Building a medium sized network

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Building a Medium-Sized Network

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Building a Medium-Sized NetworkLessons Summary:

Introducing WAN TechnologiesIntroducing Dynamic Routing ProtocolImplementing OSPF

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Introducing WAN TechnologiesWide Area Networks

A WAN is a data communications network that operates beyond the geographic scope of a LAN. WAN allows the transmission of data across greater geographic distances WANs use facilities provided by a service provider, or carrier. WANs use serial connections. An enterprise must subscribe to a WAN service provider to use WAN carrier network services.

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Point-to-Point LinksA point-to-point link provides a single, pre-established WAN communications

path from the customer premises through a carrier network, such as a telephone company, to a remote network. Point-to-point lines are usually leased from a carrier and thus are often called leased lines. For a point-to-point line, the carrier allocates pairs of wire and facility hardware to your line only. These circuits are generally priced based on bandwidth required and distance between the two connected points. Point-to-point links are generally more expensive than shared services such as Frame Relay. Figure below illustrates a typical point-to-point link through a WAN.

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Circuit SwitchingSwitched circuits allow data connections that can be initiated when needed

and terminated when communication is complete. This works much like a normal telephone line works for voice communication. Integrated Services Digital Network (ISDN) is a good example of circuit switching. When a router has data for a remote site, the switched circuit is initiated with the circuit number of the remote network. In the case of ISDN circuits, the device actually places a call to the telephone number of the remote ISDN circuit. When the two networks are connected and authenticated, they can transfer data. When the data transmission is complete, the call can be terminated. Figure below illustrates an example of this type of circuit.

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Packet SwitchingPacket switching is a WAN technology in which users share common carrier

resources. Because this allows the carrier to make more efficient use of its infrastructure, the cost to the customer is generally much better than with point-to-point lines. In a packet switching setup, networks have connections into the carrier’s network, and many customers share the carrier’s network. The carrier can then create virtual circuits between customers’ sites by which packets of data are delivered from one to the other through the network. The section of the carrier’s network that is shared is often referred to as a cloud. Some examples of packet-switching networks include Asynchronous Transfer Mode (ATM), Frame Relay, Switched Multimegabit Data Services (SMDS), and X.25. Figure below shows an example packet-switched circuit.

The virtual connections between customer sites are often referred to as a virtual circuit.

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WAN Virtual CircuitsA virtual circuit is a logical circuit created within a shared network between

two network devices. Two types of virtual circuits exist: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs).SVCs are virtual circuits that are dynamically established on demand and terminated when transmission is complete. Communication over an SVC consists of three phases: circuit establishment, data transfer,and circuit termination. The establishment phase involves creating the virtual circuit between the source and destination devices. Data transfer involves transmitting data between the devices over the virtual circuit, and the circuit termination phase involves tearing down the virtual circuit between the source and destination devices. SVCs are used in situations in which data transmission between devices is sporadic, largely because SVCs increase bandwidth used due to the circuit establishment and termination phases, but they decrease the cost associated with constant virtual circuit availability.

PVC is a permanently established virtual circuit that consists of one mode: data transfer. PVCs are used in situations in which data transfer between devices is constant. PVCs decrease the bandwidth use associated with the establishment and termination of virtual circuits, but they increase costs due to constant virtual circuit availability. PVCs are generally configured by the service provider when an order is placed for service.

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WAN Dialup ServicesDialup services offer cost-effective methods for connectivity across WANs. Two

popular dialup implementations are dial-on-demand routing (DDR) and dial backup.DDR is a technique whereby a router can dynamically initiate a call on a switched circuit when it needs to send data. In a DDR setup, the router is configured to initiate the call when certain criteria are met, such as a particular type of network traffic needing to be transmitted. When the connection is made, traffic passes over the line. The router configuration specifies an idle timer that tells the router to drop the connection when the circuit has remained idle for a certain period. Dial backup is another way of configuring DDR. However, in dial backup, the switched circuit is used to provide backup service for another type of circuit, such as point-to-point or packet switching. The router is configured so that when a failure is detected on the primary circuit, the dial backup line is initiated. The dial backup line then supports the WAN connection until the primary circuit is restored. When this occurs, the dial backup connection is terminated.

WAN DevicesWANs use numerous types of devices that are specific to WAN environments. WAN

switches, access servers, modems, CSU/DSUs, and ISDN terminal adapters are discussed in the following sections. Other devices found in WAN environments that are used in WAN implementations include routers, ATM switches, and multiplexers.

