CCNA – Semester 2

Chapter 4 –

Distance Vector Routing Protocols

CCNA Exploration version 4.0

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Objectives

• Identify the characteristics of distance vector routing

protocols.

• Describe the network discovery process of distance

vector routing protocols using Routing Information

Protocol (RIP).

• Describe the processes to maintain accurate routing

tables used by distance vector routing protocols.

• Identify the conditions leading to a routing loop and

explain the implications for router performance.

• Recognize that distance vector routing protocols are in

use today

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Introduction to Distance Vector

Routing Protocols

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Distance Vector Routing Protocols

• Dynamic routing protocols help the network administrator

overcome the time-consuming and exacting process of

configuring and maintaining static routes.

• 28 routers shown in the figure? What happens when a link goes

down? How do you ensure that redundant paths are available?

Dynamic routing is the most common choice for large networks

like the one shown.

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Distance Vector Routing Protocols

– Routing Information Protocol (RIP): RFC 1508

• Hop count is used as the metric

• Max hop is 15

• Routing updates are broadcast or multicast every 30 seconds

– Interior Gateway Routing Protocol (IGRP)

• Proprietary protocol developed by Cisco.

• Bandwidth, delay, load and reliability are used to create a composite metric.

• Routing updates are broadcast every 90 seconds,

• IGRP is the predecessor of EIGRP and is now obsolete

– Enhanced Interior Gateway Routing Protocol (EIGRP)

• It can perform unequal cost load balancing.

• It uses Diffusing Update Algorithm (DUAL) to calculate the shortest path.

• Routing updates are sent only when there is a change in the topology.

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Distance Vector Technology

• The Meaning of Distance Vector:

– A router using distance vector routing protocols knows 2 things:

Distance to final destination

Vector or direction traffic should be directed

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Distance Vector Technology

• Characteristics of Distance Vector routing protocols:

Periodic updates

Neighbors: Routing by rumor

Broadcast updates 255.255.255.255

Entire routing table is included with routing update

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Routing Protocol Algorithms

• At the core of the distance vector protocol is the algorithm. The

algorithm is used to calculate the best paths and then send that

information to the neighbors.

• Routing Protocol Algorithm:

- Defined as a procedure for accomplishing a certain task

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Routing Protocol Characteristics

• Routing Protocol Characteristics

• Criteria used to compare routing protocols includes

Time to convergence

Scalability

Classless (use of VLSM) or Classful

Resource usage

Implementation & maintenance

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Routing Protocol Characteristics

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Distance Vector Routing Protocols

• 4.1.4.2

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Network Discovery

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Cold Start

• Router initial start up

After a cold start and before the exchange of

routing information, the routers initially discover

their own directly connected networks and subnet

masks.

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Exchange of Routing Information

Initial Exchange:

• If a routing protocol is configured then

– Routers will exchange routing information

• Routing updates received from other routers

– Router checks update for new information

If there is new information:

– Metric is updated

– New information is stored in routing table

Exchange of Routing Information

• Router convergence is reached when

– All routing tables in the network contain the same network information

• Routers continue to exchange routing information

– If no new information is found then Convergence is reached

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Initial Exchange of Routing Information

10.1.0.0 10.2.0.0 10.3.0.0 10.4.0.0

10.1.0.0 0

Routing Table

10.2.0.0 0

10.2.0.0 0

Routing Table

10.3.0.0 0

10.3.0.0 0

Routing Table

10.4.0.0 0

10.3.0.0 1 10.1.0.0 1 10.2.0.0 1

10.4.0.0 1

10.4.0.0 2 10.1.0.0 2

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Convergence

• Convergence must be

reached before a network is

considered completely

operable

• Speed of achieving

convergence consists of 2

interdependent categories

– Speed of broadcasting

routing information

update

– Speed of calculating

routes

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Routing Table Maintenance

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Periodic Updates : RIPv1 & IGRP

• Periodic updates are time intervals in which a router sends out its entire routing table.

