lecture 10- routing part iii

8/2/2019 Lecture 10- Routing Part III

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LINK STATE ROUTINGLINK STATE ROUTINGLINK STATE ROUTINGLINK STATE ROUTING

Link state broadcast node learns about the path costs from its

neighbors

Inform the neighbors whenever the link cost changes

hence the name link state

Each router does the following (repeatedly):

discover neighbors, particularly, learn their networkaddressesA router learns about its neighbours by sending a special

HELLO packet to each point-to-point line. Routers on the otherend send a reply.

measure the cost to each neighbore.g. by exchanging a series of packetssending ECHO packets and measuring the average round-trip-

time

construct a link state packets

send this packet to all other routers by floodingcompute locallythe shortest path to every other router

when this information is receivedInformation will be shared when there is a change.

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Concept of Link State Routing

Cost in Link State Routing

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Link State Packet

Flooding of As LSP

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Link State Database

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The Dijkstra AlgorithmThe Dijkstra AlgorithmThe Dijkstra AlgorithmThe Dijkstra Algorithm

All the routers applies this algorithm to its link state database for

finding the shortest path between two points on a network using a

graph made up of nodes and arcs.

Nodes are of two types : networks and routers.

Arcs are connections between a router and a network(router to

network and network to router)

Cost is applied only to the arc from router to network.

The cost of the arc from network to router is always zero.

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Shortest Path Tree Dijkstra algorithm follows the following four steps what is

called the shortest path tree(routing table) for each router:

The algorithm begins to build the tree by identifying its root. The

root of each routers tree is the router itself. The algorithm then

attaches all nodes that can be reached from that root(neighbor

nodes).

The algorithm compares the trees temporary arcs and identifies

the arc with the lowest cumulative cost. This arc and node towhich it connects are now a permanent part of the shortest path

tree.

The algorithm examines the database and identifies every node

that can be reached from its chosen node.

The last two steps are repeated until every node in the network

has become a permanent part of the tree

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!

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Example of formation of shortest path tree

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Step 1 Step 3Step 2

Step 5

Contd.

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Step 6 Step 7

Step 8Step 9

Contd

Step 10 Step 11

Step 12Step 1351

Link State Routing table for Router A

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Distance Vector Vs Link State Distance Vector Protocol :

Entire routing table is sent as an update

Distance vector protocol send periodic update at every 30 or 90 second

Update are broadcasted

Updates are sent to directly connected neighbor only

Routers don't have end to end visibility of entire network.

It is proned to routing loops

Routing loop avoidance Mechanism used are as below :1) Max Hop Count 2) Split horizon 3) Route poisoning 4) Hold down Timer

Distance vector routing protocol has slow convergence due to periodic update.

Eg. RIP ,IGRP , BGP .

Link state protocol :

Updates are incremental & enitire routing table is not sent as update

Updates are triggered not periodic

Updates are multicasted

Update are sent to entire network & to just directly connected neighbor

Updates are carry SPF tree information & SPF cost Calculation information of entire

topology

Routers have visibility of entire network of that area only.

No routing loops

Convergence is fast because of triggered updates.

Eg. : OSPF , IS-IS53


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ROUTING Vs ROUTED Protocols

A routing protocol is the communication usedbetween routers. A routing protocol allows one router

to share information with other routers regarding the

networks it knows about as well as its proximity to

other routers. The information a router gets from

another router, using a routing protocol, is used to

build and maintain a routing table.

Examples of routing protocols are:

Routing Information Protocol (RIP)

Interior Gateway Routing Protocol (IGRP)

Enhanced Interior Gateway Routing Protocol (EIGRP)

Open Shortest Path First (OSPF)

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A routed protocol is used to direct user traffic. A routed

protocol provides enough information in its network

layer address to allow a packet to be forwarded from

one host to another based on the addressing scheme.

Examples of routed protocols are:

Internet Protocol (IP)

Internetwork Packet Exchange (IPX)

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Routing in the Internet

What would happen if hundreds of millions of routers

execute the same routing algorithm to compute routing

paths through the network?

Scale

large overhead

enormous memory space in the routers

no bandwidth left for data transmission

would DV algorithm converge?

Administrative autonomy

an organization should run and administer its networks as it

wishes but must be able to connect it to the outside networks

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Most routers use dynamic routingAutomatically build the routing tables

AS = Autonomous System

An autonomous system (AS) is a collection of networks

under a common administration sharing a commonrouting strategy

To the outside world, an AS is viewed as a single entity.

