3.1 Routing Fundamental

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Routing Fundamentals


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IP Routing Table

The layer 3 uses the IP routing table to send

packets from the source to destination. If not match Default Gateway


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Rout ers rout e packet s

A network must consistently represent the paths availablebetween routers

In this dynamic environment, the paths represented by somekind of addressing scheme - must be kept current

Layer 3 addresses across the entire internetwork alsoimproves the use of bandwidth by preventing unnecessary

broadcasts.


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Net w ork and host addressing

Router Determine Full Network address = Host And SubnetMask Lookup Destination Host Add. match which out going interface on Router


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Router Functions

Routing = building maps andgiving directions

Switching = moving packetsbetween interfaces

Routers are packet switches


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Routed Protocol

Routed protocolused between

routers to directuser traffic

Examples: IP, IPX

NetworkProtocol

DestinationNetwork

Protocol

Name

1.0

2.03.0

1.1

2.13.1


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Routing Protocol

Routing protocol

used only betweenrouters to maintainrouting tables

Examples: RIP, IGRP, EIGRP, etc

NetworkProtocol

DestinationNetwork

Protocol Name 1.02.03.0

Exit Portto Use

1.12.13.1


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St at ic versus dynamic rout es

Static Route Uses a programmed route that a network

administrator enters into the router

Dynamic Route

Uses a route that network routing protocol

adjusts automatically for topology or trafficchanges


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Defines a path to an IP destination

network or subnet

ip route network [ mask] { address | interface } [ distance][ permanent ]

Router (config)#

Static Routing


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Cisco A

S0

S1

S2

172.16.2.2

St at ic Rout e Ex am pleSt a t ic Rout e Ex am ple

Where

you reach

Network

want toroute

172.16.2.0

172.16.1.0/24

172.16.2.1

Cisco BE0

S1

ip route 172.16.1.0 255.255.255.0 172.16.2.1


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Cisco A

S0

S1

S2

172.16.2.2

St at ic Rout e Ex am ple (c ont )St a t i c Rout e Ex am ple (c ont )

Where

you reach

Network

want toroute

172.16.2.0

172.16.1.0/24

172.16.2.1

Cisco BE0

S1

ip route 172.16.1.0 255.255.255.0 S0


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Why dynamic routing is necessary

More flexibility Automatically updating path to reach destination network. Direct traffic from the same session over different paths in a network

for better performance. This is known as load-sharing.


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Dynamic routing operations

How-to How to send updates What knowledge is contained in these updates When to send this knowledge How to locate recipients of the update

Function

9Maintenance of

a routing table

9Timelydistribution ofknowledge, in theform of routingupdates, to other

routers


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Routing metrics are used by routing algorithms to determine

the desirability of a given route to a destination network.

Different routing protocols implement different routing

metrics.

Routing metrics represent network characteristics.

Metric information is stored in routing tables. Routing metrics include:

- Path Length- Reliability

- Delay- Bandwidth- Load- Cost

Routing Protocol Metrics


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Path Length

The path length metric is based on either path cost or hop count:

Path cost- Path cost is an arbitrary value assigned to each network link

by the network administrator. The path length is the sum of allcosts associated with all the links traversed.

Hop count

- Hop count is a value that counts the number of intermediatesystems (such as routers) through which a packet must passto travel from the source to the destination. The path length isthe sum of all hops in the path.

Routing Protocol Metrics (cont)


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ReliabilityBased on a number of network characteristics. Include:-

- Bit-Error Rate (the ratio of received bits that contain errors)

- How often each network link fails- Once down, how quickly each network link can be repaired

DelayBased on the length of time required to move a packet from the source to

a destination. Conglomeration of several variables:- Bandwidth of network links

- Queue lengths at intermediate routers

- Network congestion on network links

- Physical distance to be travelled from the source to the destination

BandwidthBased on the available traffic capacity of each network link.

- Routes through links with greater bandwidth do not necessarily

provide better routes than routes through slower links.