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WAN SwitchA WAN switch is a multiport internetworking device used in carrier networks.

These devices typically switch such traffic as Frame Relay, X.25, and SMDS, and operate at the data link layer of the OSI reference model. Figure below illustrates two routers at remote ends of a WAN that are connected by WAN switches.

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Access ServerAn access server acts as a concentration point for dial-in and dial-out

connections. Figure below illustrates an access server concentrating dial-out connections into a WAN.

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ModemA modem is a device that interprets digital and analog signals, enabling data

to be transmitted over voice-grade telephone lines. At the source, digital signals are converted to a form suitable for transmission over analog communication facilities. At the destination, these analog signals are returned to their digital form. Figure below illustrates a simple modem-to-modem connection through a WAN.

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CSU/DSUA channel service unit/digital service unit (CSU/DSU) is a digital-interface

device used to connect a router to a digital circuit like a T1. The CSU/DSU also provides signal timing for communication between these devices. Figure below illustrates the placement of the CSU/DSU in a WAN implementation.

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ISDN Terminal AdapterAn ISDN terminal adapter is a device used to connect ISDN Basic Rate

Interface (BRI) connections to other interfaces, such as EIA/TIA-232 on a router. A terminal adapter is essentially an ISDN modem, although it is called a terminal adapter because it does not actually convert analog to digital signals. Figure below illustrates the placement of the terminal adapter in an ISDN environment.

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Introducing Dynamic Routing ProtocolDynamic routing protocols play an important role in today’s

networks. The following sections describe several important benefits that dynamic routing protocols provide. In many networks, dynamic routing protocols are typically used with static routes.

Role of Dynamic Routing ProtocolWhat exactly are dynamic routing protocols? Routing protocols are used to

facilitate the exchange of routing information between routers. Routing protocols allow routers to dynamically learn information about remote networks and automatically add this information to their own routing tables, as shown below

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Routing protocols determine the best path to each network, which is then added to the routing table. One of the primary benefits of using a dynamic routing protocol is that routers exchange routing information whenever there is a topology change. This exchange allows routers to automatically learn about new networks and also to find alternate paths if there is a link failure to a current network.

Dynamic Routing Protocol OperationAll routing protocols have the same purpose: to learn about remote networks and to

quicklyadapt whenever there is a change in the topology. The method that a routing protocol

uses to accomplish this depends on the algorithm it uses and the operational characteristics of that protocol. The operations of a dynamic routing protocol vary depending on the type of routing protocol and the specific operations of that routing protocol. The specific operations of RIP, EIGRP, and OSPF are examined in later chapters. In general, the operations of a dynamic routing protocol can be described as follows:

1. The router sends and receives routing messages on its interfaces.2. The router shares routing messages and routing information with other routers that

areusing the same routing protocol.3. Routers exchange routing information to learn about remote networks.4. When a router detects a topology change, the routing protocol can advertise this

changeto other routers.

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Classifying Dynamic Routing ProtocolsRouting protocols can be classified into different groups according to their

characteristics:■ IGP or EGP■ Distance vector or link-state■ Classful or classlessThe sections that follow discuss these classification schemes in more detail.The most commonly used routing protocols are as follows:■ RIP: A distance vector interior routing protocol■ IGRP: The distance vector interior routing protocol developed by Cisco

(deprecatedfrom Cisco IOS Release 12.2 and later)■OSPF: A link-state interior routing protocol■ IS-IS: A link-state interior routing protocol■ EIGRP: The advanced distance vector interior routing protocol developed

by Cisco■ BGP: A path vector exterior routing protocol

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IGP and EGPAn autonomous system (AS)—otherwise known as a routing domain—is a

collection ofrouters under a common administration. Typical examples are a company’s

internal networkand an ISP’s network. Because the Internet is based on the autonomous

system concept,two types of routing protocols are required: interior and exterior routing

protocols. Theseprotocols are■ Interior gateway protocols (IGP): Used for intra-autonomous system

routing, that is,routing inside an autonomous system■ Exterior gateway protocols (EGP): Used for inter-autonomous system

routing, that is,routing between autonomous systems

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Distance Vector and Link-State Routing ProtocolsInterior gateway protocols (IGP) can be classified as two types:■ Distance vector routing protocols■ Link-state routing protocolsDistance vector protocols work best in situations where■ The network is simple and flat and does not require a hierarchical design.■ The administrators do not have enough knowledge to configure and

troubleshoot linkstate protocols.■ Specific types of networks, such as hub-and-spoke networks, are being

implemented.■Worst-case convergence times in a network are not a concern.Link-state protocols work best in situations where■ The network design is hierarchical, usually occurring in large networks.■ The administrators have a good knowledge of the implemented link-state

routing protocol.■ Fast convergence of the network is crucial.

