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Transcript
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
1
OSPF


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
2
OSPF

OSPF version 2 is defined in RFC 2328.

(Version 1 is no longer in use.)
Network Architecture and Design
3
OSPF

OSPF is an update-based, link-state,
open routing protocol.

These attributes make OSPF the most
commonly used IGP in use today.
Network Architecture and Design
4
OSPF

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.

This is much less bandwidth intensive
and much faster than RIP.
Network Architecture and Design
5
OSPF - Link State Protocol


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.
Network Architecture and Design
6
OSPF - Dijkstra Algorithm

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.
Network Architecture and Design
7
Dijkstra


See ospf.pdf
Slides 5 to 16.
Network Architecture and Design
8
OSPF - Flooding


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
9
When a Link Changes State
Network Architecture and Design
10
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)
 …..

Network Architecture and Design
11
Location of Different LSAs
Network Architecture and Design
12
OSPF - Link Cost

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.

Unfortunately, this does not scale well
with today’s technology.
Network Architecture and Design
13
OSPF - Link Cost

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
Network Architecture and Design
14
OSPF – Link Cost

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
15
OSPF - Convergence

When a link changes state, the LSA
flood and recalculation happen in a very
short time, usually seconds.

Because a link change is explicitly
stated, there are very few routing loops
(and for very short periods) in OSPF.
Network Architecture and Design
16
OSPF - Convergence

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
17
OSPF - Areas

To help conserve CPU and RAM, and to
limit LSA floods, areas were introduced.

Each router need only know about the
links in its area, and the link back to
Area 0 (zero).
Network Architecture and Design
18
OSPF - Areas

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).
Network Architecture and Design
19
OSPF - Area 0
Every area must be directly connected to
“Area 0”.
Area 0
Area 1
Area 3
Area 2
Network Architecture and Design
20
OSPF - Area 0

The routers in Area 0 contain the
aggregated prefixes for every area.

Area 0 is sometimes called the
“Backbone” area because all inter-area
traffic must traverse Area 0.
Network Architecture and Design
21
OSPF - Area 0


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.
Network Architecture and Design
22
OSPF - ABRs

Routers with an interface in Area 0 and
an interface in a non-zero area are
called “Area Border Routers” or ABRs.

ABRs aggregate the prefixes for a nonzero area and inject the aggregated
prefixes into Area 0.
Network Architecture and Design
23
Different Types of Routers
Network Architecture and Design
24
Location of Different Routers
Network Architecture and Design
25
OSPF - Neighbors

OSPF uses neighbor relationships to
send routing updates.

If a neighbor relationship cannot be
achieved, no routing updates will pass.
Network Architecture and Design
26
Routing Protocol Packets
Network Architecture and Design
27
OSPF - DR

On broadcast media (e.g. Ethernet),
OSPF elects a Designated Router (DR)
and a Backup Designated Router
(BDR).
Network Architecture and Design
28
OSPF - DR

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.
Network Architecture and Design
29
OSPF - BDR

If the DR is disabled or otherwise does
not respond to queries, the BDR takes
over.
Network Architecture and Design
30
DR and BDR
DR


BDR
Hellos elect DR and BDR to represent segment
Each router then forms adjacency with DR and
BDR
Network Architecture and Design
31
OSPF - Virtual Links


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
32
OSPF - Multicast

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
33
Third Intermediate Report

OSPF


BGP


I. Stergiou
A. Sgora
Deadline: 25/02/03
Network Architecture and Design
34
Third Intermediate Report

Structure






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
35
Third Intermediate Report



Report (soft and hard copy)
Use of Greek language
A related presentation (about thirty
minutes).
Network Architecture and Design
36
End of Sixth Lecture
Network Architecture and Design
37