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