Building a medium sized network

www.professordkinney.com
08/18/13
Instructional Design-Computer Networking -
Bridges Educational Group

Building a Medium-Sized Network
08/18/13

Lessons Summary:
Introducing WAN Technologies
Introducing Dynamic Routing Protocol
Implementing OSPF
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Introducing WAN Technologies
Wide 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 Links
A 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 Switching
Switched 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 Switching
Packet 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 Circuits
A 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 Services
Dialup 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 Devices
WANs 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 Switch
A 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 Server
An 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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Modem
A 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/DSU
A 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 Adapter
An 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 Protocol
Dynamic 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 Protocol
What 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 Operation
All routing protocols have the same purpose: to learn about remote networks and to quickly
adapt 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 are
using 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 change
to other routers.
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Classifying Dynamic Routing Protocols
Routing protocols can be classified into different groups according to their characteristics:
■ IGP or EGP
■ Distance vector or link-state
■ Classful or classless
The 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 (deprecated
from 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 EGP
An autonomous system (AS)—otherwise known as a routing domain—is a collection of
routers under a common administration. Typical examples are a company’s internal
network
and 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. These
protocols 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 Protocols
Interior gateway protocols (IGP) can be classified as two types:
■ Distance vector routing protocols
■ Link-state routing protocols
Distance 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 Protocols
All routing protocols can also be classified as either
■ Classful routing protocols
■ Classless routing protocols
Classful Routing Protocols
Classful routing protocols do not send subnet mask information in routing updates. The
first routing protocols, such as RIP, were classful. This was at a time when network
addresses were allocated based on classes: Class A, B, or C. A routing protocol did not
need to include the subnet mask in the routing update because the network mask could be
determined 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 Protocols
Classless routing protocols include the subnet mask with the network address in routing
updates. Today’s networks are no longer allocated based on classes, and the subnet mask
cannot be determined by the value of the first octet. Classless routing protocols are required
in 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 has
converged when all routers have complete and accurate information about the network.
Convergence time is the time it takes routers to share information, calculate best
paths, and update their routing tables. A network is not completely operable until the
network has converged; therefore, most networks require short convergence times.
Metrics
Metrics are a way to measure or compare. Routing protocols use metrics to determine
which route is the best path.
Purpose of a Metric
There are cases when a routing protocol learns of more than one route to the same
destination. To select the best path, the routing protocol must be able to evaluate and
Differentiate among the available paths.
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Metrics and Routing Protocols
Different routing protocols use different metrics. The metric used by one routing protocol
is
not comparable to the metric used by an
Metric Parameters
Two different routing protocols might choose different paths to the same destination
because 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 interface
error count or previous link failures.
Cost: A value determined either by the IOS or by the network administrator to indicate
preference for a route. Cost can represent a metric, a combination of metrics, or a policy.
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Metric Field in the Routing Table
The routing table displays the metric for each dynamic and static route
Remember 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 by
the route with the smallest composite metric value calculated from these
multiple
parameters. By default, only bandwidth and delay are used.
■ IS-IS and OSPF: Cost: Best path is chosen by the route with the lowest cost.
The
Cisco 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

Load Balancing
You 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 Distance
Before 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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Default Administrative Distances

Implementing OSPF
OSPF 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 OSPF
Enable OSPF
As 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-id
Step 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 Parameters
Our OSPF implementation allows you to alter certain interface-specific OSPF parameters,
as
needed. You are not required to alter any of these parameters, but some interface
parameters
must be consistent across all routers in an attached network. Those parameters are
controlled by the ip ospf hello-interval, ip ospf dead-interval, and ip ospf authentication
key. commands. Therefore, be sure that if you do configure any of these parameters, the
configurations 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 seconds
Strep: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, between
the 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 Type
You 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 a
ip ospf network {broadcast | non-broadcast |
point-to-multipoint} specified interface
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Configure OSPF for Nonbroadcast Networks
Because 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 networks
neighbor ip-address[priority number] [poll-interval
seconds]
You can specify the following neighbor parameters, as required:
• Priority for a neighboring router
• Nonbroadcast poll interval
• Interface through which the neighbor is reachable
Configure OSPF Not So Stubby Area (NSSA)
NSSA area is similar to OSPF stub area. NSSA does not flood Type 5 external link state
advertisements (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 NSSA
area area-id nssa [no-redistribution]
[default-information-originate]
Configure Route Summarization between OSPF Areas
Route 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 advertised
area area-id range address mask
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Generate a Default Route
You 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 OSPF
routing domain- default-information originate [always] [metric metric-value] [metric-type
type-value] [route-map map-name]
Configure Lookup of DNS Names
You 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 Metrics
In 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-bw
Monitor and Maintain OSPF
You 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] database
show 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]
[asb-summary] [link-state-id]
show ip ospf [process-id] database [external]
[link-state-id]
[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 information
show 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 information
show ip ospf virtual-links
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Lessons learned:
WAN technologies.
What is dynamic protocols and their different parameters
and functions.
Configuring OSPF.
08/18/13

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