Tài liệu học CCNA kỳ 2 mod7 distancevectorrouting

NIIT-ICT Hanoi info@niithanoi.vn 5Routing Loop Issues Routing Loops • Distance vector routing protocols are simple in their implementation and configuration, but this comes at a price..

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Module 7 – Distance Vector Routing Protocols

Overview

• Describe how routing loopscan occur in distance vector routing

• Describe several methods used by distance vector routing protocols to ensure

that routing information is accurate

• ConfigureRIP

• Use the ip classless command

• Troubleshoot RIP

• Configure RIP for load balancing

• Configure static routes for RIP

• Verify RIP

• Configure IGRP

• Verify IGRP operation

• Troubleshoot IGRP

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NIIT-ICT Hanoi info@niithanoi.vn 3

Distance Vector Routing Updates

• RIP – Hop Count

• IGRP and EIGRP – Bandwidth, Delay, Reliability, Load

• Cisco’s OSPF – Bandwidth

• IS-IS – Cost

• BGP – Number of AS or policy

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Routing Loop Issues

Routing Loops

• Distance vector routing protocols are simple in their implementation and configuration,

but this comes at a price.

• Pure distance vector routing protocols suffer from possible routing loops.

• Routing loops can cause major network problems, from packets getting lost (blackholed)

in your network, to bringing down your entire network.

• Several remedies to have been added to distance-vector algorithms to help prevent

routing loops including:

– Split horizon

– Hold-down timers

– Defining a maximum metric

What can cause routing loops?

• Routing loops can occur when there are:

– Incorrect or inconsistent routing updatesdue to slow convergence after a

topology change (Example coming up next.)

– Incorrect or incomplete routing information(see presentation on Discard

Routes)

– Static routes incorrectly configured with an intermediate addresswhich

does not become resolved in the routing table (see presentation on Static

Routes – Additional Information)

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Routing Loop Example

• Assume for the remainder of this example that Router C’s preferred path to

network 1 is by way of Router B.

• Router C’s routing table has a distance of 3 to network 1 via Router B

Network 1 Fails

• Router E sends an update to Router A

• Router A stops routing packets to network 1

• But Routers B, C, and D continue to do so because they have not yet been

informed about the failure

• Router A sends out its update

• Routers B and D stop routing to network1, (via Router A)

• However, Router C is still not updated

• To router C, network 1 is still reachable via router B

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Router C sends a periodic update to Router D

• Router C sends a periodic update to Router D indicating a path to network 1

(by way) of via Router B (4 hops).

Router D’s Routing Table information for Network 1

• Current path to Network 1 = Unreachable (down)

• Information from Router C: Network 1 : 4 hops by way of Router C

• Normally, RouterD ignores this routing information because it usually has a

better route, 2 hops, via Router A, but this route is now down

• Router D changes its routing table to reflect this (good) better, but incorrect

information, Network 1 by way of Router C (4 hops)

• Router D propagates the information to Router A

Routers A changes its routing table

• Router A adds new route to its routing table, get to Network 1 by way of Router

D (5 hops).

• Propagates the information to Routers B and E

Router B (and Router E) change their routing tables

• Router B now believes it can get to Network 1 by way of Router A (6 hops).

Wow! I was about to tell Router C that Network 1 was down via Router B, but

now I have new information!

• Propagates the incorrect information to Router C

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Router C changes its routing table

• Router C still believes it can get to Network 1 by way of Router B (7 hops).

Of course now it believes it is 7 hops instead of 3.

• Propagates the newer but still incorrect information to Router D

Here we go again!

• Data packets destined for Network 1 get caught in a routing loop, from Routers

A to D to C to B to A to D etc

• As routing updates continue between the routers, the hop count gets greater –

to infinity? (Not quite – we will see in a moment.)

Defining a Maximum

Problem: Count to infinity

Solution: Defining a Maximum

• Distance vector routing algorithms are self-correcting, but a routing loop

problem can require a count to infinity

• To avoid this prolonged problem, distance vector protocols define infinity as a

specific maximum number

• This number refers to a routing metric which may simply be the hop count

• When the metric value exceeds the maximum value, and as each router

receives this maximum metric, the network is then consideredunreachable.

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Split Horizon

• “The effect of split horizon is that a router will send out different routing

messages on different interfaces In effect a router never sends out

information on an interface that it learned from that interface.” (Lewis,

Cisco TCP/IP Routing)

Split Horizon

• This example from the curriculum is not an example of split hoizon,

but using hold-down timers.

• “Split-horizon attempts to avoid this situation If a routing update about

Network 1 arrives from Router A, Router B or Router D cannot send

information about Network 1 back to Router A Split-horizon thus

reduces incorrect routing information and reduces routing overhead.”

• Initially, this is true, but the loop is a result of Router C sending out the

updates, because it has not converged.

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Split Horizon Rule – Avoiding Routing Loops

• Routers RTA and RTB have their initial routing tables and are ready to

exchange routing information via a distance-vector routing protocol

like RIP.

