IP Routing Principles

Network-Layer Protocol Operations

• Each router provides network layer (routing) services

XY

AA

BB

CC

ApplicationPresentationSessionTransportNetworkData LinkPhysical

ApplicationPresentationSessionTransportNetworkData LinkPhysical

A B C

X Y

NetworkData LinkPhysical

NetworkData LinkPhysical

NetworkData LinkPhysical

Static versus Dynamic Routes

• Static route

Uses a protocol route that a network administrator enters into the router

• Dynamic route

Uses a route that a network routing protocol adjusts automatically for topology or traffic changes

Static Route Example

• Fixed route to address reflects administrator’s knowledge

Point-to-point orcircuit-switched

connection

“Stub” Network

Only a single network connection with no need

for routing updates BB

AA

Default (static) Route Example

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

Company X Internet

10.0.0.0

172.34.56.0

Routing TableRouting Table

No entry for destination netTry router B default route

No entry for destination netTry router B default route

CCBB

AA

Dynamic routing

Adapting to Topology Change

AA BB

CCDD

Adapting to Topology Change

• Can an alternate route substitute for a failed route?

XXAA BB

CCDD

Adapting to Topology Change

• Can an alternate route substitute for a failed route?

XXAA BB

CCDD

Dynamic Routing Operations

• Routing protocol maintains and distributes routing information

A router passes routing information to its neighbors

RoutingTable

RoutingTable

NetworkNetwork Routing ProtocolRouting Protocol

Dynamic Routing Operations

• Routing protocol maintains and distributes routing information

A router passes routing information to its neighbors

RoutingTable

RoutingTable

RoutingTable

RoutingTable

NetworkNetwork Routing ProtocolRouting Protocol Routing ProtocolRouting Protocol

Representing Distance with Metrics

• Information used to select the best path for routing

56T1

A

B

56

T1

Hop count

Ticks

Cost

Hop count

Ticks

Cost

Representing Distance with Metrics

• Information used to select the best path for routing

56T1

A

B

56

T1

Bandwidth

Delay

Load

Reliability

MTU

Bandwidth

Delay

Load

Reliability

MTU

Hop count

Ticks

Cost

Hop count

Ticks

Cost

Classes of Routing Protocols

Distance VectorDistance Vector

Hybrid RoutingHybrid Routing

Link StateLink StateCC

DD

BB

AA

CC

DD

BB

AA

• 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

One Issue: Time to Convergence

Distance Vector Concept

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

CC

DD

BB

AA

CC BB AADD

RoutingTable

RoutingTable

RoutingTable

RoutingTable

RoutingTable

RoutingTable

RoutingTable

RoutingTable

Link-State Concept

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

Link-State Packets

SPFAlgorithm

TopologicalDatabase

Shortest Path First Tree

RoutingTable

RoutingTable

CC AA

DD

BB

Comparing Distance Vector Routing to Link-State Routing

Link-State

Gets common view of entire network topology

Calculates the shortest path to other routers

Event-triggered updates: faster convergence

Passes link-state routing updates to other routers

Distance Vector

Views net topology from neighbor’s perspective

Adds hops from router to router

Frequent, periodic updates: slow convergence

Passes copies of routing table to neighbor routers

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

Hybrid Routing

Choose arouting path based on distance vectors

Converge rapidly using change-based

updates

Choose arouting path based on distance vectors

Converge rapidly using change-based

updates

Balanced Hybrid RoutingBalanced Hybrid Routing

Static Routing

• Advantages:predictable

no additional CPU overhead

no additional network overhead

easy to configure

• Disadvantages:does not scale

hard to maintain

does not adapt to network changes

Dynamic Routing

• Advantages:scalability

adaptability

• Disadvantages:increased complexity

increased bandwidth overhead

increased resource usage

Dynamic Routing

• Moral of the story:

Use static routing where you can,use dynamic routing only where you must

Autonomous Systems

• Internetwork

• The big “I” Internet

• AS = a group of routers and their networks (administered by the same owner)

• AS peering

Routing Protocols

• Where is it used?

Interior Gateway Protocols (IGP)

RIP

OSPF

EIGRP

(IS-IS)

Exterior Gateway Protocols (EGP)

BGP

Routing Protocols

• How does it work?

Distance-Vector Protocols

RIP

EIGRP

BGP

Link State

OSPF

Protocol Features

• RIPv2

hop count metric

unreliable transport

passive RIP

simple

Protocol Features

• OSPF

full CIDR support

trust

route injection

hierarchical routing

a smarter protocol

Protocol Features

• EIGRP

more information in advertisements

improved convergence properties

partial and incremental updates

no support for areas

Protocol Features

• BGP

BGP-4: RFC 1771

created to support CIDR

even more information

Policy routing

Reliable transport

can also be used as an IGP (IBGP)

high overhead

Real World Routing

• multiple routing protocols are generally necessary

• interior routing is a LOT different than exterior routing

• multi-protocol issues

Implementation Considerations

172.16

RIP

172.16

EIGRP

RIP

172.16.0.0

AS 300EIGRP

ASBR

ASBR

172.16

RIP

172.16

EIGRP

Routing feedbackSuboptimal path selectionRouting loops

Incompatible routing informationInconsistent convergence time

Controlling Routing Update Traffic

How can we preventrouting update traffic from crossing some of these links?

172.16.7.1

172.16.7.2

172.16.6.1172.16.6.2

172.16.5.2

172.16.5.1172.16.4.1

172.16.4.2

172.16.1.1 172.16.1.2

172.16.3.1

172.16.3.2172.16.2.2

172.16.2.1

172.16.9.1 172.16.10.1

172.16.11.1

TransTrans

R200R200 CenCen

R300R300 R100R100

RemRem

172.16.12.1

64Kb

64Kb

T-1

T-1Frame Relay

•Different protocols use different metrics

•Metrics are difficult to compare algorithmically

Defining Distance

•Different protocols use different metrics

•Metrics are difficult to compare algorithmically

•Therefore, need a selection process:

1—Which protocol do you believe the most?

2—Then decide which metric is the best

Defining Distance

What Protocol to Believe?

Connected Interface Connected Interface 00

Static RouteStatic Route 11

Enhanced IGRP Summary RouteEnhanced IGRP Summary Route 55

External BGPExternal BGP 2020

Internal Enhanced IGRPInternal Enhanced IGRP 9090

IGRPIGRP 100100

OSPFOSPF 110110

IS-IS IS-IS 115115

RIP RIP 120120

EGPEGP 140140

External Enhanced IGRPExternal Enhanced IGRP 170170

Internal BGPInternal BGP 200200

UnknownUnknown 255255

Route SourceRoute Source Default DistanceDefault Distance

Routers!

• Cisco CRS-1 IP/MPLS Router92 Tbps total switching capacity

1152 x OC-768c/STM-256c (40Gbps) Line cards

707 Kg, 15 KW per chassis

IPv4, IPv6, MPLS

Routers!

• Juniper T640 Internet Routing Node640 Gbps total switching capacity

770 Million packet per second forwarding

40 Gbps per slot (4 * OC-192c, 1 * OC-768c)

8 slots per rack

Routers!

• Foundry NetIron 1500 Internet Router

480 Gbps total switching capacity

178 Million packet per second forwarding

10 Gbps per slot (1 * 10G Ethernet)

15 slots per rack