RoutingRouting

Design, Design, Configuration, and Configuration, and TroubleshootingTroubleshooting

TopicsTopics

• Designing a LANDesigning a LAN• Designing a simple internetworkDesigning a simple internetwork• Designing IP addressing schemesDesigning IP addressing schemes• Configuring routers and hostsConfiguring routers and hosts• Troubleshooting routing protocols and Troubleshooting routing protocols and

addressing schemesaddressing schemes• Describing the communications processDescribing the communications process

• Packet destination addressPacket destination address• Neighbor routers to learn about remote networksNeighbor routers to learn about remote networks• Possible routes to all remote networksPossible routes to all remote networks• The best route to each remote networkThe best route to each remote network• How to maintain and verify the routing informationHow to maintain and verify the routing information

What Routers Need to Know to What Routers Need to Know to Route PacketsRoute Packets

C B AD

Basic Path SelectionBasic Path Selection• Routers consult tables stored in Routers consult tables stored in

memory to determine next hopmemory to determine next hop• The network portion of the IP address The network portion of the IP address

stored in the IP header is compared stored in the IP header is compared to network addresses stored in router to network addresses stored in router tablestables

• If no match is found a default If no match is found a default gateway is used to route the packetgateway is used to route the packet

Simple IP RoutingSimple IP Routing

172.16.1.0

BA

172.16.2.0

172.16.2.2

172.16.1.2172.16.2.1 172.16.1.1

e0 e0

>ping 172.16.1.2

B

172.16.3.1 172.16.3.2

s0s0

Host A

Host B

The IP Routing ProcessThe IP Routing Process• IP packets are created and sent out a IP packets are created and sent out a

default gatewaydefault gateway• Each intermediate router strips framing Each intermediate router strips framing

headers and trailers from packet and headers and trailers from packet and checks IP addresschecks IP address

• After consulting routing tables the packet After consulting routing tables the packet is reframed and sent to the next hopis reframed and sent to the next hop

• Once the destination network is reached, Once the destination network is reached, an ARP request identifies the MAC address an ARP request identifies the MAC address fo the destination computerfo the destination computer

Route TypesRoute Types• Static RoutesStatic Routes

– Route tables are manually configured by Route tables are manually configured by an administratoran administrator

• Dynamic RoutesDynamic Routes– A routing protocol is configured on the A routing protocol is configured on the

routerrouter– Routers then communicate with Routers then communicate with

neighboring routers to update table neighboring routers to update table informationinformation

Advantages of Static RoutingAdvantages of Static Routing

• No overhead on router CPUNo overhead on router CPU– No dynamic processing of route table No dynamic processing of route table

info requiredinfo required• No bandwidth usage between routersNo bandwidth usage between routers

– No communication needed to update No communication needed to update route tablesroute tables

• Adds securityAdds security– Administrator chooses network accessAdministrator chooses network access

Disadvantages of Static Disadvantages of Static RoutingRouting• Administrator must have a thorough Administrator must have a thorough

understanding of networks and routingunderstanding of networks and routing• Administrator must add new routes and Administrator must add new routes and

maintain all existing routes by handmaintain all existing routes by hand• Scaling is an issue – maintaining a large Scaling is an issue – maintaining a large

network becomes a full-time job for the network becomes a full-time job for the administratoradministrator

Default RoutesDefault Routes• Used to send packets with a remote Used to send packets with a remote

destination not in the routing table to destination not in the routing table to the next hop routerthe next hop router

• Used on in stub networksUsed on in stub networks– networks with only one exit path out of networks with only one exit path out of

the networkthe network

Routing Protocol BasicsRouting Protocol Basics• Routing protocols provide dynamic Routing protocols provide dynamic

route table updatingroute table updating• Two general classes existTwo general classes exist

– Distance Vector (RIP, IGRP)Distance Vector (RIP, IGRP)– Link State (OSPF, EIGRP)Link State (OSPF, EIGRP)

• Distance Vector protocols will be Distance Vector protocols will be discussed in this chapterdiscussed in this chapter

AS 1 AS2

IGPs: RIP, IGRP EGPs: BGP

Autonomous SystemsAutonomous Systems

• An autonomous system (AS) is a collection of networks under a common administrative domain.

• Interior Gateway Protocols (IGPs) operate within an AS.• Exterior Gateway Protocols (EGPs) connect different

ASes

Routing ClassesRouting Classes• ClassfulClassful

– Subnet mask not advertised with routesSubnet mask not advertised with routes– Standard classful masks are used by routersStandard classful masks are used by routers– RIP, IGRP are classfulRIP, IGRP are classful

• ClasslessClassless– masks are advertisedmasks are advertised– VLSM is supportedVLSM is supported– OSPF, EIGRP are classlessOSPF, EIGRP are classless

Administrative DistanceAdministrative Distance• A measure of trustworthiness for a routeA measure of trustworthiness for a route• For multiple routes to a remote networkFor multiple routes to a remote network

– the route with the lowest AD is placed in the route with the lowest AD is placed in routing tablerouting table

– if routes have same AD, other metrics can be if routes have same AD, other metrics can be used to determine best routeused to determine best route