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The Internet uses hierarchical routing

it is split into Autonomous Systems(AS)

routers at the border: gateways

gateways must run both intra & inter AS routing protocols

routers within AS run the same routing algorithm

the administrator can chose any Interior Gateway Protocol

Routing Information Protocol(RIP)

Open Shortest Path First (OSPF)

between AS gateways use Exterior Gateway Protocol

Border Gateway Protocol (BGP)

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Routing in an AS

IRP = Interior Routing Protocol

Also IGP ; Interior Gateway Protocol

Passes routing information between routers within AS

Can use routing metric, e.g. hop count or administrative

cost

ERP = Exterior Routing ProtocolAlso EGP; Exterior Gateway Protocol

Passes routing information between routers across AS

May be more than one AS in internet

Routing algorithms and tables may differ between

different AS

Finds a path, but cant find an optimal path since it cantcompare routing metrics via multiple AS

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Application of IRP and ERP

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RIP

Routing Information Protocol is an interior routingprotocol used inside an autonomous system. It is a very

simple protocol based on distance vector routing.

OSPF

Open Shortest Path First (OSPF) protocol is another

interior routing protocol that is gaining popularity. OSPF

uses Link State routing to update the routing tables in

an area.

BGP

Border Gateway Protocol(BGP) is an inter-autonomous

system routing protocol. It is based on a routing method

called Path vector routing.

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Autonomous system

gateways (R1, R2, R3, R4) use both interior & exterior routing

other routers use only interior routing

Note: AS routing protocols in A, B, C & D not need to be the same!

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network

router

gateway

BGP

BGP

BGPRIP &

OSPF

A B

C

D

H2

H1

The gateways are exit points

routers use default routing

each router knows all netids within AS

packets destined to another AS are sent to the default

router

default router is the border gateway to the next AS

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Why are Distance Vector Routing & Link State Routing

not good candidates?

route with the smallest hop count not the preferred one

AS not secure

Distance Vector Routing : only a number of hops known

to a destination, not the path to get there

Link State Routing : Internet too big for this routing

method

huge databases

long time to run Dijsktras algorithm

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Path Vector Routing

The principle of path vector routing is similar to distancevector routing

In path vector routing, we assume that there is one node(in practice there can be more) in each autonomoussystem that acts on behalf of the entire autonomoussystem. Let us call it the speaker node

The speaker node in an AS creates a routing table andadvertises it to speaker nodes in the neighboring Ass

A speaker node advertises the path, not the metric of thenodes, in its AS or other ASs

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Example I:

Initial routing tables in path vector routing

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Stabilized tables for three autonomous systems

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Example IIAt the beginning, each speaker node can know only the

reachability of nodes inside its autonomous system

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A speaker in an AS shares its table with immediateneighbors.

When a speaker node receives information from aneighbor, it updates its own table by adding thenodes that are not in its routing table and addingits own AS and the AS that sent the table

After a while each speaker has a table and knowshow to reach each node in other ASs

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Aggregation

The path vector routing protocols normally supportCIDR notation and the aggregation of addresses

Note that a range may also include a block thatmay not be in the corresponding AS

However, if this network exists in some other Ass,it eventually becomes part of the routing table dueto the longest prefix principle

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Internet inter-AS routing: BGP

BGP (Border Gateway Protocol): the de factostandard

BGP provides each AS a means to:1. Obtain subnet reachability information from neighboring

ASs.

2. Propagate reachability information to all AS-internal

routers.3. Determine good routes to subnets based on

reachability information and policy.

allows subnet to advertise its existence to rest ofInternet: I am here

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BGP Sessions

The exchange of routing information between 2 routers

using BGP takes place in a session

A session is a connection that is established between 2BGP routers only for the sake of exchange routing

information

The create a reliable environment, BGP uses the service of

TCP. In other words, a session at the BGP level, as an

application program, is a connection at the TCP level.

When a TCP connection is created for BGP, it can last for a

long time, until something unusual happens.

For this reason, BGP sessions are sometimes referred to

as semi-permanent connections

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Internal and external BGP sessions

BGP can have 2 types of session: external BGP(E-BGP) and internal BGP (I-BGP) sessions

The E-BGP session is used to exchangeinformation between 2 speaker nodes belonging to2 different Ass

The I-BGP session is used to exchange routinginformation between 2 routers inside an AS.

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BGP does not use traditional Interior Gateway Protocol (IGP) metrics,but makes routing decisions based on path, network policies and/orrulesets. For this reason, it is more appropriately termed a reachabilityprotocol rather than routing protocol.

Path Vector Routing

A path: an ordered list of AS that a packet should travel through toreach the destination

Path information rather than cost information!

AS #s assigned by Internet Corporation for Assigned Names andNumbers (ICANN) regional registries

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CIDRized

destination

network address

(128.119.40/24)

offers control to the

administrator!

lecture 10- routing part iii

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