static versus dynamic routes static route uses a protocol route that a network administrators enters...

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Static versus Dynamic Routes Static Route Uses a protocol route that a network administrators enters into the router Dynamic Route Uses a route that a network routing protocol adjusts automatically for topology or traffic changes

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Static versus Dynamic Routes

Static Route

Uses a protocol route that a network

administrators enters into the router

Static Route

Uses a protocol route that a network

administrators enters into the router

Dynamic Route

Uses a route that a network routing protocol adjusts automatically for topology

or traffic changes

Dynamic Route

Uses a route that a network routing protocol adjusts automatically for topology

or traffic changes

Static Route Example

AA

BB

Point-to-point or circuit-switched

connection

Only a single network connection with no need

for routing update “Stub” Network

• Fixed route to address reflects administrator’s knowledge

Default Route Example

• Use if next hop is not explicitly listed in the routing table

AA BB CC192.34.56.0

10.0.0.0

Company X Internet

Routing Table

No entry for destination net

Try router B deafult route

Adapting to Topology Change

CC

BBAA

DD

X

• Can alternate route substitute for a failed route ?

Dynamic Routing Operations

• Routing protocol maintains and distributes routing information

NetworkNetwork Routing ProtocolRouting Protocol Routing ProtocolRouting Protocol

Routingtable

Routingtable

Routingtable

Routingtable

Representing Distance with Metrics

• Information used to select the best path for routing

BB

AA

E1

E1

64

64

Bandwidth

Delay

Load

Reliability

Bandwidth

Delay

Load

Reliability

Hop count

Ticks

Cost

Hop count

Ticks

Cost

Classes of Routing Protocols

CC

BBAA

DD

CC

BBAA

DD

Distance VektorDistance Vektor

Hybrid RoutingHybrid Routing

Link StateLink State

One Issue: Time to Convergence

Convergence occurs when all routers use a consistent perspective of network topologyConvergence occurs when all routers use a consistent perspective of network topology

After a topology changes, routers must recompute routes, which disrupts routingAfter a topology changes, routers must recompute routes, which disrupts routing

The process and time required for router reconvergence varies in routing protocolsThe process and time required for router reconvergence varies in routing protocols

Distance Vector Concept

• Pass periodic copies of routing table to neighbor routers and accumulate distance vectors

CC

BBAA

DD

DD CC BB AA

RoutingTable

RoutingTable

RoutingTable

RoutingTable

RoutingTable

RoutingTable

RoutingTable

RoutingTable

Distance Vector Network Discovery

BBAA CC

W X Y Z

Routing Table

X 0

Y 0

Z 1

W 1

Routing Table

X 0

Y 0

Z 1

W 1

Routing Table

Y 0

Z 0

X 1

W 2

Routing Table

Y 0

Z 0

X 1

W 2

• Routers discover the best path to destinations from each neighbor

Routing tábla

W 0

X 0

Y 1

Z 2

Routing tábla

W 0

X 0

Y 1

Z 2

Distance Vector Topology Changes

• Updates proceed step-by-step from router to router

BB AA

TopologyTopologyChangeChangeCausesCausesRoutingRoutingTableTableUpdateUpdate

Process to Update This Routing Table

Process to Update This Routing Table

Router ARouter ASends Sends

Out ThisOut ThisUpdatedUpdatedRoutingRouting

TableTable

Process to Update This Routing Table

Process to Update This Routing Table

Problem: Routing Loops

AA EE

DD

CC

BB

X1

Alternate Route:Network 1, Distance 3

Alternate Route:Network 1, Distance 3

Network 1, UnreachableNetwork 1, Unreachable

Alternate Route: Use ANetwork 1, Distance 4

Alternate Route: Use ANetwork 1, Distance 4

• Alternate routes, slow convergence, inconsistent routing

Network 1Down

Problem:Counting to Infinity

AA EE

DD

CC

BB

X1

Network 1, Distance 5Network 1, Distance 5

Network 1, Distance 7Network 1, Distance 7

Network 1, Distance 4Network 1, Distance 4

Network 1, Distance 6Network 1, Distance 6

• Routing loops increment the distance vector

Network 1Down

Routing TableMaximum metric is 16

Network 1 is Unreachable

Routing TableMaximum metric is 16

Network 1 is Unreachable

Solution: Defining a Maximum

AA EE

DD

CC

BB

X1

Network 1, Distance 12Network 1, Distance 12

Network 1, Distance 14Network 1, Distance 14

Network 1, Distance 15Network 1, Distance 15

Network 1, Distance 13Network 1, Distance 13

• Specify a maximum distance vector metric as infinity

Network 1Down

Network 1, unreachableNetwork 1, unreachable

Solution: Split Horizon

• If you learn a protocol’s route on an interface, do not send information about that route back out that interface

