NESCOT CATC 1

Distance Vector Routing Protocols

CCNA 2 v3 – Module 7

NESCOT CATC 2

Distance Vector Routing – UpdatesUpdates occur periodically or when the topology changes.

Updates proceed systematically from router to router.

Router sends its entire routing table to each neighbor.

Distance Vector Routing – LoopsRouting loops can occur when inconsistent routing tables are not updated due to slow convergence in a changing network.

X 1. Network fails

2. E sends update to A

3. A sends update to B and D

4. C still unaware of failure, advertises route via B

5. D updates its routing table to include route from C, and forwards this incorrect information to A.

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Maximum Hop CountInvalid updates can continue to loop until some other process stops them. Each router the packet passes through increases the hop count by 1.Packets continuously looping around the network count to infinity.     The routing protocol permits the routing loop to continue until the metric exceeds its maximum allowed value. In RIP, if the hop count exceeds the maximum of 15 hops the packet is discarded and the network is considered unreachable.

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Split HorizonSplit Horizon is another mechanism used to avoid routing loops.Information about routes is prevented from being advertised out the router interface through which the information was received.

B

A

2. Router B updates its routing table3. Router B does not include Network A in update to A

1. Router A advertises route to Network A

Route PoisoningPoison Reverse updates are used to overcome large routing loops by sending explicit information when a subnet or network is not accessible. Sets the hop count to one more than the maximum. When used with triggered updates it will speed up convergence time.

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Triggered Updates and Holddown TimersRIP updates occur every 30 secs, but a triggered update is sent immediately.Router detects topology change, immediately sends update to adjacent routers – doesn’t wait for the update timer to expire.Wave of updates propagates throughout the network.Ensure all routers know of failed routes before holddown timers expire.

x

Interface down!Send triggered

update now

Route down!Send triggered

updateRoute down!Pass it on…

The count to infinity problem can be avoided by using holddown timers.When the router marks a route inaccessible it starts a holddown timer.

Update received: network inaccessible, start holddown timer

Update received: network accessible again

Isupdate from same

neighbor?

Yes Network accessible,

remove holddown

timerYesNoIgnore update,

Wait until holddown timer expires

No

Different neighbor, better

metric?

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RIP Routing Process

1. RIP v1: Classful routing protocol Does not include subnet masks in updates

2. RIP v2: Classless routing protocol Carry additional packet routing information. Authentication mechanism to secure table updates. Supports variable length subnet masking (VLSM).

30 secs.15.

Two versions of RIP:

RIP updates occur every ________The maximum number of hops in a path is ____RIP implements split horizon and holddown mechanisms.

Configuration Example:

GAD(config)# router rip

GAD(config-router)#network 192.168.13.0GAD(config-router)#network 192.168.14.0

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IP ClasslessIf one part of a major network is known, but the subnet toward which the packet is destined within that major network is unknown, the packet is dropped.

I know some 10.0.0.0/24 subnets, but not 10.2.2.0/24

Router(config)# ip classless

The router only uses the default route if the major network destination does not exist in the routing table. To forward these packets to the best supernet route possible:

Configuring ip classless on the router allows it to ignore classful boundaries of the networks in its routing table and route to the default route.

Default route

RIP Configuration IssuesRIP routers rely on neighbors for network information - Routing By Rumour. Convergence is when all routers in the Internetwork have the same routing information. Slow convergence of DV protocols results in inconsistencies.

no ip split-horizonTo disable split horizon: GAD(config-if)#

RIPs performance can be tuned to improve convergence time:

The default holddown is 180 secs. Decrease it to speed up convergence. Set the timer just longer than the longest possible update time for the network:GAD(config-router)# timers basic 30 180 180 240

The default RIP update interval is 30 secs. Longer intervals can conserve bandwidth, shorter intervals may decrease convergence time:GAD(config-router)#

To disable sending routing updates on specified interfaces:GAD(config-router)# passive-interface f0/0

To configure the router to send and receive packets from only version 2:GAD(config-router)# version 2

To control how packets received from an interface are processed:GAD(config-if)# ip rip receive version 1 2

Update Invalid Holdtime Flush

update-timer 40

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Verifying RIP ConfigurationDublin# show ip protocols

RIP routing is configured

Interfaces sending and receiving RIP updates

Router is advertising the correct networks

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Verifying RIP ConfigurationDublin# show ip route

Route received from RIP neighbor is in the

routing table

Additional commands to check RIP configuration:

Command Definition

show interface

Interface's IP information and status

show running-config

Statistics for all interfaces configured on router

show ip interface

Current configuration in RAM

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Troubleshooting RIPTypical RIP configuration errors: incorrect network statement discontiguous subnets split horizons

Other commands to troubleshoot RIP:

Summary of interface status and parameters

debug ip rip {events}

View the routing table

show ip protocols {summary}

Summary of entries in RIP routing database

DefinitionCommand

Pretoria#

To analyse RIP update issues:

show ip rip database

show ip route

Check routing updates are being sentshow ip interface brief

Data for each routing protocol active on router

debug ip rip

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RIP Load BalancingLoad balancing allows a router to simultaneously use multiple paths to a destination. RIP can load balance over 6 equal-cost paths, (default 4 paths).