Split Horizon disabled

• If split horizon were disabled the routing updates would include all of

the networks in their routing tables including their directly connected

networks and any networks learned from any interface.

10.1.2.0/24 0 s0

Simple Split Horizon

Initial routing tables

Split Horizon Disabled

• After the initial exchange of updates everything in the routing tables

look fine.

• Because split horizon disabled, the 10.1.2.0/24 network is sent by both

routers, but neither router includes the other’s route to 10.1.2.0/24 (1

hop) in the routing table, because it has a current route with a better

metric of 0.

Initial routing tables

New routing tables

10.1.2.0/24 network is included because split horizon has been disabled

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• After the next exchange of updates everything in the routing tables look fine and

the routing tables are converged

• Because split horizon disabled, the besides the 10.1.2.0/24 network, the networks

learned from the other router in the previous update is also sent by both routers

• However, neither router includes the those networks, because it has a current

route with a better metric of 0

10.1.2.0/24 1 10.1.1.1

Routing Update

Next-hop Net Hops Address

10.1.3.0/24 1 10.1.2.2

Routing Table

Net Hops Ex-Int

10.1.1.0/24 0 e0 10.1.3.0/24 1 10.1.2.2

Routing Table

Net Hops Ex-Int

10.1.2.0/24 0 s0 10.1.1.0/24 1 10.1.2.1

Routing Table

Net Hops Ex-Int

10.1.1.0/24 0 e0 10.1.3.0/24 1 10.1.2.2

Routing Table

Net Hops Ex-Int

10.1.2.0/24 0 s0 10.1.1.0/24 1 10.1.2.1

Previous routing tables

Networks in red were included because split horizon has been disabled

Split Horizon Disabled – 10.1.3.0/24 down

• Note: Routing tables are not sent at the exactly same time We will

learn about this in Ch 12 Routing Protocols, that this is done on

purpose to avoid collisions on broadcast networks like Ethernet.

• Here, the 10.1.3.0/24 network fails, and before RTB sends out its

routing update, RTB receives a routing update from RTA.

10.1.2.0/24 1 10.1.1.1

Routing Table

Net Hops Ex-Int

10.1.1.0/24 0 e0 10.1.3.0/24 1 10.1.2.2

Routing Table

Net Hops Ex-Int

10.1.2.0/24 0 s0 10.1.1.0/24 1 10.1.2.1

Routing Table

Net Hops Ex-Int

10.1.1.0/24 0 e0 10.1.3.0/24 1 10.1.2.2

Routing Table

Net Hops Ex-Int

10.1.2.0/24 0 s0

10.1.3.0/24 2 10.1.2.1

X

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• RTB notices that it has a route to 10.1.3.0/24 via RTA Even though it is 2

hops it is certainly better than its current situation of “unreachable” so it

accepts this better, but incorrect information from RTA

• RTB now forwards all packets destined for 10.1.3.0/24 to RTAat 10.1.2.1

• RTA receives these packets and forwards them to RTB at 10.1.2.2

• RTB forwards them back to RTA at 10.1.2.1

• And so on! The packets get blackholed in this routing loop

10.1.2.0/24 0 s0 10.1.1.0/24 1 10.1.2.1

10.1.2.0/24 0 s0

10.1.3.0/24 2 10.1.2.1

X

• Meanwhile, its RTB’s turn to send its routing update.

• RTB increments the hop count to 10.1.3.0/24 to 3 hops and sends it to

RTA.

• When RTA sends out its next routing table it will increment the hop

count to 10.1.3.0/24 to 4 hops and sends it to RTB.

• And on and on, until “infinity” which in RIP is 16 hops.

10.1.2.0/24 0 s0

10.1.3.0/24 2 10.1.2.1

Routing Update

10.1.3.0/24 3 10.1.2.2

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• Once both routers have 16 hops for 10.1.3.0/24, they will both mark

this network as unreachable and discontinue forwarding , drop, packets

to this network.

• This temporary routing loop can be easily avoided by enabling split

horizon on the serial 0 interfaces.

• Split horizon rule states that router never sends out information on an

interface that it learned from that interface

10.1.1.0/24 0 e0

10.1.3.0/24 16 10.1.2.2

Routing Table

Net Hops Ex-Int

10.1.2.0/24 0 s0

10.1.3.0/24 16 10.1.2.1

Simple Split Horizon

10.1.1.0/24

Routing Table

Net Hops Ex-Int

10.1.2.0/24 0 s0

Routing Update

Routing Table

Net Hops Ex-Int

10.1.1.0/24 0 e0 10.1.3.0/24 1 10.1.2.2

Routing Table

Net Hops Ex-Int

10.1.2.0/24 0 s0 10.1.1.0/24 1 10.1.2.1

Routing Update

Split

Horizon

Enabled

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Split Horizon Enabled

• As you can see, with split horizon enabled, RTA does not send RTB (out s0)

information about 10.1.3.0/24 because it learned it from RTB (same s0), and

RTB does not send RTA (out s0) information about 10.1.1.0/24 to RTA

because it learned it from RTA (same s0) (This also includes the common

network between them

Routing Table

Net Hops Ex-Int

10.1.2.0/24 0 s0

Routing Update

Routing Table

Net Hops Ex-Int

10.1.1.0/24 0 e0

10.1.3.0/24 1 10.1.2.2

Routing Table

Net Hops Ex-Int

10.1.2.0/24 0 s0

10.1.1.0/24 1 10.1.2.1

Routing Update

Split Horizon Enabled – 10.1.3.0/24 down

• RTB notices 10.1.3.0/24 is down and puts this route into hold-down state in

its routing table (hold-down coming next)

• RTB immediately sends out a triggered update for only this route (if there were

others in the routing table) with a metric of infinity, 16

• RTA receives the triggered update and puts the route for 10.1.3.0/24 into

hold-down state.

