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
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. - PowerPoint PPT Presentation
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Objectives
After completing this chapter you will be able to:
Describe hierarchical routing in OSPFDescribe the 3 protocols in OSPF, the Hello, Exchange
and Flooding protocolsDescribe the OSPF databaseOutline the advantages and disadvantages of OSPF
Functional Requirements of OSPF
A more descriptive routing metric was introducedOSPF can discover multiple best paths to a given
destinationOSPF supports a 2 level routing hierarchyOSPF supports Variable Length Subnet Masks (VLSM)OSFP packets have a space reserved for authenticationOSPF is an example of a link state algorithm that adjusts
to network changes quicker than RIP and is more robust
Hierarchical Routing
Area 1
Area 2
Area 3
Hierarchical OSPF Internetwork
Area 1
Router 4
Router 1
Router 5
H2Router 8
Router 7
Router 11
Router 13
H1
Area 2
Area 3
Router 12
Area 0(Backbone)
Router 2Router 3
Router 6
Router 9
Router 10
Hierarchical Routing in OSPF
Area 0.0.0.0
Area 0.0.0.1
Area 0.0.0.2
Area 0.0.0.3
X
Y Z
W
Hierarchical OSPF Network Example
LAN 4
Router 1
Router 2
Router 3
Router 6
Router 4Router 5
LAN 3
LAN 5
LAN 2Router 2Router 3
Area 3.0.0.0
Area 0.0.0.0
Area 4.0.0.0External Network
ABR
ABR
ASBR
Multiple copies of algorithm
Inter area routing
Intra area routing
Intra area routing
OSPF Area Types
Transit AreasA transit area includes any area capable of propagating or originating Type-5 AS external LSAs
Stub AreasStub 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
Version(1) Type Message Length
Router IP address
Area ID
Checksum
Authentication (octets 0-3)
0 8 16 24 31
Authentication Type
Authentication (octets 4-7)
Type Meaning1 Hello2 Database description3 Link status request4 Link status update5 Link status
acknowledgementIP protocol 89
IP Packet
24 byte header
Sending and Receiving an OSPF Packet
Version(1) Type Message Length
Router IP address
Area ID
Checksum
Authentication (octets 0-3)
Authentication Type
Authentication (octets 4-7)
Version = 2
Type = 1 (hello)
Length = entire length
Router IP address = 192.168.10.1
Area ID = 3.0.0.0
Checksum = 16 bit checksum of entire packet
Authentication = 64 bits
Check on IP level
Check IP protocol number (89)
OSPF version must be 2
IP address must be on same network as receiving interface
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