Open Shortest Path First
Chang-Keun Park
[email protected]
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Content
 History
 Routing protocols taxonomy
 Terminology
 OSPF Router Types
 Features of OSPF
 Types of Link
 Link State Advertisements (LSAs)
 Operation of OSPF
– Dijkstra Algorithm
 OSPF Packet Types
 OSPF v3
 Problems
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History
 Development began 1987 by IETF
 OSPFv2 established in 1991 with RFC 1247
 Goal: a link state protocol more efficient and scalable than
RIP
 Latest revision is RFC 2328, April 1998
 OSPFv3 with IPv6 support is defined in RFC 2740
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Routing protocols taxonomy
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Terminology (1/3)
 Link: Interface on a router
 Link state: Description of an interface and of its
relationship to its neighboring routers
 Metric: cost assigned for passing through a network
– RIP uses metric based on hop-count
– OSPF uses metric based on type of services (Throughput, delay)
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Terminology (2/3)
 Area: collection of networks, hosts, and routers all
contained within an autonomous system
– All networks inside an area must be connected
– Router inside an area floods the area with routing information
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Terminology (3/3)
 On broadcast media, OSPF elects a Designated Router
(DR) and a Backup Designated Router (BDR)
 All other routers in the LAN, establish adjacencies only
with DR and BDR
– send Link State Advertisements (LSAs) only to DR/BDR
– help reduce routing update traffic
 BDR becomes DR, only if the DR fails
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OSPF Router Types
 Area Border Router (ABR)
– Routers with interfaces attached to other areas
 Backbone Router (BR)
– Routers inside backbone (area 0)
 Autonomous System Boundary Router (ASBR)
– Routers that have at least one interface connected to an external
internetwork (another autonomous system)
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Features
 OSPF is link-state routing protocol
–
RIP, IGRP and EIGRP are distance-vector routing protocol
 OSPF has fast convergence
– RIP and IGRP hold-down timers can cause slow convergence
 Cisco’s OSPF metric is based on bandwidth
–
–
RIP is based on hop count
IGRP/EIGRP is based on bandwidth, delay, reliability, load
 OSPF only sends out changes when they occur.
–
RIP sends entire routing table every 30 seconds, IGRP every 90 seconds
 OSPF uses the concept of areas to implement hierarchical
routing
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Types of Link (1/4)
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Types of Link (2/4)
 Point-to-Point Link
– Connects two routers without any other host or router
– Telephone line or T-line
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Types of Link (3/4)
 Transient Link
– A network with several routers attached to it
– LAN and WAN with two or more router
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Types of Link (4/4)
 Stub Link
– A network that is connected to only one router
 Virtual Link
– When the link between two routers is broken, the administration
create a virtual link between them using longer path
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Examples of an Internet
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Graphical representation
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Link State Advertisements (1/6)
 To share information about neighbors, each router
distributes link state advertisements (LSAs)
 LSA announces states of entity links
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Link State Advertisements (2/6)
 Router Link
– Generated by each router for each area it belongs to
• Flooded inside area
– Describes the states of the links in the area to which this router
belongs
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Link State Advertisements (3/6)
 Network Link
– Defining the links of a network
– Generated by the Designated router on every multi-access
network
– Designated Router (DR)
• Act as a central point of contact
• Each router sends LSAs only to DR/BDR (Backup DR)
• DR forwards LSAs to all other routers in network
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Link State Advertisements (4/6)
 Summary link to network
– Originated by the ABR
– Describes links state between ABR and Internal Routers of the
Local Area
– Flood throughout the backbone and to other ABRs.
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Link State Advertisements (5/6)
 Summary link to AS boundary router
– Originated by the ABR
– Flood throughout the backbone area to the other ABRs
– Describes the reachability to the ASBRs
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Link State Advertisements (6/6)
 External link
– Originated by the ASBR
– Describes destination networks external to the Autonomous
System
– Used to know which networks are available outside the
autonomous system
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OSPF Operations
1 – Flooding of link-state
information
3 – SPF Algorithm
2 – Building a
Topological
Database
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5 – Routing Table
4 – SPF Tree
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Link State Database
 Every router in an area receives the router link and
network link LSAs from every other router and forms Link
State database
 Tabular representation of the topology in an area
 Show the relationship between each router and its
neighbors including the metrics
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Dijkstra Algorithm
 calculating the shortest path between two points on a
network using a graph made up of nodes and edges
 divide the nodes into two set
– tentative and permanent
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Shortest Path Calculation
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Shortest Path Calculation
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Shortest Path Calculation
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Routing Table
 Each router uses the shortest path tree method to
construct its routing table
 Showing the cost of reaching each network in the area
 To find the cost of reaching networks outside of the area,
the routers use the summary link to network, the
summary link to boundary router, and the external link
advertisements.
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Link state routing table for router A
Network
------N1
N2
N3
N4
N5
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Cost
-----5
7
10
11
15
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Next Router
---------C
D
B
D
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OSPF packet header
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Types of OSPF packets
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Hello packet
 Used for creating neighborhood relationships and
to test the reachability of neighbors before
flooding link state information
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Database description packet
 Used when a router is connected to the system for
the first time or after a failure
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Link state request packet
 Used to request link state information about a
specific route or routers
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Link state update packet
 used by a router to advertise the states of its links
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Link state acknowledgment packet
 Used by a router to acknowledge LSAs to make
flooding of LSAs reliable
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OSPFv3
 OSPF for IPv6
 Based on OSPFv2, with enhancements
 Similarities
– Basic packet types
– Mechanisms for neighbor discovery and adjacency formation
– Nearly identical LSA types
 Differences
– OSPFv3 runs over a link, rather than a subnet
– OSPFv3 uses standard authentication mechanism supporting IPv6
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Problems
 Scalability
– OSPF is very processor intensive
– OSPF maintains multiple copies of routing information, increasing
the amount of memory needed
 OSPF is not as easy to learn as some other protocols
 Link cost is traffic carried: can cause route oscillation
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