OSPF Theory

3. How does OSPF work ?

Three Table Step

1. Router send Hello Message (Hello Protocol) to Build Neighbor Table

2. Only neighbors send LSAs to each other, build Topology Table

3. Router run SFP to calculate best-route, place its in to Routing Table

vs Distance Vector, only 1 table: Routing Table

Building Link-State Database -Topology Table for neighbors only

Seven OSPF Router States

Down, Init, 2Way, Exstart, Exchange, Loading, Full

Building OSPF Routing Table

Router run SPF to calculate best-route(lowest cost) and pla'ce it in routingtable ( coded "O")

4. OSPF: Other Information

Router Types

Backbone Router: Have at least 1interface connect to Area 0

Internal Router: like R5, R2, R1

ABR - Area Border Router

ASBR

Hello protocol

Send to 224.0.0.5

To become a neighbor, two route musthave Hello/dead-interval, Area ID,Authentication the same

Area TypesArea 0: back-bone Area

Others: Normal Area, have to connect to Area 0Stub Area

NSSA

Basic LSAs Types

LSA1, 2 within Area

LSA3: summary routes of an area, sendto other area, run in ABR

LSA4: run in ABR, send summary routefrom ASBR to other area

LSA5: send routes from other AS

2. OSPF Design

Area Design(Hierarchical Model)

Single AreaMust be area 0

Use for small network

Multi-Area

Use for huge network

LSAs processing confined to an area, wehave some LSAs types

Small routing table (if summarization isused - for areas)

Area Types

Router Types

LSA Types

think about Distance Vector: it is FLAT design

Router ID

The number by which the routes is know to OSPF

Can set manually using router-id command

If router-id is not configure, the highest IP address on the activeloopback interface (if not have loopback active, using physicalactive interface) at the moment of OSPF process startup isselected as a router id

Five Type of Packet

Hello : for Building Neighbor Table

DBD: Database Description

LSR : Link-state Request, for requesting routing information

LSU : Link-state Update, it is contained in LSAs

LSAck : Like ACK in TCP, confirm what they received

Multi Access Network(Ethernet)

Electing Design Router (DR) and BackupDR (BDR) in Multi-access network,remain routers named OTHER

DR is Router have highest "ospfpriority", default = 1, max = 255, if ospfpriority = 0 then router is OTHER only

"ospf priority" can be set manually usingcommand "ip ospf priority"

If 2 router have the same "ospf priority",Router have highest router-id will be DR

The second-highest will be BDR

OTHER router is only full adjacency(FULL State) with DR, BDR

OTHER will be 2Way state with other OTHER

OTHER router send LSAs Type 1 to DRand BDR (ip dest = 224.0.0.6)

DR send LSA Type 2 to all (ip dest = 224.0.0.5)

1. OSPF Overview

What's OSPF ?

Open Shortest Path First (OSPF) is a link-state routing protocol forInternet Protocol (IP) networks. It uses a link state routing algorithm andfalls into the group of interior routing protocols, operating within a singleautonomous system (AS)

Why's OSPF ?

History

RIP: for small network

EIGRP: for large network, Cisco device only

OSPF: for large network, for any vendor

huge network, multi-vendor devices:OSPF is best routing solution

Link-state vs Distance Vector

The basic concept of link-state routing is that every node constructs amap of the connectivity to the network , in the form of a graph, showingwhich nodes are connected to which other nodes. Each node thenindependently calculates the next best logical path from it to everypossible destination in the network. The collection of best paths will thenform the node's routing table .

Distance-vector routing protocols, which work by having each node shareits routing table with its neighbors. In a link-state protocol the onlyinformation passed between nodes is connectivity related.

OSPF general parameters

AD = 110

Classless, VLSM

Trigger update and 30 minute period update

Multicast update (better than broadcast update)

routing information update: LSAs (link-stateadvertisement), LSA contain LSU - Link-state Update

Metric: Cost = 10^8/BWconfig

Use Layer 4: OSPF, protocol number 89, not TCP, UDP

Training

OSPF Basic Theory-1.mmap - 26/06/2013 - Mindjet