Link-State Routing ProtocolsRouting Protocols and Concepts – Chapter 10 pps

– Link-state routing protocols are more like a road map because they create a topological map of the network and because they create a topological map of the network and each router uses

Link-State Routing

Protocols

Routing Protocols and Concepts – Chapter 10

routing protocols.

– Distance vector routing protocols are like road signs because routers must make preferred path decisions based

because routers must make preferred path decisions based

on a distance or metric to a network

– Link-state routing protocols are more like a road map because they create a topological map of the network and

because they create a topological map of the network and each router uses this map to determine the shortest path to each network

The ultimate objective is that every router receives all of the

– The ultimate objective is that every router receives all of the link-state information about all other routers in the routing area With this link-state information, each router can create its own topological map of the network and independently

its own topological map of the network and independently calculate the shortest path to every network

ƒ List the benefits and requirements of link-state routing

protocols.

Link-State Routing

ƒ Link state routing protocols

ƒ Link state routing protocols

-Also known as shortest path first algorithms -These protocols built around Dijkstra’s SPF

ƒ Dikjstra’s algorithm also known as the shortest path first (SPF) algorithm

–This algorithm accumulates costs along each path, from source to destination

Link-State Routing

ƒ The shortest path to a destination is not necessarily the path with the least number of hops

1 Each routers learns about its own directly connected networks

1 Each routers learns about its own directly connected networks

– interface is in the up state

2 Each router is responsible for meeting its neighbors on directly

t d t kconnected networks

– exchange hello packet to other directly connected link state routers.

3 Each router builds a Link-State Packet (LSP) containing the state of ( ) geach directly connected link

– recording all the pertinent information about each neighbor, including neighbor ID, link type, and bandwidth.

4 Each router floods the LSP to all neighbors, who then store all LSPs received in a database

– Each router stores a copy of each LSP received from its neighbors in

to determine the best path to each network

ƒ Link state routing protocols use a hello protocol

ƒ Link state routing protocols use a hello protocol

Purpose of a hello protocol:

-To discover neighbors (that use the same link state routing protocol) on its link

Link-State Routing:

ƒ Connected interfaces that are

Link-State Routing:

step 2 - Sending Hello Packets to Neighbors

ƒ Connected interfaces that are

using the same link state

routing protocols will exchange

routing protocols will exchange

hello packets.

ƒ Once routers learn it has

ƒ Once routers learn it has

neighbors they form an

adjacency

– 2 adjacent neighbors will

exchange hello packets

– These packets will serve as a

keep alive function

ƒ Contents of LSP:

ƒ Contents of LSP:

– State of each directly connected link

– Includes information about

neighbors such as neighbor ID link

neighbors such as neighbor ID, link

type, & bandwidth.

ƒ A simplified version of the LSPs from

Link-State Routing:

ƒ Once LSP are created they are

Link-State Routing:

step 4 - Flooding LSPs to Neighbors

ƒ Once LSP are created they are

forwarded out to neighbors.

–Each router floods its link-state ac ou e oods s s a e

information to all other link-state

routers in the routing area

Whenever a router receives an LSP

–Whenever a router receives an LSP

from a neighboring router, it

immediately sends that LSP out all

other interfaces except the interface

other interfaces except the interface

that received the LSP

–This process creates a flooding effect p g

of LSPs from all routers throughout

the routing area

LSP t t d th f ll i diti

– Initial router start up or routing process

– When there is a change in topology

• including a link going down or coming up, or a neighbor adjacency being established or brokenj y g

Link-State Routing:

ƒ Routers use a database to

Link-State Routing:

step 5 - Constructing a link state data base

ƒ Routers use a database to

construct a topology map of the

network

–After each router has propagated its

own LSPs using the link-state

flooding process each router will

flooding process, each router will

then have an LSP from every

link-state router in the routing area

–These LSPs are stored in the

link-state database

–Each router in the routing area canEach router in the routing area can

now use the SPF algorithm to

construct the SPF trees that you saw

router R1 has learned the link-state

information for each router in its

p

Link-State Routing:

ƒ Process begins by examining R2’s LSP information

Link-State Routing:

Example - How R1 constructs its SPF tree.

Process begins by examining R2 s LSP information

–R1 can ignore the first LSP, because R1 already knows that it is

connected to R2 on network 10.2.0.0/16 with a cost of 20

–R1 can use the second LSP and create a link from R2 to another

router, R5, with the network 10.9.0.0/16 and a cost of 10 This

information is added to the SPF tree

Using the third LSP R1 has learned that R2 has a network

–Using the third LSP, R1 has learned that R2 has a network

10.5.0.0/16 with a cost of 2 and with no neighbors This link is

added to R1's SPF tree

ƒ Process begins by examining R3’s LSP information Process begins by examining R3 s LSP information

–R1 can ignore the first LSP, because R1 already knows that it is

connected to R3 on network 10.3.0.0/16 with a cost of 5

–R1 can use the second LSP and create a link from R3 to the

router R4, with the network 10.7.0.0/16 and a cost of 10 This

information is added to the SPF tree

Using the third LSP R1 has learned that R3 has a network

–Using the third LSP, R1 has learned that R3 has a network

10.6.0.0/16 with a cost of 2 and with no neighbors This link is

added to R1's SPF tree

Link-State Routing:

ƒ Process begins by examining R4’s LSP information

Link-State Routing:

Example - How R1 constructs its SPF tree.

