%%category%% Archives %%page%% - Network Ninja

Archive for the 'OSPF' Category

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 2

Published

on June 16, 2009

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

. 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.

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.

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 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 – 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

. 9 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.

Open Shortest Path First – OSPF Fundamentals – Basics

Published

Deon Botha

on October 7, 2008

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

. 0 Comments

Open Shortest Path First (OSPF) is an open standard routing protocol, defined in detail in many Internet Engineering Task Force (IETF) Request For Comments (RFCs)Â including RFC 2328.

OSPF uses the Shortest Path First (SPF) Algorithm to calculate the best path to a given destination. OSPF builds loop-free paths that converge quickly, but often requires more processor power and memory than distance vector routing protocols (EIGRP).

OSPF can be more complicated because there are many topology and configuration options to consider versus EIGRP that has is has less of a learning curve.

OSPF is designed to offer flexibility in network design (OSPF is an open standard vs EIGRP that is Cisco Proprietary) which IOW allows OSPF to supports linking between different vendors Cisco, HP, etc.

OSPF Basics

OSPF as mentioned before is a Link-State routing protocol (basics) that is based on the Dijkstra Shortest Path First (SPF) Algorithm.

When one compares distance-vector routing to link-state routing;

link-state routing processes more information locally (on the router meaning more memory & CPU use) to reduce network bandwidth use.
Link-State routing protocols record all possible routes thus avoiding many of the techniques needed by distance-vector routing protocols to avoid loops.
Distance-vector routing protocols advertise routes to neighbours while link-state routing protocols advertise a list of connections.
In link-state routing, a neighbour is a directly connected router (or a router on the opposite side of a WAN link with the same network address).

OSPF is used within an Autonomous-System (AS). It has advantages over distance-vector routing protocols:

OSPF is classless + allows summarization
Converges quickly
OSPF is a standard, and fairly widely support can be found in a heterogeneous environment
Conserves bandwidth
uses multicast and not broadcast
sends incremental change-based updates
uses cost as the metric
KB is fairly widely available and less restricted than EIGRP

When a link goes up or down in a link-state routing protocol network, a link-state advertisement (LSA) is generated. LSAs are shared with neighbours and a topological database (a.k.a link-state database (LSDB) or Topology Table) is built.

LSAs are marked with sequence numbers so that older and/or newer versions of advertisements can be recognized (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 Topology Database (LSDB) is the routers view of the network within the AS it operates. This includes every OSPF router within that area and all connected networks.

To view the current status of the link state database,
Router#show ip ospf database

The Topology Database is updated by LSAs and each router in a AS has exactly the same topology database. All routers must have the same view of the network otherwise routing loops or loss of connectivity will occur.

When a router realizes there has been a change to the network topology, the router is responsible for informing the rest of the routers in the area with a LSA. This happens mostly due to:

A router losing physical or data-link layer connectivity on a connected network
A router does not receive a predetermined number of consecutive OSPF hello messages
A router receives a LSA update from a adjacent neighbour, informing it of the change in the network topology

In any of the above cases, the router will generate an LSA and flood it to all neighbours with the following stipulations:

If the LSA is new, the route is added to the database, the route is flooded out other links so other routers are updated, SPF is rerun.
If the sequence number is the same as the current entry in the Topology Database, the router ignores the advertisement.
If the sequence number is older, the router sends the newer copy (in memory) back to the advertiser to make sure that all neighbours have the latest LSA.

All OSPF operations centre around populating and maintaining

Neighbour Table
Topology Table
Routing Table

Notes and Notices:

BSCI Design Foundation – Routing Protocols

Published

Deon Botha

on July 25, 2008

in BGP, BSCI, BSCI Notes, CIDR, Certification, Cisco Systems, Concepts and Constructs, EIGRP, IGRP, IS-IS, OSPF, RIP, RIPv2 and VLSM

. 2 Comments

Routing protocols employ one of two basic strategies to communicate/propagate routing information:

Distance vector routing protocols work by passing copies of their routing tables to their neighbours (a.k.a routing by rumour).
Link State routing protocols work by advertising a list of neighbours and the network attachment state to their neighbours until all routers have a copy of all the lists, routers then run the Shortest Path First Algorithm to analyse all paths and determine the best paths available.

Distance vector routing are less processor and memory intensive than link state routing, but can have loops because routing decisions are made on incomplete information.

Link state routing is loop-proof because routers know all possible routes, but link state routing requires more CPU time and memory.

Classless and Classful Routing

An important characteristic of routing protocols is how they advertise their routes. Older routing protocols (RIP and IGRP) assumed the subnet mask the same as the one the receiving on the interface or that it is the default one (Class A is /8, Class B is /16 and Class C is /24). This is called classful because the assumption is based on the Class of the IP address.

