Module 3 - Review of OSPF Fundamentals and Features potx

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BSCI Module 3 Lesson 1 - Review of OSPF
Fundamentals and Features
OSPF
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Objective
 Link-State Routing Protocols
 Overview of OSPF Operations
 OSPF Data Structures
 OSPF Adjacency Database
 OSPF Area Structure
 OSPF Router Types
 OSPF Terminology
 Calculation the OSPF Metric
 Link-State Data Structure
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Link-State Protocols
 Each router has a full picture of the network topology,
including bandwidth information for links to remote
networks. Therefore, each router is able to
independently select a loop-free and efficient pathway,
based on cost, to reach every network in the area.
 OSPF is a classless routing protocol
 Support VLSM

 Support Discontigous
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Link-State Routing
 Routers with link-state routing protocols use a Hello protocol
to discover any neighbors on its links. A neighbor is any
other router that is enabled with the same link-state routing
protocol.
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Sending Hello Packets to Neighbors
 Connected interfaces that are using
the same link state routing protocols
will exchange hello packets.
 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.
Link-State Routing
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Link-State Routing
Building the
Link State Packet
 Each router builds its
own Link State Packet
(LSP)
 Contents of LSP:
–State of each
directly connected
link
–Includes
information about
neighbors such as
neighbor ID, link
type, & bandwidth.
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 Directly Connected Networks.
 Link: this is an interface on a router.
 Link state: this is the information about the state of the links.
Link-State Routing
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Link-State Routing
Flooding LSPs to Neighbors
 Once LSP are created they are forwarded out to neighbors.
–After receiving the LSP the neighbor continues to forward it
throughout routing area.
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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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Metrics and Routing Protocols
The Metric Parameters
 Different routing protocols use different metrics. Two different routing
protocols might choose different paths to the same destination due to
using different metrics.
 Metrics used in IP routing protocols include:Hop count, Bandwidth, Load,
Delay, Reliability, Cost
RIP
OSPF
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Overview of OSPF Operations
 Link-state routing protocols have
the following characteristics:
–Respond quickly to network changes
–Send triggered updates when a
network change occurs
–Send periodic updates, known as
link-state refresh, at long intervals,
such as every 30 minutes

 LSAs are reliable (ACK)
 LSAs have a sequence number
 LSAs are propagated to all
neighboring devices using the
reserved class D multicast
address 224.0.0.5.
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OSPF Data Structures
–Neighbor table:
•Also known as the adjacency database
•Contains list of recognized neighbors
–Topology table:
•Typically referred to as LSDB
•Contains all routers and their attached links in the area or
network
•Identical LSDB for all routers within an area
–Routing table:
•Commonly named a forwarding database
•Contains list of best paths to destinations
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OSPF Adjacency Database
–Routers discover neighbors by exchanging hello packets
224.0.0.5
–Routers declare neighbors to be up after checking

certain parameters or options in the hello packet.
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Forming OSPF Adjacencies
–Point-to-point WAN links:
•Both neighbors become fully adjacent.
–LAN links:
•Neighbors form a full adjacency with the DR and BDR.
•Routers maintain two-way state with the other routers
(DROTHERs).
–Routing updates and topology information are passed only
between adjacent routers.
–Once an adjacency is formed, LSDBs are synchronized by
exchanging LSAs.
–LSAs are flooded reliably throughout the area (or network).
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OSPF Area Structure
 To reduce the SPF calculations, link-state routing
protocols can partition networks into sub-domains
called areas.
 An area is a logical collection of OSPF networks,
routers, and links that have the same area
identification.
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OSPF area
characteristics:
• Minimizes routing
table entries
• Localizes impact of
a topology change
within an area
• Detailed LSA
flooding stops at
the area boundary
• Requires a
hierarchical
network design
OSPF Area Structure
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OSPF Router Types
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• Routers A and B are
backbone routers.
• Backbone routers
make up area 0.
• Routers C, D, and E are

known as area border
routers (ABRs).
• ABRs attach all other
areas to area 0.
Area Terminology
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OSPF Terminology
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OSPF Calculation
 Routers find the best paths to destinations by applying
Dijkstra’s SPF algorithm to the link-state database as
follows:
–Every router in an area has the identical
link-state database.
–Each router in the area places itself into
the root of the tree that is built.
–The best path is calculated with respect to the
lowest total cost of links to a specific destination.
–Best routes are put into the forwarding database (routing
table).
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SPF Calculation
Assume all links are Ethernet, with an OSPF cost of 10.
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Link-State Data Structures: LSA
Operation
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Activity
 3.1.2 Check Box: RIPv1 versus OSPF
–Upon completion of this activity, the student will be able to
compare routing issues between RIPv1 and OSPF.
 3.1.6 Drag and Drop: OSPF Router Types
–Upon completion of this activity, the student will be able to
identify the four different types of OSPF routers.