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CCNA 2 v3.1 Module 7

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CCNA 2

Module 7

Distance Vector Routing Protocols

RIP and IGRP

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Distance vector routing updates

• Routing table updates occur periodically or when topology change

• Routers send their entire routing table to all adjacent neighbors

• Routing table contains info about routing metrics

Hop count

Bandwidth

Cost

Load

Reliability

Delay

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Distance vector routing loop

• Loops occur when inconsistent routing tables are not updated due to slow convergence in a changing network

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• Count to infinity

Packets loop continuously around the network

Packet never reach their destination

May occur because

Network is down

Interface not configured correctly

• To overcome count to infinity

Specify a maximum hop count

Split Horizon

Route Poisoning

Hold Down timers

Count To Infinity

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• Used to overcome count to infinity

• Hop count

The number of routers a packet goes through

It increases each time a packet goes through a router

• RIP maximum hop count is 15

Packet is discarded if the hop count of 16 is reached

This is because the network 1 is unreachable

Maximum hop count

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How does the Maximum Hop Count Work?

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• Solution to count to infinity

• Do not send routing updates to the router you learned the information about

Reduces incorrect routing information

Reduces routing overhead

Split Horizon

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Route poisoning

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• Poison reverse

• When route poisoning is used with triggered updates it will speed up convergence time because neighboring routers do not have to wait 30 seconds before advertising the poisoned route.

• Failed routes are advertised with infinite metrics.

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Avoiding routing loops with triggered updates

• Triggered updates

Sent immediately when topology changes

Used in conjunction with route poisoning

Ensure all routers know of failed routes before any holddown timers can expire

i.e., Don’t wait for the time limit to expire

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Preventing routing loops with holddown timers

• When a router receives an update saying a route is down, it marks the route as inaccessible and starts a hold-time timer

• If at any time before the holddown timer expires an update is received from:the same neighbour indicating the network is accessible again

Router marks network as accessible + removes holddown timer

A different neighbour with a better metric

Router marks network as accessible + removes holddown timer

A different neighbour with a poorer metric

The update is ignored

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RIP Routing Protocol

• Open standard RFC 1058 and Internet Standard (STD) 56

• Documented by:

Request for Comments (RFC) 1058

Internet Standard (STD) 56.

• RIP Version 1 (RIP v1)

Classful Routing Protocol

• RIP Version 2 (RIP v2)

Classless Routing Protocol

Ability to carry additional packet routing information.

Authentication mechanism to secure table updates.

Supports variable length subnet masking (VLSM)

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• RIP has slow convergence which results in

Routing loops

Count to infinity

• To reduce these RIP uses

Count-to-infinity

Split horizon

Poison reverse

Holddown counters

Triggered updates

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Configuring RIP

• Enter global configuration mode

Router#config t

• Specify RIP as the routing protocol

Router(config)#router rip

• Specify directly connected networks

Router(config-router)#network 10.10.10.0

• A router running RIP can be configured to

Send triggered updates independently the scheduled updates (every 30 seconds)

Router(config-if)#ip rip triggered

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RIP Configuration Issues• Count-to-infinity

Does not have to be configured with RIP

• To disable Split Horizon

Router(config-if)#no ip split-horizon

• Default holddown for RIP is 180 seconds

• The default RIP update 30 seconds

Can be configured for longer intervals

To conserve bandwidth

Can be configured for for shorter intervals

To decrease convergence time

• Command

Router(config)#router rip

Router(config-router)#update-timer seconds

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• To prevent RIP updates leaving an interface

Router(config-router)#passive-interface Fa0/0

• To configure RIP to exchange routing information in a non-broadcast network (Frame Relay)

RIP needs to be told of other neighboring RIP routers

Router(config-router)#neighbor <ip address>

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•To configure software to send and receive Rip V1 and V2 packets•Router(config-router)#version{1|2}

•To configure an interface to send RIP V1 packets•Router(config-if)#ip rip send version 1

•To configure an interface to send RIP V2 packets•Router(config-if)#ip rip send version 2

•To configure an interface to send RIP V1 and V2 packets•Router(config-if)#ip rip send version 1 2

•To configure and interface to receive RIP V1 packets•Router(config-if)#ip rip receive version 1

•To configure and interface to receive RIP V2 packets

•Router(config-if)#ip rip receive version 2

•To configure and interface to receive RIP V1 and V2 packets

•Router(config-if)#ip rip receive version 1 2

Configure Router to Send/Receive Packets

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Verifying RIP configuration • show ip protocols

Shows which routing protocols are carrying IP traffic on the router

Confirms

RIP routing is configured

The correct interfaces are sending and receiving RIP updates

The router is advertising the correct networks

• show ip route

Displays the routing table

Verify that routes received by RIP neighbours are installed in the routing table

• Additional commands to check RIP configuration

show interface interface

show ip interface interface

show running-config

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Troubleshooting RIP update issues

• debug ip rip

Displays RIP routing updates as they are sent and received

Problems such as discontiguous subnets or duplicate networks can be diagnosed with this command

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Discontiguous subnet

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Duplicate Subnets Duplicate Subnets

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Other commands to troubleshoot RIP:

• show ip rip database

• show ip protocols {summary}

• show ip route

• debug ip rip {events}

• show ip interface brief

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Preventing routing updates through an interface

