Transcript No Slide Title
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
Area 1
Hierarchical Routing
Area 2 Area 3
Router 1 Hierarchical OSPF Internetwork
H2
Router 8 Router 6 Router 2 Router 3
Area 1 Area 2
Router 7 Router 9 Router 4 Area 0 (Backbone) Router 11 Router 5 Router 12
Area 3
Router 13 Router 10
H1
Hierarchical Routing in OSPF
Area 0.0.0.2
Area 0.0.0.1
W X Y
Area 0.0.0.0
Z
Area 0.0.0.3
Hierarchical OSPF Network Example External Network Router 5 LAN 5
ASBR Multiple copies of algorithm ABR
Area 3.0.0.0
LAN 3
Router 3
Router 3 LAN 4 Router 4
Intra area routing
Area 4.0.0.0
ABR
Router 6 Area 0.0.0.0
Router 1
Inter area routing Intra area routing
Router 2
Router 2
LAN 2
OSPF Area Types
Transit Areas A transit area includes any area capable of propagating or originating Type 5 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
Type 1 2 3 4 5 Meaning Hello Database description Link status request Link status update Link status acknowledgement 0 Version(1) 8 Checksum 16 24 31 Type Message Length Router IP address Area ID Authentication Type Authentication (octets 0-3) Authentication (octets 4-7) IP Packet IP protocol 89
Sending and Receiving an OSPF Packet
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 Version(1) Authentication = 64 bits Checksum Type Message Length Router IP address Area ID Authentication Type Authentication (octets 0-3) Authentication (octets 4-7) 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
DR Hello Protocol
Priority = 1
BDR
Priority = 2 Priority = 7 Priority = 4
HELLO
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
OSPF Hello 10.1.1.7
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 ……..
I M S 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
--0---EB ---0--- Link ID Link Data Number of links 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 TOS=y 0 TOS x metric 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 x metric External route tag (x)
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.