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Classful and Classless Routing ProtocolsAll routing protocols can also be classified as either■ Classful routing protocols■ Classless routing protocols

Classful Routing ProtocolsClassful routing protocols do not send subnet mask information in routing

updates. Thefirst routing protocols, such as RIP, were classful. This was at a time when

networkaddresses were allocated based on classes: Class A, B, or C. A routing protocol

did notneed to include the subnet mask in the routing update because the network

mask could bedetermined based on the first octet of the network address. Classful routing

protocols can still be used in some of today’s networks, but because they do not include the subnet mask, they cannot be used in all situations. Classful routing protocols cannot be used when a network is subnetted using more than one subnet mask. In other words, classful routing protocols do not support variable-length subnet masks (VLSM).

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Classless Routing ProtocolsClassless routing protocols include the subnet mask with the network address in routingupdates. Today’s networks are no longer allocated based on classes, and the subnet

maskcannot be determined by the value of the first octet. Classless routing protocols are

requiredin most networks today because of their support for VLSM, discontinuous network.Convergence when the routing tables of all routers are at a state of consistency. The network hasconverged when all routers have complete and accurate information about the network.Convergence time is the time it takes routers to share information, calculate bestpaths, and update their routing tables. A network is not completely operable until thenetwork has converged; therefore, most networks require short convergence times.MetricsMetrics are a way to measure or compare. Routing protocols use metrics to determinewhich route is the best path.Purpose of a MetricThere are cases when a routing protocol learns of more than one route to the samedestination. To select the best path, the routing protocol must be able to evaluate andDifferentiate among the available paths.

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Metrics and Routing ProtocolsDifferent routing protocols use different metrics. The metric used by one routing

protocol isnot comparable to the metric used by anMetric ParametersTwo different routing protocols might choose different paths to the same destinationbecause of using different metrics. other routing protocol.Metrics used in IP routing protocols include the following:

Hop count: A simple metric that counts the number of routers a packet must traverse.

Bandwidth: Influences path selection by preferring the path with the highest bandwidth. Load: Considers the traffic utilization of a certain link. Delay: Considers the time a packet takes to traverse a path.Reliability: Assesses the probability of a link failure, calculated from the interfaceerror count or previous link failures. Cost: A value determined either by the IOS or by the network administrator to

indicatepreference for a route. Cost can represent a metric, a combination of metrics, or a

policy.

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Metric Field in the Routing TableThe routing table displays the metric for each dynamic and static

routeRemember that static routes always have a metric of 0.The list that follows defines the metric for each routing protocol:■ RIP: Hop count: Best path is chosen by the route with the lowest

hop count.■ IGRP and EIGRP: Bandwidth, delay, reliability, and load: Best path

is chosen bythe route with the smallest composite metric value calculated from

these multipleparameters. By default, only bandwidth and delay are used.■ IS-IS and OSPF: Cost: Best path is chosen by the route with the

lowest cost. TheCisco implementation of OSPF uses bandwidth to determine the

cost. IS-IS is discussed in CCNP.

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The metric associated with a certain route can be best viewed

using the show ip route command. The metric value is the second

value in the brackets for a routing table entry

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Load BalancingYou now know that individual routing protocols use metrics to

determine the best route to reach remote networks. But what happens when two or more routes to the same destination have identical metric values? How will the router decide which path to use for packet forwarding? In this case, the router does not choose only one route. Instead, the router load balances between these equal-cost paths. The packets are forwarded using all equal-cost paths.

Purpose of Administrative DistanceBefore the routing process can determine which route to use when forwarding a

packet, it must first determine which routes to include in the routing table. There can be times when a router learns a route to a remote network from more than one routing source. The routing process will need to determine which routing source to use. Administrative distance is used for this purpose.

Administrative distance (AD) defines the preference of a routing source. Each routing source—including specific routing protocols, static routes, and even directly connected networks—is prioritized in order of most to least preferable using an administrative distance value. Cisco routers use the AD feature to select the best path when they learn about the same destination network from two or more different routing sources.