Process begins by examining R4 s LSP information

– R1 can ignore the first LSP because R1 already knows that it is

connected to R4 on network 10.4.0.0/16 with a cost of 20

– R1 can also ignore the second LSP because SPF has already learned R1 can also ignore the second LSP because SPF has already learned

about the network 10.6.0.0/16 with a cost of 10 from R3

– However, R1 can use the third LSP to create a link from R4 to the router

R5, with the network 10.10.0.0/16 and a cost of 10 This information is

dd d t th SPF t

added to the SPF tree

– Using the fourth LSP, R1 learns that R4 has a network 10.8.0.0/16 with a cost of 2 and with no neighbors This link is added to R1's SPF tree.

ƒ Process begins by examining R5’s LSP informationProcess begins by examining R5 s LSP information

–R1 can ignore the first two LSPs (for the networks 10.9.0.0/16 and 10.10.0.0/16), because SPF has already learned about these links and added them to the SPF tree

–R1 can process the third LSP learning that R5 has a network

10.11.0.0/16 with a cost of 2 and with no neighbors This link is

added to the SPF tree for R1

Link-State Routing

ƒ Determining the shortest path

– The shortest path to a destination

determined by adding the costs & finding the

O th SPF l ith h

ƒ Once the SPF algorithm has

determined the shortest path

routes, these routes are placed in

routes, these routes are placed in

the routing table.

ƒ The routing table will also include The routing table will also include

all directly connected networks

and routes from any other

sources, such as static routes

Packets will now be forwarded

according to these entries in the

according to these entries in the

routing table.

Link-State Routing Protocols

Advantages of a Link State Routing Protocol

Routing

protocol

Builds Topological map

Router can independently determine the shortest path

Convergence

Event driven routing updates

Use

of LSP

to every network.

Distance No No Slow Generally No No

t l protocols

ƒ Builds a Topological Map

• Link-state routing protocols create a topological map, or SPF tree of the network topology

• Using the SPF tree each router can independently determine the shortest path to every network Using the SPF tree, each router can independently determine the shortest path to every network.

• Distance vector routing protocols do not have a topological map of the network

• Routers implementing a distance vector routing protocol only have a list of networks, which includes the cost (distance) and next-hop routers (direction) to those networks

• After the initial flooding of LSPs, link-state routing protocols only send out an LSP when there is a change

in the topology The LSP contains only the information regarding the affected link

• Unlike some distance vector routing protocols, link-state routing protocols do not send periodic updates.

ƒ Hierarchical Design

• Link-state routing protocols such as OSPF and IS-IS use the concept of areas Multiple areas create a hierarchical design to networks allowing for better route aggregation (summarization) and the isolation of hierarchical design to networks, allowing for better route aggregation (summarization) and the isolation of routing issues within an area

Link-State Routing Protocols

– This should only occur during initial startup of routers, but can also be an issue on unstable networks

i i i h ff CPU d

minimize the effects on memory, CPU, and

bandwidth

• The use and configuration of multiple areas can reduce

the size of the link-state databases Multiple areas can

the size of the link state databases Multiple areas can

also limit the amount of link-state information flooding in

a routing domain and send LSPs only to those routers

that need them.

• For example when there is a change in the topology For example, when there is a change in the topology,

only those routers in the affected area receive the LSP

and run the SPF algorithm

• This can help isolate an unstable link to a specific area

in the routing domain

in the routing domain

ƒ In the figure, If a network in Area 51 goes down, the

LSP with the information about this downed link is

only flooded to other routers in that area.

only flooded to other routers in that area

• Routers in other areas will learn that this route is down,

but this will be done with a type of link-state packet that

does not cause them to rerun their SPF algorithm

Note: Multiple areas with OSPF and IS-IS are discussed in

Link-State Routing Protocols

ƒ 2 link state routing protocols used for routing IP

ƒ 2 link state routing protocols used for routing IP

-Open Shortest Path First (OSPF) -Intermediate System-Intermediate System (IS-IS)

ƒ Link State Routing protocols are also known as

Shortest Path First protocols

ƒ Summarizing the link state process

-Routers 1ST learn of directly connected networks Routers 1ST learn of directly connected networks -Routers then say “hello” to neighbors

-Routers then build link state packets -Routers then flood LSPs to all neighbors Routers use LSP database to build a network topology -Routers use LSP database to build a network topology map & calculate the best path to each destination

-Cost associated with link -Neighboring routers on the link

ƒ Link State Packets

After initial flooding, additional LSP are sent out when a change in topology occurs

ƒ Examples of link state routing protocols

-Open shortest path first -IS-IS