– if routes have same AD and same metrics, if routes have same AD and same metrics, load balancing is implementedload balancing is implemented

Distance Vector ProtocolsDistance Vector Protocols• RIP, IGRPRIP, IGRP• Distance – number of hops to destinationDistance – number of hops to destination• Vector – direction (path) to destinationVector – direction (path) to destination• Entire routing table is advertised to Entire routing table is advertised to

neighboring routers for update purposesneighboring routers for update purposes– called “Routing by rumor” because validity called “Routing by rumor” because validity

of route tables is assumedof route tables is assumed

Distance Vector IssuesDistance Vector Issues• Periodic table advertisementPeriodic table advertisement• Slow ConvergenceSlow Convergence• Pinhole CongestionPinhole Congestion• Routing loopsRouting loops• Restricted metricsRestricted metrics• Load balancing techniquesLoad balancing techniques• Note: Severity depends upon specific Note: Severity depends upon specific

Distance Vector implementationDistance Vector implementation

Slow Convergence Routing Slow Convergence Routing LoopLoop

Pinhole CongestionPinhole Congestion

Distance Vector Problem Distance Vector Problem SolutionsSolutions• Maximum hop countMaximum hop count

– fix for count to infinity problemfix for count to infinity problem• Split horizonSplit horizon

– fix for routing loop problemsfix for routing loop problems• Route poisoningRoute poisoning

– fix for inconsistent updatesfix for inconsistent updates• HolddownsHolddowns

– prevent regular update messages from prevent regular update messages from reinstating problem routesreinstating problem routes

Discovering RoutesDiscovering Routes

Discovering RoutesDiscovering Routes

RIP OverviewRIP Overview• Hop count metric selects the path, Hop count metric selects the path,

16 is unreachable16 is unreachable• Full route table broadcast every 30 Full route table broadcast every 30

secondsseconds• Load balance maximum of 6 equal Load balance maximum of 6 equal

cost paths (default = 4)cost paths (default = 4)• RIPv2 supports VLSM and RIPv2 supports VLSM and

Discontiguous networksDiscontiguous networks

Router(config)#router rip Router(config-router)#network network-number*

network 172.16.0.0network 192.168.10.0

router RIPnetwork 172.16.0.0network 10.0.0.0

router RIP

172.16.10.0 192.168.10.010.3.5.0

RIP Routing ConfigurationRIP Routing Configuration

RIP Version 2RIP Version 2• Allows the use of variable length subnet masks Allows the use of variable length subnet masks

(VLSM) by sending subnet mask information with (VLSM) by sending subnet mask information with each route updateeach route update

• Distance Vector – same AD, and timersDistance Vector – same AD, and timers• Easy configuration, just add the command Easy configuration, just add the command

“version 2” during router configuration“version 2” during router configuration

router ripnetwork 10.0.0.0version 2

Discontiguous AddressingDiscontiguous Addressing• Two networks of the same class separated by Two networks of the same class separated by

a different network addressa different network address

192.168.10.0/24

10.1.1.0/24

192.168.10.0/24

•Classful RIPv1 and IGRP cannot support Classful RIPv1 and IGRP cannot support discontiguous subnetsdiscontiguous subnets

•Classless OSPF, EIGRP, and RIPv2 support Classless OSPF, EIGRP, and RIPv2 support discontiguous subnetsdiscontiguous subnets

Passive Interface CommandPassive Interface Command• Allows a router to receive updates Allows a router to receive updates

on an interface, but not send on an interface, but not send updates via that interfaceupdates via that interface– Router(config)#Router(config)#router riprouter rip– Router(config-router)#Router(config-router)#passive-passive-

interface serial0interface serial0

S0 GatewayInternet

UpdatesX

Verifying RIPVerifying RIP

• show ip protocolsshow ip protocols• show protocolsshow protocols• show ip routeshow ip route• debug ip ripdebug ip rip• undebug all (un all)undebug all (un all)

IGRPIGRP• Interior Gateway Routing Protocol - Cisco ProprietaryInterior Gateway Routing Protocol - Cisco Proprietary• Uses bandwidth and delay of the line by default to select best pathUses bandwidth and delay of the line by default to select best path

AutonomousSystem

Router D

Router BRouter A

Router C

Requires Autonomous System number for activation

IGRP vs. RIPIGRP vs. RIP

• IGRPIGRP– is more scalability than RIP is more scalability than RIP – responds faster to network changesresponds faster to network changes– uses a more sophisticated metricuses a more sophisticated metric– supports multiple-pathssupports multiple-paths

Router(config)#router igrp autonomous-systemRouter(config-router)#network network-number*

network 172.16.0.0network 192.168.10.0

router IGRP 10network 172.16.0.0network 10.0.0.0

router IGRP 10

172.16.10.0 192.168.10.010.3.5.0

Configuring IGRPConfiguring IGRP

Verifying IGRP Verifying IGRP

• show ip routeshow ip route• show ip protocols show ip protocols • debug ip igrp transactiondebug ip igrp transaction• debug ip igrp eventsdebug ip igrp events• undebug all (un all)undebug all (un all)