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The AD value can also be verified with the show ip protocols command. This command displays all pertinent information about routing protocols operating on the router.

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Building a Medium-Sized NetworkDefault Administrative Distances

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Implementing OSPFOSPF typically requires coordination among many internal routers, area

border routers (routers connected to multiple areas), and autonomous system boundary routers. At a minimum, OSPF-based routers or access servers can be configured with all default parameter values, no authentication, and interfaces assigned to areas. If you intend to customize your environment, you must ensure coordinated configurations of all routers.

To configure OSPF, complete the tasks in the following sections. Enabling OSPF is mandatory; the other tasks are optional, but might be required for your application.

Enable OSPF• Configure OSPF Interface Parameters• Configure OSPF over Different Physical Networks• Configure OSPF Area Parameters• Configure OSPF Not So Stubby Area (NSSA)• Configure Route Summarization between OSPF Areas• Configure Route Summarization when Redistributing Routes into OSPF• Create Virtual Links• Generate a Default Route

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• Configure Lookup of DNS Names• Force the Router ID Choice with a Loopback Interface• Control Default Metrics• Configure OSPF on Simplex Ethernet Interfaces• Configure Route Calculation Timers• Configure OSPF over On Demand Circuits• Log Neighbor Changes• Monitor and Maintain OSPFEnable OSPFAs with other routing protocols, enabling OSPF requires that you create an

OSPF routing process, specify the range of IP addresses to be associated with the routing process, and assign area IDs to be associated with that range of IP addresses. Perform the following tasks, starting in global configuration mode.

Step 1 Enable OSPF routing. router ospf process-idStep 2 Define an interface on which OSPF runs and define the area ID for

that interface.network address wildcard-mask area area-id

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Configure OSPF Interface ParametersOur OSPF implementation allows you to alter certain interface-specific OSPF

parameters, asneeded. You are not required to alter any of these parameters, but some interface

parametersmust be consistent across all routers in an attached network. Those parameters arecontrolled by the ip ospf hello-interval, ip ospf dead-interval, and ip ospf

authenticationkey. commands. Therefore, be sure that if you do configure any of these parameters,

theconfigurations for all routers on your network have compatible values.Step 1: Explicitly specify the cost of sending a packet on an OSPF interface.ip ospf cost cost.Step 2: Specify the number of seconds between link state advertisement

retransmissions for adjacencies belonging to an OSPF interface.ip ospf retransmit-intervalseconds.Step 3: Set the estimated number of seconds it takes to transmit a link state update

packet on an OSPF interface.ip ospf transmit-delay secondsStrep:4 Set priority to help determine the OSPF designated router for a network.ip ospf priority number

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Step 5: Specify the length of time, in seconds, betweenthe hello packets that the Cisco IOS software sends on an OSPF interface.ip ospf hello-intervalseconds

Step 6: Set the number of seconds that a device’s hello packets must not have been seen before its neighbors declare the OSPF router down.

ip ospf dead-intervalseconds

Step 7: Assign a specific password to be used by neighboring OSPF routers on a network segment that is using OSPF’s simple password authentication.

ip ospf authentication-key key

Step 8: Enable OSPF MD5 authentication. ip ospf message-digest-key keyid md5 key

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Configure Your OSPF Network TypeYou have the choice of configuring your OSPF network type as either

broadcast or nonbroadcast multiaccess, regardless of the default media type. Using this feature, you can configure broadcast networks as nonbroadcast multiaccess networks when, for example, you have routers in your network that do not support multicast addressing. You also can configure nonbroadcast multiaccess networks (such as X.25, Frame Relay, and SMDS) as broadcast networks. Configuring nonbroadcast, multiaccess networks as either broadcast or nonbroadcast assumes that there are virtual circuits from every router to every router or fully meshed network. This is not true for some cases, for example, because of cost constraints, or when you have only a partially meshed network. In these cases, you can configure the OSPF network type as a point-to-multipoint network.

Routing between two routers not directly connected will go through the router that has virtual circuits to both routers. Note that you must not configure neighbors when using this feature.

Configure the OSPF network type for aip ospf network {broadcast | non-broadcast |point-to-multipoint} specified interface

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Configure OSPF for Nonbroadcast NetworksBecause there might be many routers attached to an OSPF network, a designated router

is selected for the network. It is necessary to use special configuration parameters in the designated router selection if broadcast capability is not configured.

