Download - OSPF Basic Theory
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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