chapter 5 – routing protocols: igrp. building a network to be reliable – provide error detection...
TRANSCRIPT
Building a Network
To Be Reliable – provide error detection and ability to correct errorsTo Provide Connectivity – incorporate a variety of hardware & software products so they can function togetherTo Be Easy To Use – users need to have no concern for the network’s structure and implementationTo Be Easy To Modify – allow itself to evolve and adapt as needs changeBe Easy To Implement – must follow industry standards and allow a variety of configurations
Path Determination
Enables a router to evaluate the available paths to a destination and establish best path for the packetMost routing protocols use simply use the shortest and best pathAnalogy – Packet routing is like driving a car: Routers through the use of protocols make path decisions based on routing tables, and people driving cars determine their paths by reading road signs
Metrics
Routing tables must be updated and accurateEach routing protocol interprets the “best path” in its own wayProtocol generates a value, METRIC, for each path through the networkSmaller the metric the better the pathMetrics differ depending on the protocol being used
Router Forwarding Decisions
Router examines the packet’s destination protocol address – it either knows or does not know how to forward to the next hopDoes not know and no default route assigned - the packet is typically droppedNetwork portion of IP address used by router within the network cloudThe source and destination IP addresses never changes as a packet traverses the network
Router Forwarding Decisions
Network portion of the address is used to make path selectionRouter responsible for passing the packet to the next network along the path Switching function allows the router to accept a packet on one interface and forward it another interface
Routing Protocols
ROUTED protocols – moved over a network
TCP/IPIPXAppleTalk
ROUTING protocols – route routed protocols through a network
IGRPEIGRPOSPFBGPOSI routingAdvanced Peer-to-Peer Networking(APPN)Intermediate System-to-Intermediate System(IS-IS)RIP
Routing Protocols Continued
End systems (computers) use routed protocols to talk to each other
IP
Routers (intermediate systems) use routing protocols to talk to each other – about networks and paths
Multi-Protocol Routing
Routers are capable of supporting multiple independent routing protocols
IGRPRIP
Allows routers to deliver packets from several routed protocols over the same data links
TCP/IPIPXApple Talk
Differentiating Routing Protocols from One Another
Can be differentiated from one another by:
Particular goals of the designerVarious types - each has a different effect on the networkRouting protocols use a variety of metrics to identify best paths
Routing Protocols
Interior ProtocolsUsed for routing information within networks that are under a common administrationAll IP interior protocols must be specified with a list of associated networks before routing can occurCISCO supports RIP and IGRP
Exterior ProtocolsUsed to exchange information between networksRequire the following before routing can begin
List of neighbor routers List of networks to advertise as directly reachable
EGP and BGP
Goals of Routing Protocols
Optimal RouteSelect best routeDepends on metric and metric weighting
Simplicity and EfficiencyEfficiency is important when software implementing the routing protocol must run on a computer with limited resources
RobustnessShould perform correctly at all times
Hardware failures, high load conditions and incorrect implementation
Goals of Routing Protocols Continued
Rapid ConvergenceMust converge rapidly
Speed and ability of a group of devices to agree on the topology after a change has occurred
FlexibilityQuickly and accurately adapt to a variety of network circumstances
Routing Loops
Packet continues to bounce back and forth between to devices until:
Device is updated Packet is switched the maximum number of times allowed
Different routing protocols have different maximums
IGRP has a maximum hop count of 255 it defaults to 100Usually set to 50 or less
Classifications of Routing Protocols
Most can be classified into three basic approaches
Distance vector routingDetermines the direction and distance to any link in the network
IGRP and RIP
Link-state routing (shortest path first)Re-creates exact topology of the entire network
OSPF, IS-IS, NLSP
Hybrid approach Combines both aspect of link-state and distance vectoring
EIGRP
IP Routing Configuration
Any routing protocol must follow two steps:
Create routing process with one of the router commandsConfigure the protocol specifics
Interior routing protocols also must have a list of networks specified before routing beginsIGRP requires an AS (Autonomous System) number
IP Routing Configuration
Choosing a routing protocol – consider the following:
Network size and complexityNetwork traffic levelsSecurity needsReliability needsNetwork delay characteristicsOrganizational policiesOrganizational acceptance of change
IGRP Metrics
CISCO proprietary – developed to supercede RIPDistance vector interior routing protocolUses a combination of metricsNetwork delay, bandwidth, reliability and load all factored into the routing decision
IGRP Continued
Provides a wide range for its metricsFor example
Reliability – value between 1 and 255Bandwidth – values reflecting speeds from 1200 bps to 10 GbpsDelay – value from 1 to 224
Default values related to weightings for IGRP give the most importance to bandwidth – makes IGRP superior to RIPContrast with RIP(does not weigh metrics) – it only uses one
IGRP Operation
Goal of IGRP was create a robust protocol for routing within an ASIGRP uses a combination of user-configurable metricsIGRP advertises three types of routes:
InteriorSystemExterior
IGRP Operation–Interior Routes
Interior routes – between subnets in the network attached to a router interfaceIf the network attached to router is not subnetted – IGRP does NOT advertise interior routesSubnet information is not included in IGRP updates
IGRP Operation-System Routes
Routes to other major networks within ASRouter derives system routes from directly connected interfaces and system route information from other routers that use IGRPSystem routes do NOT include subnetting information
IGRP Operation-Exterior Routes
Routes to networks outside the AS that are considered when identifying the gateway of last resortRouter uses the gateway of last resort if it does not have a better routeIf AS has more than one connection to an external network - different routers can choose different exterior routers as the gateway of last resort
Features that Enhance Stability of IGRP - Holddowns
Used to avoid routing loops in the networkEffect of increasing topology convergence timeUsed to prevent regular update messages from reinstating a route that may have gone badTell routers to hold down any changes that might effect routes for some period of timePeriod of time is calculated to be just greater than time to update entire network
Features that Enhance Stability of IGRP – Split Horizons
Occur when router tries to send information about a route back in the direction that it cameHelps prevent routing loopsImplemented in IGRP
Features that Enhance Stability of IGRP – Poison Reverse Updates
Intended to defeat larger routing loopsPoison reverse updates are sent to remove the route and place it in holddownPoisoning the route can speed convergence
IGRP Metrics and routing Updates
IGRP uses several types of metric informationFor each path through an AS IGRP records:
Segment with lowest bandwidthAccumulated delaySmallest maximum transmission unit (MTU)Reliability and load
IGRP Metrics and routing Updates
Bandwidth by default is given the most importanceRouter running IGRP sends updates every 90 secondsDeclares route inaccessible if an update from first router is not sent within 3 update periods (270 seconds) after 5 update periods (450 seconds) router removes the route from the routing tableUses flash update and poison reverse to speed up convergence
IGRP Metrics and routing Updates
Flash UpdateSending of update sooner than standard periodic update interval
Poison Reverse UpdatesIntended to defeat larger routing loopsSent to remove a route and place it in holddown
IGRP – Maximum Hop Count
Maximum hop count 255Normally set lower than the default of 100Should be number at least greater than the maximum number of routers a route might have to go through