Routing With a Link-State Protocol

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Routing With a
Link
-
State Protocol


Introducing Routing and Switching in the Enterprise



Chapter 6

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Objectives


Describe and plan a network using OSPF.


Design and configure a network using single
-
area
OSPF.


Work with multi
-
protocol environments.


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Describe and Plan a Network Using OSPF


Link
-
state routing protocol


Open standard


Classless interior gateway protocol


Divides the network into areas for scalability



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Describe and Plan a Network Using OSPF


DOES NOT send frequent periodic updates of the entire
routing table


Sends an update only when a change in the topology
occurs, such as a link going down.


In addition, OSPF performs a full update every 30 minutes.



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Describe and Plan a Network Using OSPF


Compared with distance vector protocols, link
-
state
routing protocols:


Requires more complex network planning and configuration


Requires increased router resources


Requires more memory for storing multiple tables


Requires more CPU and processing power for the complex
routing calculations

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Describe and Plan a Network Using OSPF


RIP


receive updates from their immediate neighbors, but with no
details about the network as a whole.


OSPF


generate a complete map of the network from their own
viewpoint


OSPF does not automatically summarize at major network
boundaries

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Describe and Plan a Network Using
OSPF


Bandwidth determines the cost of a link


Higher bandwidth = lower cost = desirable route


Cost = 100,000,000 / bandwidth of link in bps


Manual cost configuration at an interface: ip ospf cost


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Describe and Plan a Network Using OSPF


SPF algorithm (Diijkstra’s Algorithm)


Each router has a topological tree (map of network)


SPF tree information stored in topology database

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Describe and Plan a Network Using OSPF


OSPF


Establish and maintain neighbor relationships, or adjacencies,
with other connected OSPF routers (advanced form of
neighborship between routers that are willing to exchange
routing information)


When routers initiate an adjacency with neighbors, an
exchange of link
-
state updates begins.


Routers reach a FULL state of adjacency when they have
synchronized views on their link
-
state database




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Describe and Plan a Network Using OSPF

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Describe and Plan a Network Using OSPF


OSPF


Hello protocol is used to initially establish and maintain
adjacencies


sends hello packets to directly connected OSPF
routers using the address of 224.0.0.5


Packets are sent every 10 seconds on Ethernet and broadcast
links and every 30 seconds for non
-
broadcast links


Settings shared include the hello interval, dead interval, and
network type, as well as the authentication type and
authentication data if configured




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Describe and Plan a Network Using OSPF


Designated router (DR)


Backup designated router (BDR)


DROther

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Describe and Plan a Network Using OSPF


DR and BDR reduces the number of updates sent,
unnecessary traffic flow, and processing overhead on all
routers


All routers accept updates from the DR only


On broadcast network segments there is only one DR and
BDR.


All other routers must have a connection to the DR and BDR.


When a link fails, the router with information about the link
sends the information to the DR, using the multicast address
224.0.0.6.


The DR is responsible for distributing the change to all other
OSPF routers, using multicast 224.0.0.5..


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Describe and Plan a Network Using OSPF

The router ID is an IP address that is determined by:


1. The value configured with the router
-
id command


2. If no value is set with the router
-
id command, the highest
configured IP address on any loopback interface


3. If no loopback interface is configured, the highest IP
address on any active physical interface


The router ID can be viewed using the following show
commands:


show ip protocols, show ip ospf
, or
show ip ospf interface


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Within a local network, the router with the highest router ID is
elected the DR. The second highest is elected as the BDR.


An administrator can force the DR and BDR election by
configuring a priority using the interface configuration command:
ip ospf priority number


By default, OSPF routers have a priority value of 1.

Highest priority setting will win the election for DR, regardless of
highest router ID.


The highest value that can be set for router priority is 255.


A value of 0 signifies that the router is ineligible to be DR or BDR.

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EXERCISE 6.1.3.5

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Area 0


Backbone Area

Other areas up to 65,635


must connect to Area 0


Roles of routers in an OSPF Autonomous System:

Area Border Router (ABR)
-

connects an area to the backbone

Autonomous System Border Router (ASBR)
-

?router that
connects an area to a different routing protocol

Activity 6.1.4.2



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Design and Configure a Network Using
Single
-
Area OSPF


OSPF configuration steps:


Enable OSPF + process ID


Advertise networks + wildcard mask and area ID


Activity 6.2.1.3


Lab 6.2.1.4


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Design and Configure a Network Using
Single
-
Area OSPF


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Design and Configure a Network Using
Single
-
Area OSPF


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Design and Configure a Network Using
Single
-
Area OSPF


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Design and Configure a Network Using
Single
-
Area OSPF


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Design and Configure a Network Using
Single
-
Area OSPF


LSPs

do not need to be sent periodically.