10.1.2.0/24 0 s0 10.1.1.0/24 1 10.1.2.1

10.1.3.0/24 16 10.1.2.2

Routing Table

Net Hops Ex-Int

10.1.2.0/24 0 s0 10.1.1.0/24 1 10.1.2.1

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Split Horizon Enabled – 10.1.3.0/24 down

• Notice that RTA never sends RTB a routing update for 10.1.3.0/24,

because split horizon is enabled on these interfaces.

10.1.2.0/24 0 s0 10.1.1.0/24 1 10.1.2.1

10.1.3.0/24 16 10.1.2.2

Routing Table

Net Hops Ex-Int

10.1.2.0/24 0 s0 10.1.1.0/24 1 10.1.2.1

Split Horizon with Poison Reverse

• Many vendor implementations of distance vector routing protocols like Cisco’s RIP and

IGRP apply a special kind of split horizon, called split horizon with poison reverse.

• “Split horizon with poison reverse means that, instead of not advertising routes to the

source, routes are advertised back to the source with a metric of 16 , which will make the

source router ignore the route It is perceived that explicitly telling a router to ignore a

route is better than not telling it about the route in the first place.” (Lewis, Cisco TCP/IP

Routing)

• One drawback is that routing update packet sizes will be increased when using Poison

Reverse, since they now include these routes

10.1.1.0/24 0 e0 10.1.3.0/24 1 10.1.2.2

Routing Table

Net Hops Ex-Int

10.1.2.0/24 0 s0 10.1.1.0/24 1 10.1.2.1

Routing Update

10.1.1.0/24 1 10.1.1.1

10.1.3.0/24 16 10.1.2.1

Routing Update

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Split Horizon Enabled by Default

Split horizon with poison reverse is enabled by default for all interfaces except:

• Physical interfaces or multipoint sub-interfaces using Frame Relay or SMDS

encapsulation (CCNA Semester 4 and CCNP)

To disable split horizon on an interface:

10.1.1.0/24 0 e0 10.1.3.0/24 1 10.1.2.2

Routing Table

Net Hops Ex-Int

10.1.2.0/24 0 s0 10.1.1.0/24 1 10.1.2.1

Routing Update

10.1.1.0/24 1 10.1.1.1

10.1.3.0/24 16 10.1.2.1

Routing Update

10.1.3.0/24 1 10.1.2.2

10.1.1.0/24 16 10.1.2.2

“Poisoned”

routes in red

Route poisoning

• 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.

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Preventing routing loops with holddown timers

• The main function of holddown timers is to prevent the distance vector

routing protocol from establishing routing loops during periods of network

transition (topology changes)

• “The rule: Once a route is marked unreachable, it must stay in this state for a

period of time assumed sufficient for all routers to receive new information

about the unreachable network In essence, we instruct the routers to let the

rumors calm downand then to pick up the truth.” (Zinin, Cisco IP Routing)

• The amount of time a router remains in “this state” is determined by the

holddown timer.

Curriculum

• A count to infinity problem can be avoided by using holddown timers.

• When a router receives an update from a neighbor indicating that a

previously accessible network is now inaccessible, the router marks

the route as inaccessible and starts a hold-down timer

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Same Route from same neighbor : Network is back up (Correct News)

• If at any time before the hold-down timer expires an update is received

from the same neighbor indicating that the network is again accessible ,

the router marks the network as accessible and removes the

hold-down timer

Better Route from different neighbor (Correct News)

• If at any time before the hold-down timer expires an update arrives

from a different neighboring router with a better metric than originally

recorded for the network, the router marks the network as accessible

and removes the hold-down timer.

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Poorer Route from a different neighbor (Incorrect News)

• If at any time before the hold-down timer expires an update arrives from a

different neighboring router with a poorer metric than originally recorded for the

network the update is ignored and the hold-down timer continues

• Ignoring an update with a poorer metric when a hold-down is in effect allows

more time for the knowledge of a disruptive change to propagate through the

entire network

Additional Information on Holddown Timers

Flapping routes

• Holddown timers not only help prevent routing loopsduring transient periods

but also help network stability by dampening unstable, flapping routes(routes

which continuously go up and down)

Holddown Time

• As we will see with both RIP and IGRP, the amount of time the router remains

in the holddown state can be modified (with caution!), even set to 0

• We will look at this later in the presentations on RIP and IGRP

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