ASBR).

OSPF Type Packets

As OSPF link-state information is shared between areas, an intricate set of mechanisms is followed, relying on a number of different OSPF packet types. All OSPF traffic is transmitted inside IP Packets. Receivers recognize OSPF traffic because it is marked as IP Protocol (89).

OSPF includes five packet types:

Hello Packets – Establish communication with directly attached neighbors.
Database Descriptor (DBD) - Sends a list of router IDs from whom the router has an Link State Advertisements (LSA) and the current sequence number. This information is used to compare information about the network.
Link State Requests (LSR) – Follow the Database Descriptors (DBDs) to ask for any missing Link State Advertisements (LSAs)
Link State Update (LSU) – Replies to a link-state request with the requested data.
Link State acknowledgments (LSAck) - Confirm receipt of link-state information.

All OSPF packets have a common format that contains the following nine fields:

Version – All packets are assumed to be Version 2 (at least for this part of Cisco stuff)
Type - There are five packet types, numbered 1 to 5
Packet Length - The length in bytes
Router ID – 32-bit identifier for the router
Area ID – 32-bit identifier for the area
Checksum - Standard 16-bit check sum
Authentication Type - OSPFv2 supports three authentication methods:
1. no authentication
2. plain text passwords
3. MD5 hashes
Authentication Data – 64-bit data, either empty, with a plain-text word, or with a “message digest” of a shared secret
Data – Values being communicated

And this took me almost 2 weeks. Shame on me.

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.

CCIE Command Memorizer

Published

Deon Botha

on September 12, 2008

in Asides, Off-Topic, Support and Vine

. 2 Comments

About a two weeks ago David Bombal from Configure Terminal contacted me about doing a write up for CCIE Command Memorizer. As a current CCNP student there is obviously some of the CCIE content that still goes over my head but most of the stuff I am fairly okay with. David didn’t seem to mind that I wasn’t a CCIE as yet and I sure didn’t mind giving CCIE Command Memorizer a spin so below are my thoughts and feelings.

During my initial research on the CCIE Command Memorizer I found that CCIE Command Memorizer was geared towards the CCIE Routing and Switching (R&S) Lab. When I received the application Monday two weeks ago my initial impression was that it covers without many frills and spills in an effective and efficient manner the commands in a Do-It-Yourself fill in the blanks task to objective orientated format. There is no round-about, search for things “what now” moments; everything is straight and to the point. Another bonus is that the download isn’t enormous either.

The CCIE Command Memorizer application itself is written in an e-book format, which works for the content and in that format. If like myself you like using the keyboard when you get going on command line (IOW dislike moving between the mouse+keyboard) some of the shortcuts might feel “off” and take a bit to get use to. Take heart that once you get going it all starts flowing like second nature.

My feelings on how the CCIE Command Memorizer would fit into my studies it that as a study tool / study aid as it helps you practice commands anywhere at any time as long as you have your notebook handy (face it as a CCNP I realize I am a Geeks and my notebook goes on holiday with me so this pretty much means anywhere). My current situation with a LAB is that I am busy getting racks, cabling, and other kit together to put a LAB up, the actual LAB is non-existent. In my situation the CCIE Command Memorizer is my stop-gap for actual “command” time practice as I don’t really have kit and at times no block time to zero in on a specific Technology Area and do some quick revision work with a specific focus only on that technology. This is different from the LAB where you need to setup and configure peripheral services and technologies to get to the meat close to the bone, CCIE Command Memorizer lets you focus ONLY on those parts to give you practice on those parts.

In my opinion students that would find the CCIE Command Memorizer useful would be individuals who have problems with instant recall of information when under stress or duress (people with classic test condition issues that forget as they walk through the door when in normal day-to-day they are Uber networkers) and students who need to drill things to remember (repetition, repetition, repetition) information to get it to pass from short term into long term memory (this one would be me). Most other students may also find it handy to bridge the book theory and lab practical (where you just went through the theory to quickly go over the practical again to cement the knowledge just a bit more).

From my perspective as a CCNP student the CCIE Command Memorizer shares most (if not all) of the content material with the CCNP that I cared to check up on. In the two weeks I have been working through EIGRP and OSPF (notes to follow shortly) for the BSCI. Everything I covered in theory the CCIE Command Memorizer covered + some more.

To close off for those who want a one-size-fits-all study solution don’t mistake the CCIE Command Memorizer for something it wasn’t built to be. The CCIE Command Memorizer is a STUDY TOOL / STUDY AID and not a “Complete Blended Solution” or All-In-One (aio) Product like the guys from IP Expert offer. This will mean that you will probably either attend classes from a Cisco Learning Partner and/or do labs at home or through a Rack Rental Company and/or also have Cisco Press books that will ADD TO THE VALUE the CCIE Command Memorizer.

In summation I suggest taking a serious look at the CCIE Command Memorizer and how to integrate it into your study plans. Taking all things into consideration I would seriously recommend the CCIE Command Memorizer even to current CCNP level students. The value add is easy to find and it helps to have something that you can take along as you travel that does command line study effectively without the need to spend hours on “setting up other things” before you get to play with advanced topics.

Network Ninja

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CCIE Command Memorizer

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