An LSP only needs to be sent:


During initial startup of the router or of the routing protocol process


Whenever there is a change in the topology, including a link going down or
coming up, or a neighbor adjacency being established or broken


Other information included in the LSP


sequence numbers and aging information
-

used by each router to determine if
it has already received the LSP from another router
-

allows a router to keep
only the most current information in its link
-
state database.

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Design and Configure a Network Using
Single
-
Area OSPF


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Design and Configure a Network Using
Single
-
Area OSPF



Before two routers can form an OSPF neighbor adjacency, they must
agree on three values: Hello interval, Dead interval, and network type


Every 10 seconds on multiaccess and point
-
to
-
point segments


OSPF Hello packets are sent as multicast to an address reserved for
ALLSPFRouters at 224.0.0.5

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Design and Configure a Network Using
Single
-
Area OSPF



The router selects the DR based on the highest value of any one of the following parameters, in
the sequence listed:



1. Interface Priority: The interface priority is set with the priority command.



2. Router ID: The router ID is set with the OSPF router
-
id configuration command.



3. Highest Loopback Address: The loopback interface with the highest IP address is used as the
router ID by default. OSPF favors loopback interfaces since they are logical interfaces and not
physical interfaces. Logical interfaces are always up.



4. Highest Physical Interface Address: The router uses the highest active IP address from one of
its interfaces as the router ID. This option poses a problem if interfaces go down or are
reconfigured.



After changing the ID of a router or interface priority, reset neighbor adjacencies. Use the clear ip
ospf process command. This command ensures that the new values take effect.

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Design and Configure a Network Using
Single
-
Area OSPF


Tune OSPF parameters:


Interface priority




Router ID



Loopback and interface addresses


Bandwidth (cost )


Use the
clear ip ospf process
command to ensure
that the new values take effect



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Design and Configure a Network Using
Single
-
Area OSPF



When the DR is elected, it remains the DR until one of
the following conditions occurs:

The DR fails.

The OSPF process on the DR fails.

The multiaccess interface on the DR fails.


If the DR fails, the BDR assumes the role of DR and an
election is held to choose a new BDR.


If a new router enters the network after the DR and BDR
have been elected, it will not become the DR or the BDR
even if it has a higher OSPF interface priority or router
ID than the current DR or BDR.

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Design and Configure a Network Using
Single
-
Area OSPF



Packet Tracer Lab


DR & BDR



Exploration 3


11.4.3.2



Packet Tracer Lab


Fine Tuning OSPF


Exploration 3


11.5.2.3



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Design and Configure a Network Using
Single
-
Area OSPF


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Design and Configure a Network Using
Single
-
Area OSPF


Verification and troubleshooting commands:


show ip ospf neighbor


show ip protocols, show ip route


show ip ospf, show ip ospf interface



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The default configuration of OSPF exchanges information
between neighbors in plain text.


A hacker on a network could use packet sniffing software to
capture and read OSPF updates and determine network
information.


When authentication is enabled in an area, routers will only
share information if the authentication information matches.


Authentication protects integrity of routing information


Type 1
-

Simple password authentication


Type 2
-

Message Digest 5 (MD5)




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Type 2
-

Message Digest 5 (MD5)


Requires a key (password) and a key ID on each router.


Lab


6.2.2.2




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Work with Multi
-
Protocol Environments


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Work with Multi
-
Protocol Environments



Default route configured on ASBR


ASBR distributes the route into the OSPF network



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Work with Multi
-
Protocol Environments


Benefits of OSPF summarization:


Reduces number of networks advertised


Reduces memory requirements


Reduces number of entries in router updates


Isolates flapping and other problems to their location



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Work with Multi
-
Protocol Environments


To configure an OSPF
ABR router to summarize
these networks to another
OSPF area, issue the
following command in
router configuration mode:


area area
-
id range ip
-
address ip
-
address
-
mask

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Work with Multi
-
Protocol Environments


OSPF limitations:


Increased router memory and processing demands


Strict design requirements


Knowledgable administrator required


Initial discovery process takes up network bandwidth



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Work with Multi
-
Protocol Environments



Administrative distance (AD) determines routes that
appear in routing table




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Summary


OSPF is a classless interior link
-
state routing protocol
used in enterprise networks


OSPF uses bandwidth to generate the cost metric


OSPF routers elect a DR and BDR on multi
-
access
networks


Router ID or router priority can be used to dictate the
selection of DR and BDR


The OSPF network command uses a wildcard mask


Default route distribution and inter
-
area route
summarization are used in OSPF networks


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