AA EE

DD

CC

BB

X1

Network 1Down

B:Do not update router Aabout routes to network 1

D: Do not update router Aabout routes to network 1

Network 1route to network 1has infinite Cost

Network 1route to network 1has infinite Cost

Solution: Route Poisoning

• Router keeps an entry for the network down state, allowing time for other routers to recompute for this topology change

AA EE

DD

CC

BB

X1

Network 1Down

Solution: Hold-Down Timers

• Routers ignore network update information for some period

AA EE

DD

CC

BB

?,X1

Network 1 DownNetwork 1 Down

Update afterHold-Down Time

Update afterHold-Down Time

Network 1Down

Update afterHold-Down Time

Update afterHold-Down Time

Update afterHold-Down Time

Update afterHold-Down Time

Update afterHold-Down Time

Update afterHold-Down Time

Link-State Concept

• After initial flood, pass small event-triggered link-state updates to all other routers

AA

DD

CC

BB

Link-State Packets

TopologicalDatabase

Shortest Path First Tree

RoutingTable

RoutingTable

SPFAlgorithm

Link-State Network Discovery

BBAA CC

W X Y Z

Link-State Packet

W 0

X 0

Link-State Packet

W 0

X 0

Link-State Packet

X 0

Y 0

Link-State Packet

X 0

Y 0

Link-State Packet

Y 0

Z 0

Link-State Packet

Y 0

Z 0

TopologicalDatabase

TopologicalDatabase

TopologicalDatabaseSPF SPF SPF

A RoutingTable

A RoutingTable B Routing

Table

B RoutingTable

C RoutingTable

C RoutingTable

SPF Tree SPF Tree SPF Tree

• Routers calculate the shortest path to destinations in paralell

Link-State Topology Changes

Topology Topology Change Change in Link- in Link- State State UpdateUpdate Process to

Update This Routing Table

Process to Update This Routing Table

• Update processes proceed using the same link-state update

Process to Update This Routing Table

Process to Update This Routing Table

Process to Update This Routing Table

Process to Update This Routing Table

Link-State Concerns

TopologicalDatabase

SPF

RoutingTable

RoutingTable

SPF Tree

• Processing and memory required for link-state routing

• Bandwidth consumed for initial link state „flood”

Problem: Link-State Updates

AA

DD

CC

BB

Network 1, Back Up NowNetwork 1, Back Up Now

Network 1, UnreachableNetwork 1, Unreachable

Network 1, UnreachableNetwork 1, Unreachable

Network 1, UnreachableNetwork 1, Unreachable

X,ok

Slow path update Slow path update arriveslast

Network 1goes down then comes up

Fast path updates arrive first

Which SPF tree to use for routing?

• Unsynchronized updates, inconsistent path decisions

Link-State Update Problems (cont.)

• Synchronizing large networks-which network topology updates are correct?

• Router startup-order of start alters the topology learned

• Partitioned regions-slow updating part separated from fast updating part

Solution: Link-State Mechanisms

• Reduce the need for resources

”Dampen” update frequncy

Target link-state updates to multicast

Use link-state area hierarchy for topology

Exchange route summaries at area borders

• Reduce the need for resources

”Dampen” update frequncy

Target link-state updates to multicast

Use link-state area hierarchy for topology

Exchange route summaries at area borders

• Coordinate link-state updates

Use time stamps

Update numbering and counters

Manage partitioning using an area hierarchy

• Coordinate link-state updates

Use time stamps

Update numbering and counters

Manage partitioning using an area hierarchy

Comparing Distance Vector Routing to Link-State Routing

Distance Vector Link-State Views net topology from Gets common view ofneighbor’s perspective entire network topology

Adds distance vectors Calculates the shortestfrom router to router path to other routers

Frequent, periodic updates: Event-triggered updates:slow convergence faster convergence

Passes copies of routing Passes link-state routing updatestable to neighbor routers to other routers

Distance Vector Link-State Views net topology from Gets common view ofneighbor’s perspective entire network topology

Adds distance vectors Calculates the shortestfrom router to router path to other routers

Frequent, periodic updates: Event-triggered updates:slow convergence faster convergence

Passes copies of routing Passes link-state routing updatestable to neighbor routers to other routers

Hybrid Routing

Choose a

routing path based

on distance vectors

Convergence rapidly using

change-based

updates

Balanced Hybrid RoutingBalanced Hybrid Routing

• Share attributes of both distance-vector and link-state routing

EIGRP