Transcript Link-State Routing Protocols

Link-State Routing
Protocols
Routing Protocols and
Concepts – Chapter 10
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Objectives

Describe the
basic features &
concepts of link-state routing protocols.

List the
benefits and
requirements of link-state routing protocols.
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Link-State Routing
 The shortest path to a destination is not necessarily the
path with the least number of hops
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Link-State Routing -Definitions
 Directly Connected Networks
 Link: This is an interface on a
router
 Link state : This is the information
about the state of the links

IP address
Subnet mask
Type of network
Cost associated with link
Neighboring routers on the link
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Link-State Routing
Sending Hello Packets to Neighbours
 Link state routing protocols use a hello protocol
Purpose of a hello protocol:
-To discover neighbors (that use the same
link state routing protocol) on its link
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Hello packets
Hello Packet contains
Router ID of transmitting router
 OSPF Hello Intervals
–Usually multicast (224.0.0.5)
–Sent every 10 seconds on
–Multiaccess networks and
Point to point links
 OSPF Dead Intervals
–This is the time that must
transpire before the neighbor
is considered down
–Default time is 4 times
the hello interval
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Link-State Routing
Sending Hello Packets to
Neighbors
 Once routers learn it has
neighbors they form an
Adjacency
-2 adjacent neighbors will
exchange hello packets
-These packets will serve
as a keep-alive function
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Link-State Routing
Routers also exchange Link
State Packets (LSP) with
neighbours which contain
-State of each directly
connected link
-Includes information
about neighbors such as
neighbor ID, link type, &
bandwidth.
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Link-State Routing
 LSPs (Link State Packets) are sent out under the
following conditions
-Initial router start up or routing process
-When there is a change in topology
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Link-State Routing
Constructing a link state data base
 Routers use a database to construct a topology map of
the network
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Link-State Routing
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Link-State Routing
 Determining the shortest path
The shortest path to a destination determined by adding the
costs & finding the lowest cost
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Link-State Routing
 Once the SPF algorithm has determined the shortest
path routes, these routes are placed in the routing
table.
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Link-State Routing Protocols
Advantages of a Link-State Routing Protocol
Routing
protocol
Builds
Topological
map
Router can
independently
determine the
shortest path to
every network.
Distance
vector
No
Link State
Yes
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Convergence
A periodic/
event driven
routing updates
Use
of
LSP
No
Slow
Generally No
No
Yes
Fast
Generally Yes
Yes
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Link-State Routing Protocols
Requirements for using a link state routing protocol
 Memory requirements
Typically link state routing protocols use more
memory
 Processing Requirements
More CPU processing is required of link state
routing protocols
 Bandwidth Requirements
Initial startup of link state routing protocols can
consume lots of bandwidth
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Link-State Routing Protocols
 2 link state routing protocols used for routing IP
-Open Shortest Path First (OSPF)
-Intermediate System-Intermediate System (IS-IS)
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OSPF Metric
 OSPF uses cost as the metric for determining the
best route
-The best route will have the lowest cost
-Cost is based on bandwidth of an interface
Cost is calculated using the formula
108 / bandwidth
-Reference bandwidth
defaults to 100Mbps
can be modified using
auto-cost reference-bandwidth command
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Basic OSPF Configuration
Lab Topology
 Topology used for this chapter
Discontiguous IP addressing scheme
Since OSPF is a classless routing protocol the subnet mask is
configured in
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Basic OSPF Configuration
The router ospf command
 To enable OSPF on a router use the following
command
R1(config)#router ospf process-id
Process id
 A locally significant number between 1 and 65535
-this means it does not have to match other OSPF
routers
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Basic OSPF Configuration
 OSPF network command
-Requires entering: network address
wildcard mask - the inverse of the subnet
mask
area-id - area-id refers to the OSPF area.
OSPF area is a group of routers that
share link state information
-Example: Router(config-router)#network network-address
wildcard-ask area area-id
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Basic OSPF Configuration
Verifying OSPF
 Use the show ip ospf command to verify & trouble
shoot OSPF networks
Command will display the following:
 Neighbor adjacency
-No adjacency indicated by Neighboring router’s Router ID is not displayed
A state of full is not displayed
-Consequence of no adjacencyNo link state information exchanged
Inaccurate SPF trees & routing tables
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Basic OSPF Configuration
Verifying OSPF - Additional Commands
Command
Show ip protocols
Show ip ospf
Show ip ospf interface
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Description
Displays OSPF process ID,
router ID, networks router is
advertising & administrative
distance
Displays OSPF process ID,
router ID, OSPF area information
& the last time SPF algorithm
calculated
Displays hello interval and dead
interval
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Summary
 Link State Routing protocols are also known as
Shortest Path First protocols
 Summarizing the link state process
-Routers 1ST learn of directly connected networks
-Routers then say “hello” to neighbors
-Routers then build link state packets
-Routers then flood LSPs to all neighbors
-Routers use LSP database to build a network topology
map & calculate the best path to each destination
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