• network command

If it is issued for a given network using RIP or IGRP

router immediately begin sending advertisements out all interfaces within the specified network address range

Router(config)#router RIP

Router(config-router)#network 192.101.21.0

• passive interface command

Stops the router from sending updates through a particular interface to a neighbor

Router continues to listen and use routing updates from that neighbor

Prevent other systems from learning about routes dynamically

Router(config)#router RIP

Router(config-router)#passive-interface fa0/0

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Load balancing with RIP • Allows a router to take advantage of

multiple best paths to a given destination

Calculated by dynamic routing protocol

Entered statically by administrator

• RIP performs “round robin” load balancing

over 6 equal-cost paths, with 4 paths being default

takes turns forwarding packets over the parallel paths

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• Interface pointer cycles through the interfaces and routes in a deterministic fashion such as 1-2-3-4-1-2-3-4-1

• RIP metric is hop count

no regard is given to the speed of the links

56 Kbps path will be given the same preference as the 155 Mbps path

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• Equal cost routes can be found by using the “show ip route “

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• When a router learns multiple routes to a specific network, the route with the lowest administrative distance is installed in the routing table

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• When a router has multiple routes to a network

route with lowest admin distance is installed in routing table

• If the router has many routes, learned via the same routing process with the same admin distance

router chooses the path with the lowest cost or metric

• If the router has multiple paths to a destination with the same admin distance and cost

load-balancing can occur

• Cisco IOS allows up to 6 equal cost routes in a routing table

EIGRP allows up to 4 equal cost routes

Static routes always install 6 routes

BGP by default allows only 1 route

IGRP can load balance up to 6 unequal links

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• Router(config-router)#maximum-paths [no.]

• Cisco IOS offers two methods of load balancing:

per-packet:If process switching is enabled,

Router will alternate paths on a per-packet basis

per-destination: If fast switching is enabled

Only one route will be used for that destination address

All packets bound for a specific host take the same path

Packets bound for a different host on the same network may use an alternate route

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Integrating static routes with RIP • Static routes

User-defined routes

Force packets to take a specific path

Useful for specifying a “gateway of last resort”; a default route

• Administrative Distance (AD)

Each dynamic routing protocol has a default AD

A static route can be defined as less desirable than a dynamically learned route, as long as the AD of the static route is higher than that of the dynamic route

• floating static route

A route configured on a router to take the place of the RIP route in the event that the RIP routing process fails

• RIP can advertise static routes using the redistribute static command

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IGRP features • Distance vector routing protocol

• Developed by Cisco

• Sends routing updates every 90 second

Sends entire table

• Advertising networks for an autonomous system

• Metrics used are normally bandwidth and delay

Bandwidth

Delay – amount of time to get to the destination

Reliability – reliability of link determined by exchange of keepalives

Load

MTU – maximum transmission unit of the path

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• IGRP converges faster than RIP

thereby avoiding the routing loops

• IGRP does not share the hop count limitation of RIP.

• Useful Commandsshow interface interface

show running-config

show running-config interface interface

show running-config | begin interface interface

show running-config | begin igrp

show ip protocols

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Metric K1 represents bandwidth by default is set to 1Metric K3 represents delay and by default is set to 1Metric K2, K4 and K5 are set to 0.

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show ip route shows the IGRP metric values in brackets.A link with a higher bandwidth will have a lower metricA route with a lower cumulative delay will have a lower metric

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IGRP Routes• GRP advertises three types of routes:

Interior

Routes between subnets of a network attached to a router interface

If the network attached to a router is not subnetted, IGRP does not advertise interior routes

System

Routes to networks within an autonomous system number

The IOS derives system routes from directly connected network interfaces

System routes information provided by other IGRP routers.

System routes do not include subnet information

Exterior

Routes to networks outside the autonomous system that are considered when identifying a gateway of last resort.

Gateway of last resort is used if

a better route is not found

The destination is not a connected network

Exterior routes are also referred to as "candidate default"

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IGRP Routes

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IGRP stability features

Holddowns

Prevent regular update messages from inappropriately reinstating a route that may not be up

Split horizons

• It is usually not useful to send information about a route back in the direction from which it came

• Prevent routing loops

Poison reverse

• Poison reverse updates then are sent to remove the route and place it in holddown.

• With IGRP, poison reverse updates are sent only if a route metric has increased by a factor of 1.1 or greater.

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IGRP Timers• update timer specifies

how frequently routing update messages should be sent

Default is 90 sec

• invalid timer specifies

how long a router should wait in the absence of routing-update messages about a route before declaring that route invalid

Default is 3 times the update period (270 sec)

• holddown timer specifies

the amount of time for which information about poorer routes is ignored

Default is 3 times the update timer period + 10 sec (280 sec)

• flush timer indicates

how much time should pass before a route is flushed from the routing table

Default is 7 times the routing update timer (after 630 sec)

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Configuring IGRP

• To set IGRP routing on

Router# config t

Router(config)# router igrp 101

Router(config-router)#network 192.101.2.0

• To turn IGRP routing off

Router(config)# no router igrp 101

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Troubleshooting IGRP

• Problems to look out for

mistyped network statement

discontiguous subnets

Incorrect Autonomous System Number

• Useful commands

show ip protocols

show ip route

debug ip igrp events

debug ip igrp transactions

ping

traceroute