Modern routing protocols (OSPF, IS-IS, and EIGRP) explicitly advertise the mask. There is no assumption made with regard to the mask, it is clearly indicated. This is called classless because no assumption is made and an address alone is not a good indicator subnet mask.

Variable Length Subnet Masks (VLSM) refers to the property of a network that allows different subnet masks to be mixed throughout the network.

Classless Interdomain Routing (CIDR) is a property of a network that allows classful networks to be aggregated.

Classless routing protocols support both VLSM and CIDR.

Interior and Exterior Gateway Protocols

Most protocols are “Interior Gateway”, meaning that they are designed to be run inside a network (inside the trusted boundaries of the company).

BGP on the other hand is an exterior gateway protocol (EGP) and is used for routing between autonomous systems (AS) on the Internet (outside the trusted boundaries of the company). As BGP is the only EGP you will have to consider using it if you connect your network to the Internet.

Convergence Times

A distinguishing characteristic of routing protocols is the speed of convergence times. To explain convergence, when a routing protocol is forwarding data, it is converged. In this state the routing protocol has shared routing table information and each router in the topology knows the best paths available. If there was a change (a router going down, another router being added, etc) this would require all routers to share information again because there are routes they do not have information on. The time between network change and forwarding would be “convergence”. This is generally classed as either slow or fast.

Fast convergence would mean that the routing protocol is able to recognize a problem on the network and fix that problem faster than a user can call to report a given problem.

Slow protocols, such as RIP and IGRP, can take up to minutes to converge when a problem occurs.

Fast protocols (OSPF, IS-IS, EIGRP) generally take less than 10 seconds to converge.

Proprietary and Open Standard Protocols

The important aspects to look for in routing protocols is speed of convergence and whether the protocol is classless (OSPF, IS-IS, and EIGRP). While OSPF and IS-IS are open standards (plays well with other vendors kit), EIGRP is Cisco proprietary (Cisco Only). Of the three protocols EIGRP is the easiest to configure and maintain but requires a pure Cisco environment to run.

Routing Protocol and the ECNM

The ECNM mentioned in previous posts can assist in showing where a particular routing protocol will run in the enterprise. Using information discussed above and using the ECNM the above diagram shows what the advanced routing protocols (EIGRP, OSPF, IS-IS) are best suited for when considering size of network, speed of convergence, VLSM, open or proprietary, and support staff knowledge needs.

The object (ideal) is to have a single routing protocol running throughout the enterprise (reality however is another story) where the enterprise edge will require BGP as the only EGP and at least one if not more of the IGPs within the enterprise boundaries depending on needs/requirements of end-points or design specifications.

In Summation

Older routing protocols (RIP, RIPv2 and IGRP) are slow because they send a full copy of their information periodically, these periodic transmissions act as both routing advertisement and keepalive message. In addition to being slow they consume a lot of bandwidth relative to their function (RIP every 30 seconds).

More modern routing protocols are faster because they separate the routing advertisements and the keepalive messages. Updates are only sent out when new networks need to be advertised or old networks need to be withdrawn; otherwise routers just need to verify that neighbours are still alive (EIGRP every 5 seconds).

RIP and IGRP

These are older distance vector routing protocols that are slow and classful. Some legacy systems (UNIX) expect to learn their default gateway by eavesdropping on RIP advertisements. If you deploy RIP use RIPv2 which is classless.

EIGRP

A modern distance vector routing protocol. It is classless and fast as well as being easy to configure and maintain. Some organizations refuse to implement proprietary standards though (EIGRP provides equivalent performance to OSPF but is easier to implement and maintain).

OSPF

OSPF is a modern classless and fast link-state routing protocol. OSPF has a steep learning curve and uses more processor time and memory than EIGRP. This is the open standard if an organization supports a heterogeneous mixture of routers or has a philosophical problem with proprietary standards.

IS-IS

This routing protocol was developed to compete with OSPF and the two are more similar than they are dissimilar. It is moderately difficult to find anyone who has experience working with IS-IS even if it is open, fast, and classless. There is still however some interest in IS-IS because it can be adapted to support MPLS and IPv6.

BGP

BGP is a routing protocol used between AS on the Internet and you will have to use it to connect your network to the Internet.

Resources:

Internetworking Technology Handbook Routing Basics

Internetworking Technology Handbook RIP

Internetworking Technology Handbook IGRP

Internetworking Technology Handbook OSPF

Internetworking Technology Handbook EIGRP

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 can’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

Archive for the 'OSPF' Category

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 2

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

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

Open Shortest Path First – OSPF Fundamentals – Multiple Areas

Open Shortest Path First – OSPF Fundamentals – DR and BDR

Open Shortest Path First – OSPF Fundamentals – Neighbours and Adjacencies

Open Shortest Path First – OSPF Fundamentals – Basics

BSCI Design Foundation – Routing Protocols

Search

About

Latest

Archives

Categories