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Advanced IP Routing

Open Shortest Path First (OSPF) Protocol
Reference
D. E. Comer, Internetworking with TCP/IP, ISBN 1-13018380-6, 4th Ed., Vol. 1, Ch. 15.
Network Architecture and Design
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OSPF
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OSPF is the Open Shortest Path First
protocol.
That means it is an “Open” version of
the “Shortest Path First” algorithm, it
does not mean it tries to open the
shortest path first.
Network Architecture and Design
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OSPF
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OSPF version 2 is defined in RFC 2328.
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(Version 1 is no longer in use.)
Network Architecture and Design
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OSPF
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OSPF is an update-based, link-state,
open routing protocol.
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These attributes make OSPF the most
commonly used IGP in use today.
Network Architecture and Design
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OSPF
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OSPF sends its entire routing table
upon startup, then sends a keep alive
every 10 seconds. If a topology change
occurs, only the changes are sent.
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This is much less bandwidth intensive
and much faster than RIP.
Network Architecture and Design
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OSPF - Link State Protocol
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OSPF is a “link state” protocol.
This means that each router keeps a
database of all the links in its “area”,
and calculates the “shortest path” to
each destination network from that
database.
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OSPF - Dijkstra Algorithm
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In a nutshell, the Dijkstra Algorithm has
each router imagine itself as the root of
a tree, and calculates each successive
link as a branch in the tree.
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Dijkstra
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See ospf.pdf
Slides 5 to 16.
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OSPF - Flooding
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In each area, every time a link
changes state, every router is
“flooded” with Link State
Advertisements (LSAs) describing the
change.
Every router must run the Dijkstra
algorithm to re-calculate every route in
the area.
Network Architecture and Design
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When a Link Changes State
Network Architecture and Design
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OSPF - LSAs
Different types of LSAs.
Examples
 Router link (LSA type 1)
 Network link (LSA type 2)
 Network summary (LSA type 3)
 External (LSA type 5)
 …..
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Network Architecture and Design
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Location of Different LSAs
Network Architecture and Design
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OSPF - Link Cost
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Each link is assigned a cost. By default
this cost is 100,000,000 / (speed of link
in bps). So, the default cost for a FDDI
link is 1, 10BaseT is 10, and a T1 is 64.
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Unfortunately, this does not scale well
with today’s technology.
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OSPF - Link Cost
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Fortunately, the cost of a link can be set
manually. In cisco, this is done under
each interface:
interface serial 0
ip ospf cost 10
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OSPF – Link Cost
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A “total” cost is then calculated for each
destination prefix. Each prefix is
installed into the routing table with a
next hop relating to the lowest cost
path.
Network Architecture and Design
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OSPF - Convergence
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When a link changes state, the LSA
flood and recalculation happen in a very
short time, usually seconds.
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Because a link change is explicitly
stated, there are very few routing loops
(and for very short periods) in OSPF.
Network Architecture and Design
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OSPF - Convergence
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This may seem CPU and memory
intensive, and it does take a great deal
more memory and CPU than RIP, but it
also allows OSPF to converge routes in
seconds even over large and complex
networks.
Network Architecture and Design
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OSPF - Areas
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To help conserve CPU and RAM, and to
limit LSA floods, areas were introduced.
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Each router need only know about the
links in its area, and the link back to
Area 0 (zero).
Network Architecture and Design
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OSPF - Areas
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Areas are defined as a 32-bit number,
either straight decimal (e.g. 123456) or
as a “dotted decimal” (e.g. 10.0.0.1).
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OSPF - Area 0
Every area must be directly connected to
“Area 0”.
Area 0
Area 1
Area 3
Area 2
Network Architecture and Design
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OSPF - Area 0
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The routers in Area 0 contain the
aggregated prefixes for every area.
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Area 0 is sometimes called the
“Backbone” area because all inter-area
traffic must traverse Area 0.
Network Architecture and Design
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OSPF - Area 0
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Area 0 places an extreme burden on a
network designer using OSPF.
Many networks “grow” in non-elegant
ways, making a truly hierarchical
network difficult or even impossible.
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OSPF - ABRs
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Routers with an interface in Area 0 and
an interface in a non-zero area are
called “Area Border Routers” or ABRs.
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ABRs aggregate the prefixes for a nonzero area and inject the aggregated
prefixes into Area 0.
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Different Types of Routers
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Location of Different Routers
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OSPF - Neighbors
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OSPF uses neighbor relationships to
send routing updates.
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If a neighbor relationship cannot be
achieved, no routing updates will pass.
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Routing Protocol Packets
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OSPF - DR
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On broadcast media (e.g. Ethernet),
OSPF elects a Designated Router (DR)
and a Backup Designated Router
(BDR).
Network Architecture and Design
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OSPF - DR
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When updates are sent, each router on
the LAN sends the updates to the DR
(and the BDR), which sends one copy
to each router.

This is much better than each router
sending a copy of each update to each
other router.
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OSPF - BDR
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If the DR is disabled or otherwise does
not respond to queries, the BDR takes
over.
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DR and BDR
DR
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BDR
Hellos elect DR and BDR to represent segment
Each router then forms adjacency with DR and
BDR
Network Architecture and Design
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OSPF - Virtual Links
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A tunnel, or “virtual link”, can be used
when direct physical connectivity cannot
be achieved.
This is not the preferred method.
Network Architecture and Design
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OSPF - Multicast
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OSPF uses multicast (224.0.0.x) to
propagate its routing updates, not
broadcast. This reduces the CPU
requirement on other hosts on the LAN
as they do not have to process the
multicast packet if they are not part of
the multicast group.
Network Architecture and Design
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Third Intermediate Report
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OSPF
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BGP
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I. Stergiou
A. Sgora
Deadline: 25/02/03
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Third Intermediate Report
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Structure
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Overview of examined technology
Focus on open research points
Related to open points works - State of the
art behind open points
Your own interests - Ideas
Conclusions
References
Network Architecture and Design
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Third Intermediate Report
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Report (soft and hard copy)
Use of Greek language
A related presentation (about thirty
minutes).
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End of Sixth Lecture
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