Static versus Dynamic Routes

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

Static Route Example Point-to-point or circuit-switched connection A Only a single network connection with no need for routing update B “Stub” Network • Fixed route to address reflects administrator’s knowledge

Default Route Example Internet Company X 192.34.56.0 A C B 10.0.0.0 Routing Table No entry for destination net Try router B deafult route • Use if next hop is not explicitly listed in the routing table

Adapting to Topology Change A B X C D • Can alternate route substitute for a failed route ?

Dynamic Routing Operations Network Routing Protocol Routing Protocol Routing table Routing table • Routing protocol maintains and distributes routing information

Representing Distance with Metrics 64 A Hop count Ticks Cost E1 64 Bandwidth Delay Load Reliability E1 B • Information used to select the best path for routing

Classes of Routing Protocols A B Distance Vektor C D Hybrid Routing A B Link State C D

One Issue: Time to Convergence Convergence occurs when all routers use a consistent perspective of network topology After a topology changes, routers must recompute routes, which disrupts routing The process and time required for router reconvergence varies in routing protocols

Distance Vector Concept A B C D D B C A Routing Table Routing Table Routing Table Routing Table • Pass periodic copies of routing table to neighbor routers and accumulate distance vectors

Distance Vector Network Discovery Y X Z W A B C Routing Table X 0 Y 0 Z1 W 1 Routing tábla W 0 X 0 Y1 Z 2 Routing Table Y 0 Z 0 X1 W 2 • Routers discover the best path to destinations from each neighbor

Distance Vector Topology Changes Process to Update This Routing Table Process to Update This Routing Table Router A Sends Out This Updated Routing Table Topology Change Causes Routing Table Update B A • Updates proceed step-by-step from router to router

Problem: Routing Loops B Network 1, Unreachable C A E 1 X D Alternate Route: Network 1, Distance 3 Network 1 Down Alternate Route: Use A Network 1, Distance 4 • Alternate routes, slow convergence, inconsistent routing

Problem:Counting to Infinity B Network 1, Distance 6 Network 1, Distance 7 C A E 1 X D Network 1, Distance 5 Network 1, Distance 4 Network 1 Down • Routing loops increment the distance vector

Solution: Defining a Maximum B Network 1, Distance 13 Network 1, Distance 14 C A E 1 X Network 1, Distance 15 D Network 1, Distance 12 Network 1 Down Routing Table Maximum metric is 16 Network 1 is Unreachable • Specify a maximum distance vector metric as infinity

Solution: Split Horizon B:Do not update router A about routes to network 1 B Network 1, unreachable C A E 1 X D Network 1 Down D: Do not update router A about routes to network 1 • If you learn a protocol’s route on an interface, do not send information about that route back out that interface

Solution: Route Poisoning Network 1 route to network 1 has infinite Cost B C A E 1 X D Network 1 Down • Router keeps an entry for the network down state, allowing time for other routers to recompute for this topology change

Solution: Hold-Down Timers Update after Hold-Down Time Update after Hold-Down Time B Network 1 Down C A E 1 Update after Hold-Down Time Update after Hold-Down Time ?,X D Network 1 Down • Routers ignore network update information for some period

Link-State Concept B C A Topological Database D Link-State Packets Routing Table SPF Algorithm Shortest Path First Tree • After initial flood, pass small event-triggered link-state updates to all other routers

SPF SPF SPF Link-State Network Discovery Y X Z W A B C Link-State Packet W 0 X 0 Link-State Packet X 0 Y 0 Link-State Packet Y 0 Z 0 Topological Database Topological Database Topological Database SPF Tree SPF Tree SPF Tree A Routing Table C Routing Table B Routing Table • Routers calculate the shortest path to destinations in paralell

Link-State Topology Changes Process to Update This Routing Table Process to Update This Routing Table Topology Change in Link- State Update Process to Update This Routing Table • Update processes proceed using the same link-state update

SPF Link-State Concerns • Processing and memory required for link-state routing Topological Database • Bandwidth consumed for initial link state „flood” SPF Tree Routing Table

Problem: Link-State Updates Slow path update arriveslast Slow path update Network 1, Unreachable Network 1, Unreachable B Which SPF tree to use for routing? C A X,ok Network 1 goes down then comes up Fast path updates arrive first D Network 1, Unreachable Network 1, Back Up Now • 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 • 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 of neighbor’s perspective entire network topology Adds distance vectors Calculates the shortest from 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 updates table to neighbor routers to other routers

Hybrid Routing EIGRP Choose a routing path based on distance vectors Convergence rapidly using change-based updates Balanced Hybrid Routing • Share attributes of both distance-vector and link-state routing

Static versus Dynamic Routes