Configure routers or access servers interconnecting to nonbroadcast networksneighbor ip-address[priority number] [poll-intervalseconds]You can specify the following neighbor parameters, as required:• Priority for a neighboring router• Nonbroadcast poll interval• Interface through which the neighbor is reachableConfigure OSPF Not So Stubby Area (NSSA)NSSA area is similar to OSPF stub area. NSSA does not flood Type 5 external link stateadvertisements (LSAs) from the core into the area, but it has the ability of importing AS

external routes in a limited fashion within the area.NSSA allows importing of Type 7 AS external routes within NSSA area by redistribution.

These Type 7 LSAs are translated into Type 5 LSAs by NSSA ABR which are flooded throughout the whole routing domain. Summarization and filtering are supported during the translation.Use NSSA to simplify administration if you are an Internet service provider (ISP), or a network administrator that must connect a central site using OSPF to a remote site that is using a different routing protocol.

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Define an area to be NSSAarea area-id nssa [no-redistribution][default-information-originate]Configure Route Summarization between OSPF AreasRoute summarization is the consolidation of advertised addresses. This

feature causes a single summary route to be advertised to other areas by an ABR. In OSPF, an ABR will advertise networks in one area into another area. If the network numbers in an area are assigned in a way such that they are contiguous, you can configure the ABR to advertise a summary route that covers all the individual networks within the area that fall into the specified range.To specify an address range, perform the following task in router configuration mode:

Specify an address range for which a single route will be advertisedarea area-id range address mask

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Generate a Default RouteYou can force an autonomous system boundary router to generate a default route

into an OSPF routing domain. Whenever you specifically configure redistribution of routes into an OSPF routing domain, the router automatically becomes an autonomous system boundary router. However, an autonomous system boundary router does not, by default, generate a default route into the OSPF routing domain.

To force the autonomous system boundary router to generate a default route,

perform the following task in router configuration mode:Force the autonomous system boundary router to generate a default route into the

OSPFrouting domain- default-information originate [always] [metric metric-value]

[metric-type type-value] [route-map map-name]Configure Lookup of DNS NamesYou can configure OSPF to look up Domain Naming System (DNS) names for use in

all OSPF show command displays. This feature makes it easier to identify a router, because it is displayed by name rather than by its router ID or neighbor ID.

To configure DNS name lookup, perform the following task in global configuration mode:

Configure DNS name lookup. - ip ospf name-lookup

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Control Default MetricsIn Cisco IOS Release 10.3 and later, by default, OSPF calculates the OSPF metric

for an interface according to the bandwidth of the interface. For example, a 64K link gets a metric of 1562, while a T1 link gets a metric of 64.The OSPF metric is calculated asref-bw divided by bandwidth, with ref-bw equal to 108 by default,and bandwidth determined by the bandwidth command. The calculation gives FDDI a metric of 1.

If you have multiple links with high bandwidth, you might want to specify a larger number to differentiate the cost on those links. To do so, perform the following task in router configuration mode:

Differentiate high bandwidth links - ospf auto-cost reference-bandwidth ref-bwMonitor and Maintain OSPFYou can display specific statistics such as the contents of IP routing tables, caches,

and databases. Information provided can be used to determine resource utilization and solve network problems. You can also display information about node reachability and discover the routing path your device’s packets are taking through the network.

To display various routing statistics, perform the following tasks in EXEC mode:Display general information about OSPF routing processes- show ip ospf [process-

id]

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Display lists of information related to the OSPF database

show ip ospf [process-id area-id] databaseshow ip ospf [process-id area-id] database [router][link-state-id]show ip ospf [process-id area-id] database [network][link-state-id]show ip ospf [process-id area-id] database [summary][link-state-id]show ip ospf [process-id area-id] database[asb-summary] [link-state-id]show ip ospf [process-id] database [external][link-state-id]show ip ospf [process-id area-id] database[database-summary]

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Display the internal OSPF routing table entries to Area Border Router (ABR) and Autonomous System Boundary Router (ASBR).

show ip ospf border-routers

Display OSPF-related interface informationshow ip ospf interface [interface-name]

Display OSPF-neighbor infrmation on a per-interface basis.show ip ospf neighbor [interface-name] [neighbor-id]Detail

Display OSPF-related virtual links informationshow ip ospf virtual-links

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Lessons learned:WAN technologies.What is dynamic protocols and their different

parameters and functions.Configuring OSPF.

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