distance vector linkstate

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DISTANCE VECTORROUTING



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Distance vector algorithm

• Routing algorithms pass periodic copies of a routing table from router to router.

• Each router receives a routing table from

its directly connected neighbors.• Distance-vector algorithms do not allow a

router to know the exact topology of an

internetwork.



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Distance vector concepts



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Exchange routing tables



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Topology changes

• When the topology in a distance-vector protocol network changes, routing tableupdates must occur.

• Topology change updates proceed step-by-step from router to router.

• Distance-vector algorithms call for each

router to send its entire routing table toeach of its adjacent neighbors.



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Updating process



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Routing Loop

Network 1

unreachable Network 1

Distance 3

• A stop routing packets to Net1



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Counting to Infinity

Network 1

Distance 3

Network 1

Distance 4

Network 1

Distance 5



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The problem of routing loops

• Routing loops can occur if a network'sslow convergence on a new configurationcauses inconsistent routing entries.

• Solutions: – Defining a Maximum

– Split Horizon

–

Split Horizon with Poison Reverse – Route Poisoning

– Hold-down Timers



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Loop prevention: Defining a

Maximum

• Distance-vector protocols define infinity (vô tận)as a specific maximum number.

•

This number refers to a routing metric (e.g. asimple hop count).

• The routing protocol permits the routing loopto continue until the metric exceeds its

maximum allowed value.



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Loop prevention: Defining a

Maximum

MaximumMetric is 16

Network 1

unreachable

Network 1

Distance 14

Network 1

Distance 15



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Loop prevention: Split horizon•

A routing loop occurs when incorrectinformation that has been sent back to a

router contradicts (mâu thuẩn) the correct

information that it sent.

• The routing protocol advertises routes out an

interface only if they were not learned from

updates entering that interface.



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Loop prevention: Split horizon

Network 1

Distance 1

Network 1

unreachable

Network Router Distance

1 C 2

Network Router Distance

1 A 3



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Loop prevention: Poison

reverse

• The routing protocol advertises all routes out

an interface, but those learned from earlier

updates coming in that interface are markedwith infinite distance metrics.



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Loop prevention: Poison

reverse

Network 1

unreachable

Network 1

Distance 16



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Loop prevention: Route

poisoning

• When a distance vector routing protocol

notices that a particular route is no longer

valid, it advertises that route, but with an

infinite metric, signifying that the route is

bad.



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Loop prevention: Route

poisoning

Network 1

Distance 16



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Loop prevention: Hold-down

timer • After knowing that a route to a subnet has

failed, a router waits a certain period of time

before believing any other routing

information about that subnet.



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Loop prevention: Hold-down

timer

Network 1

unreachable

Network 1

Distance 2



LINK STATE ROUTING



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Link state routing basics

• Link-state based routing algorithms, also knownas SPF (shortest path first) algorithms, maintaina complex database of topology information.

•

Link-state routing algorithm maintains fullknowledge of distant routers and how theyinterconnect.

• Link-state routing uses: –

Link-state advertisements (LSAs) – A topological database

– The SPF algorithm, and the resulting SPF tree

– A routing table of paths and ports to each network



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Link state concepts



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Exchange routing tables

• Routers exchange LSAs with each other.

• The router constructs this logical topologyas a tree, with itself as root.

• The SPF algorithm computes networkreachability. It then sorts these pathsshortest path first (SPF).

•The router lists its best paths, and theports to these destination networks, in therouting table.



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Link state network discovery



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Topology changes

• Whenever a link-state topology changes,the routers first send information to allrouters in the internetwork.

• Each time an LSA packet causes achange to the link-state database, theshortest path first algorithm (SPF)recalculates the best paths and updatesthe routing table.



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Link state update

• Keeps track of its neighbors: name, status,and the cost.

• Constructs an LSA packet that lists its

neighbor router names and link costs.• Sends out this LSA packet so that all other

routers receive it.

•When it receives an LSA packet, recordsthe LSA packet in its database.

• Completes a map and then computesroutes by the SPF algorithm.



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LS concerns: Processing and

Memory• Network administrators must ensure that

the routers are capable of providing thesenecessary resources.

• For link-state routing, their memory mustbe able to hold information from variousdatabases, the topology tree, and therouting table.

• Using Dijkstra's algorithm to compute theSPF requires a processing task.



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LS concerns: Bandwidth

• During the initial discovery process, allrouters using link-state routing protocolssend LSA packets to all other routers.

• After that, link-state routing protocolsgenerally require only minimal bandwidthto send infrequent (không thường xuyên) or event-triggered LSA packets that reflect (mang lại)topology changes.



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Link state updates



THE CONTEXT OF

DIFFERENT ROUTING

PROTOCOLS



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Distance Vector vs. Link State

routing



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Hybrid routing protocols

• Examples:

– IS-IS (OSI)Intermediate System-

to-Intermediate System – EIGRP (Cisco)

Enhanced Interior Gateway Routing

Protocol



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Hybrid routing protocols

• Combines aspects of both distance-vector and link-state routing.

• Use distance vectors with more accurate

metrics to determine the best paths todestination networks.

• Using topology changes to trigger routing

database updates.• Using fewer resources such as bandwidth,

memory, and processor overhead.



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LAN-to-LAN routing

• The network layer must understand andbe able to interface with various lower layers.

• Routers must be capable of seamlesslyhandling packets encapsulated intovarious lower-level frames withoutchanging the packets' Layer 3 addressing.

• Although the lower-layer framing mustchange when the router passes packettraffic, the Layer 3 addressing for source

and destination remains the same.



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LAN-to-LAN routing



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LAN-to-WAN routing

• The network layer must interface with

various lower layers for LAN-to-WAN

traffic.

• The path taken by a packet may encounter

several relay points and a variety of data

link types.

• Routers enable packet flow by keeping thelayer 3 source and destination addresses

constant while encapsulating the packet in

data link frames.



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LAN-to-WAN routing



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Multiple protocols and media



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Summary

• Functions of the layer 3 include network

addressing and best path selection.

• Routed protocols allow routers to direct

user traffic.

• Routing protocols work between routers to

maintain routing tables.

• DVR, routers exchange of routing tables.

• LSR, routers calculate the shortest paths.

•

Hybrid routing uses both LSR and DVR.



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distance vector linkstate

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