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Load

Based on the degree to which a network resource(such as a router) is busy. Load is calculatedaccording to factors such as:

9 CPU utilization

9 Packets processed per second.

Cost

Based on the monetary cost of using each networklink. For example, a slower company-owned link canbe configured as preferable over faster public links

that cost money for usage time.



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Three classes of rout ing protocols

shortest path first


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Time to convergence

Convergence:The knowledge needs to reflect an

accurate, consistent view of the new topologyafter network fail or restructure.

Convergence occurs when all routers use a

consistent perspective of network topology After topology changes, Routers must re-compute

routes, which disrupts routing

The process and time required for router re-convergence varies with routing protocols


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An autonomous system is a collection of networksunder a common administrative domain.

IGPs operate within an autonomous system.

EGPs connect different autonomous systems.

Autonomous Systems: Interior orExterior Routing Protocols


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Administrative Distance:Ranking Routes


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Administrative Distance


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Routers pass periodic copies of routing table to neighborrouters and accumulate distance vectors.

Distance Vector Routing Protocols


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Routers discover the best path todestinations from each neighbor.

Sources of Information andDiscovering Routes


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Selecting theBest Route with Metrics


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Updates proceed step-by-stepfrom router to router.

Maintaining Routing Information


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Each node maintains the distance from itself to eachpossible destination network.

Inconsistent Routing Entries


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Slow convergence produces inconsistent routing.

Inconsistent Routing Entries (Cont.)


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Router C concludes that the best path to network10.4.0.0 is through router B.



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Router A updates its table to reflect the new buterroneous hop count.



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Hop count for network 10.4.0.0 counts to infinity.

Count to Infinity


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Define a limit on the number of hops to preventinfinite loops.

Defining a Maximum


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Packets for network 10.4.0.0 bounce (loop) betweenrouters B and C.

Routing Loops


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It is never useful to send information about a routeback in the direction from which the original informationcame.

Split Horizon


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Routers advertise the distance of routes that have gonedown to infinity.

Route Poisoning


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Poison reverse overrides split horizon.

Poison Reverse


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The router keeps an entry for the networks possibledown state, allowing time for other routers to recomputefor this topology change.

Holddown Timers


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The router sends updates when a change in its routingtable occurs.

Triggered Updates


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Distance Vector Operation


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Distance Vector Operation (Cont.)


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After initial flood, pass small event-triggered link-state

updates to all other routers

Link-State Routing Protocols


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Link-State Routing Features

Li k St t N t k Hi h E l


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Minimizes routing table entries

Localizes impact of a topology change within

an area

Link-State Network Hierarchy Example


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Link-State Routing Protocol Algorithms


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Benefits of Link-State Routing

Fast convergence: changes are reportedimmediately by the source affected.

Robustness against routing loops: Routers know the topology. Link-state packets are sequenced and acknowledged.

By careful (hierarchical) network design, you canutilize resources optimally.

C t f Li k St t R ti


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Caveats of Link-State Routing

Significant demands for resources:

Memory (three tables: adjacency, topology, forwarding) CPU (Dijkstras algorithm can be intensive, especially when a lot

of instabilities are present.)

Requires very strict network design (when more areas

area routing) Problems with partitioning of areas

Configuration generally simple but can be complex

when tuning various parameters and when the design iscomplex

Troubleshooting easier than in distance vector routing


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Drawbacks to Link-State Routing Protocols

Initial discovery may cause flooding.

Memory- and processor-intensive.

Distance-vector versus link-state


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rout ing protocols

Distance-Vector View network technology

from neighbors perspective

Adds distance vectors from

router to router Frequent, periodics updates:

Slow convergence

Passes copies of routing

tables to neighbors router

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

Hybrid rout ing protocols


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Hybrid rout ing protocols

Cisco's EIGRP (Enhanced Interior GatewayRouting Protocol)

LAN t o LAN rout ing


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LAN-t o-LAN rout ing

LAN to WAN Routing


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LAN to WAN Routing

Path select ion and sw it ching ofl i l l d di


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mult iple prot ocols and media


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3.1 Routing Fundamental

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