Objectives
After completing this chapter you will be able to:
 Describe hierarchical routing in OSPF
 Describe the 3 protocols in OSPF, the Hello, Exchange
and Flooding protocols
 Describe the OSPF database
 Outline the advantages and disadvantages of OSPF
Functional Requirements of OSPF
 A more descriptive routing metric was introduced
 OSPF can discover multiple best paths to a given
destination
 OSPF supports a 2 level routing hierarchy
 OSPF supports Variable Length Subnet Masks (VLSM)
 OSFP packets have a space reserved for authentication
 OSPF is an example of a link state algorithm that adjusts
to network changes quicker than RIP and is more robust
Hierarchical Routing
Area 2
Area 1
Area 3
Hierarchical OSPF Internetwork
Router 1
Router 6
H2
Router 8
Router 2
Router 3
Router 7
Area 1
Area 2
Router 4
Area 0
(Backbone)
Router 9
Router 10
Router 11
Router 5
H1
Router 12
Router 13
Area 3
Hierarchical Routing in OSPF
Area 0.0.0.2
X
W
Area 0.0.0.1
Area 0.0.0.3
Area 0.0.0.0
Y
Z
Hierarchical OSPF Network Example
Area 4.0.0.0
External
Network
LAN 4
Router 4
Router 5
Intra area routing
ABR
LAN 5
ASBR
Multiple copies of
algorithm
Router 6
Area 0.0.0.0
Inter area routing
Router 1
ABR
Intra area routing
Area 3.0.0.0
Router 3
LAN 3
Router
3
Router 2
Router
2
LAN 2
OSPF Area Types
 Transit Areas
A transit area includes any area capable of propagating or originating Type5 AS external LSAs
 Stub Areas
Stub areas do not import external route information( External LSAs Type 5).
Instead, network traffic to destinations not local to the area or AS is
directed to the closest area border router advertising a default route
 Not-so-stubby areas (NSSA)
The NSSA (not-so-stubby-area) defines a new OSPF area similar to the stub
area in that External LSAs (Type-5) are not propagated into the area nor
may they originate in a stub area (via an ASBR). The area may contain an
AS border router that may inject NSSA LSAs (Type-7) into the area.
OSPF Packet Header Format
24 byte header
0
8
Version(1)
16
Type
24
Message Length
Router IP address
Area ID
Checksum
Authentication Type
Authentication (octets 0-3)
Authentication (octets 4-7)
Type
1
2
3
4
5
Meaning
Hello
Database description
Link status request
Link status update
Link status
acknowledgement
IP Packet
IP protocol 89
31
Sending and Receiving an OSPF Packet
Version(1)
Type
Message Length
Router IP address
Area ID
Checksum
Authentication Type
Authentication (octets 0-3)
Authentication (octets 4-7)
Version = 2
Check on IP level
Type = 1 (hello)
Check IP protocol number (89)
Length = entire length
OSPF version must be 2
Router IP address = 192.168.10.1
IP address must be on same
network as receiving interface
Area ID = 3.0.0.0
Checksum = 16 bit checksum of
entire packet
Authentication = 64 bits
Area ID must be = 3.0.0.0
Checksum must be successful
Authentication must be successful
The Protocols within OSPF
 Hello protocol
-To check that the links are operational.
-To elect the Designated Router (DR) and the Backup Designated Router
(BDR).
 Exchange protocol
-Performs initial OSPF database synchronisation between two adjacent
routers.
 Flooding protocol
-The flooding protocol is used to maintain the two databases in adjacent
routers in synchronisation
Hello Protocol
DR
Priority = 1
BDR
Priority = 2
HELLO
Priority = 7
Priority = 4
Priority = 3
Hello Protocol - OSPF Hello Message Format
OSPF Header with Type = 1
Network Mask
Dead Timer
Hello Int
Gway Prio
Designated router
Backup Designated Router
Neighbour(1) IP Address
Neighbour(2) IP Address
Neighbour(n) IP Address
……..
Database Synchronisation
10.1.1.5
10.1.1.7
OSPF Hello
OSPF Hello: I heard 10.1.1.5
Database Description: Seq=x
Database Description: Seq=x, 5 LSA headers
Database Description: Seq=x+1
Database Description: Seq=x+1,1 LSA header
Link State Request Packet
Link State Update Packet
Link State Update Packet
Exchange Protocol - OSPF Database
Description Message
OSPF Header with Type = 2
Must be Zero
Database Sequence Number
Link Type
Link ID
Advertising Router
Link Sequence Number
Link Checksum
Link Age
……..
IMS
The fields
starting at
Link Type to
Link Age are
repeated for each
link
Exchange Protocol & Flooding Protocol OSPF Link Status Request Message Format
OSPF Header with Type = 3
Link Type
Link ID
Advertising Router
………
Exchange Protocol & Flooding Protocol OSPF Link Status Update Message Format
OSPF Header with Type = 4
Number of Link Status Advertisements
Link Status Advertisement (1)
………………
Link Status Advertisement (n)
Exchange Protocol & Flooding Protocol Header Format used for all Link State
Advertisements
Link Age
Link Type
Link ID
Advertising Router
Link Sequence Number
Link Checksum
Length
The OSPF Database - Router Links State
Record
Number of links
--0---EB ---0---Link ID
Link Data
Type
#TOS
TOS
0 metric
Type
#TOS
TOS
x metric
The OSPF Database - The Network Links
State Record
Network mask
Attached router
--------Attached router
The OSPF Database - The Summary Links
State Record
Network Mask
TOS=0
0
TOS
0 metric
…….
……
TOS=x
0
TOS
x metric
TOS=y
0
TOS
y metric
………..
The OSPF Database - The External Links
State Record
Network Mask
E,TOS=0
0
TOS
0 metric
External route tag (0)
E,TOS =x
0
TOS
External route tag (x)
x metric
Calculation of the Routing Table
 The present routing table is invalidated.
 The intra-area routes are calculated by building the
shortest path tree into each attached area.
 The inter-area routes are calculated through the
examination of summary LSAs.
 In ABRs connected to one or more transit areas, the
transit area’s summary LSAs are examined to see if
better paths exist using transit areas than were found in
steps 2 and 3 above.
 Routes to external destinations are calculated, through
the examination of AS external LSAs.
Advantages of OSPF
 OSPF is a standard protocol that all vendors can
implement interoperability.
 It provides rapid, deterministic calculation of internet
routes. It uses Link State Advertisements.
 It facilitate separate administration of differing parts of
the internet.
 It facilitate hiding of detailed information about the
internet.
 It provides a more advanced use of metrics.
 With OSPF one can isolate misconfigured or
malfunctioning routers in the internet and route around
them.
 OSPF provides for the effective use of information
derived from other routing protocols
Disadvantages of OSPF
 Link-state protocols use large amounts of router memory to
store topological databases, as each router keeps a map of
the entire network.
 When a network experiences frequent changes, link-state
routers use a large portion of network bandwidth by sending
out LSAs at each network change.