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 1 Area 2 Area 3

Hierarchical OSPF Internetwork Area 1 Router 4 Router 1 Router 5 H2 Router 8 Router 7 Router 11 Router 13 H1 Area 2 Area 3 Router 12 Area 0 (Backbone) Router 2 Router 3 Router 6 Router 9 Router 10

Hierarchical Routing in OSPF Area Area Area Area X Y Z W

Hierarchical OSPF Network Example LAN 4 Router 1 Router 2 Router 3 Router 6 Router 4 Router 5 LAN 3 LAN 5 LAN 2 Router 2 Router 3 Area Area Area External Network ABR ASBR Multiple copies of algorithm Inter area routing Intra area routing

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 Version(1) Type Message Length Router IP address Area ID Checksum Authentication (octets 0-3) Authentication Type Authentication (octets 4-7) IP 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 = Area ID = 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 = 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 BDR DR HELLO Priority = 1 Priority = 2 Priority = 7 Priority = 4Priority = 3

Hello Protocol - OSPF Hello Message Format Network Mask Backup Designated Router Designated router OSPF Header with Type = 1 Neighbour(1) IP Address …….. Dead Timer Hello Int Gway Prio Neighbour(2) IP Address Neighbour(n) IP Address

Database Synchronisation OSPF Hello OSPF Hello: I heard Database Description: Seq=x Database Description: Seq=x, 5 LSA headers Database Description: Seq=x+1,1 LSA header Database Description: Seq=x+1 Link State Request Packet Link State Update Packet

Exchange Protocol - OSPF Database Description Message Must be Zero Link ID Link Type OSPF Header with Type = 2 Advertising Router Link Sequence Number …….. Link Checksum Database Sequence Number 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 ChecksumLength

The OSPF Database - Router Links State Record EB Link ID Link Data Type #TOSTOS 0 metric Type #TOSTOS x metric Number of links

The OSPF Database - The Network Links State Record Attached router Network mask 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 E,TOS=x 0 TOS x metric External route tag (0) 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.