Here each path from GAD to BHM is considered equal by

RIP metric (2 hops)

Equal cost routes can be found by using #show ip route.Each route is represented by a routing descriptor block. An asterisk (*) next to one of the entries corresponds to the active route.

Router(config-router)# maximum-paths 5

RIP performs what is referred to as “round robin” load balancing: If process switching is enabled, paths alternate on a ___________ basis. If fast switching is enabled, paths alternate on a _______________ basis.

per-packetper-destination

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Redistributing Static Routes into RIP1. Static routes are important for destinations not included in dynamic routing

processes. They are also useful for specifying a ____________.

2. Each dynamic routing protocol has a default ________________________

3. A static route can be defined as less desirable than a dynamically learned route if its ____ is higher than the dynamic route’s.    

4. If a static route points to an interface that is not part of the RIP process (as defined with a network command) RIP will not advertise the route unless configured to:

5. _____________________ are routes with an AD set greater than the AD of the dynamic routing protocol in use.

6. Static routes are removed from the routing table when their corresponding interface ___________ or when the next hop is ________________.

7. Static routes can be removed using the ______________ global configuration command.

Router(config)#

default route

administrative distance (AD).

AD

router ripRouter(config-router)# redistribute static

Floating Static routes

goes down no longer valid

no ip route

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IGRP can be configured to use a combination of variables for its metric: Bandwidth Delay Reliability Load MTU

IGRP Features

Key design characteristics of IGRP are a follows: The versatility to automatically handle indefinite, complex topologies The flexibility to handle different bandwidth and delay characteristics Scalable to large networks

Interior Gateway Routing Protocol is a DV protocol proprietary to Cisco.IGRP sends routing updates at 90 second intervals, advertising networks for a particular AS.

– based on lowest bandwidth value in the path– the cumulative interface delay along the path

– of the link based on exchange of keepalives– amount of traffic on a link based on bits per second– Maximum Transmission Unit value of the path

Default metric components

These parameters are considered only if enabled via configuration

show ip protocols – displays parameters including metric values K1 to K5. K1= bandwidth, K3= delay.

show ip route – displays metric values in brackets for each route.

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IGRP Routes

Type Definition

Interior

Routes to networks outside AS.Used to identify default gateway.Different routers may choose different routes as the gateway of last resort.

System

Routes between subnets of a network attached to a router interface.

If the network is not subnetted, IGRP does not advertise them.

Exterior

Routes to networks within the AS. Derived from directly connected interfaces and information from other IGRP-speaking devices. Do not include subnet information.

Match the IGRP route type to its definition:

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IGRP Stability Features

Holddowns Split horizons Poison reverse updates

Like RIP, IGRP has a number of features designed to enhance its stability:

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

Router# show ip protocolsRouting protocol is “IGRP 101”Sending updates every 90 seconds, next due in 51 secondsInvalid after 270 seconds, holddown 280 seconds, flushed after 630 seconds< output omitted >

IGRP default timer values

How frequently routing update messages should be sent

How long to wait in the absence of specific updates before declaring a route invalid (Default: 3 x U)

Amount of time for which information about poorer routes is ignored (Default: 3 x U + 10)

Time before a route is flushed from the routing table (Default: 7 x U)

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Configuring IGRP

A B

Consider this network on which RIP is already running:

192.168.1.0/24192.168.2.0/24

192.168.3.0/24

A# show ip route< output omitted >C 192.168.1.0/24 is directly connected, FastEthernet0/0C 192.168.2.0/24 is directly connected, Serial0/0R 192.168.3.0/24 [120/1] via 192.168.2.2, 00:00:29, Serial0/0

IGRP is then configured on both routers, example:

A(config)#

Once A has received an IGRP update from B:A# show ip route< output omitted >C 192.168.1.0/24 is directly connected, FastEthernet0/0C 192.168.2.0/24 is directly connected, Serial0/0I 192.168.3.0/24 [100/80135] via 192.168.2.2, 00:00:69, Serial0/0

router igrp 101A(config-router)# network 192.168.1.0A(config-router)# network 192.168.2.0

AD and metric

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Verifying and Troubleshooting IGRP

Command Related Useshow ip route

To check if IGRP is enabled

show running-config

Filter the tail of the output from full command

show interface

Filter interface specific information from full command

debug ip igrp events

IGRP activity including route update details

ping

Hop by hop path determination utility

Check routing table for any ‘I’ routes

show ip protocolsVerify the router is configured for IGRP networks

show running-config begin …Verify an interface is properly configured

show running-config int …Overview of IGRP activity, updates sent and received

debug ip igrp transactionsEnd to end connectivity test at layer 3

traceroute