• Changes may occur for several reasons, including:

– Failure of a link

– Introduction of a new link

– Failure of a router

– Change of link parameters

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Periodic Updates : RIPv1 & IGRP

• RIP uses 4 timers

– Update timer 30s

– Invalid timer 180s

– Holddown timer 180s

– Flush timer 240s

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Bounded Updates: EIGRP

• EIRPG routing updates are

– Partial updates

– Triggered by topology changes

– Bounded: meaning the propagation of partial updates

are automatically bounded so that only those routers

that need the information are updated.

– Non periodic

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Triggered Updates

• Conditions in which triggered updates are sent

– Interface changes state

– Route becomes unreachable

– Route is placed in routing table

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Random Jitter

• Synchronized updates

– A condition where multiple

routers on multi access LAN

segments transmit routing

updates at the same time.

Problems with synchronized updates

• Bandwidth consumption

• Packet collisions

• Solution to problems with synchronized updates

Used of random variable called RIP_JITTER, 0% to 15%

of the specified update interval (25 to 30 seconds for the

default 30-second interval)

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Routing Loop

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Definitions & Implications

• Routing loop is a condition in which a packet is continuously transmitted within a series of routers without ever reaching its destination.

• The IP protocol has its own mechanism to prevent the possibility of a packet traversing the network endlessly. IP has a Time-to-Live (TTL) field and its value is decremented by 1 at each router. If the TTL is zero, the router drops the packet.

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Definitions & Implications

• Routing loops may be caused by:

– Incorrectly configured static routes

– Incorrectly configured route redistribution

– Slow convergence

– Incorrectly configured discard routes

• Routing loops can create the following issues

– Excess use of bandwidth

– CPU resources may be strained

– Network convergence is degraded

– Routing updates may be lost or not processed in a timely manner

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Count to Infinity

• This is a routing loop whereby packets bounce infinitely

around a network.

4.4.2.1

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Setting a maximum

• Distance Vector routing protocols set a specified

metric value to indicate infinity

– Once a router “counts to infinity” it marks the route

as unreachable

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Preventing Routing Loops with Holddown

Timers

• Network is unstable: a interface resets as up, then down,

then up again in rapid succession. The route is flapping.

Using triggered updates, the routers might react too quickly

and unknowingly create a routing loop. A routing loop could

also be created by a periodic update that is sent by the

routers during the instability. Holddown timers prevent

routing loops from being created by these conditions.

Holddown timers also help prevent the count to infinity

condition.

• Holddown timers allow a router to not accept any changes

to a route for a specified period of time

• Point of using holddown timers

– Allows routing updates to propagate through network

with the most current information.

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Holddown timers

Holddown timers work in the following way:

1. A router receives an update from a neighbor indicating

that a network that previously was accessible is now no

longer accessible.

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Holddown timers

2. The router marks the network as possibly down and starts

the holddown timer.

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Holddown timers

3. If an update with a better metric for that network is

received from any neighboring router during the holddown

period, the network is reinstated and the holddown timer is

removed.

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Holddown timers

4. If an update from any other neighbor is received during

the holddown period with the same or worse metric for

that network, that update is ignored. Thus, more time is

allowed for the information about the change to be

propagated.

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Holddown timers

5. Routers still forward packets to destination networks that

are marked as possibly down. This allows the router to

overcome any issues associated with intermittent

connectivity. If the destination network truly is unavailable

and the packets are forwarded, black hole routing is

created and lasts until the holddown timer expires.

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Split Horizon Rule

• Split Horizon rule:

– A router should not advertise a network through the

interface from which the update came.

4.4.5.1

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Split horizon with Poison Reverse or Route

Poisoning

• Route poisoning is used to mark the route as unreachable in a routing update that is sent to other routers.

• Split horizon with poison reverse:

– The rule states that once a router learns of an unreachable route through an interface, advertise it as unreachable back through the same interface

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IP & TTL

• Purpose of the TTL field

– The TTL field is found in an IP header and is used

to prevent packets from endlessly traveling on a

network

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IP & TTL

• How the TTL field works

– TTL field contains a numeric value

– The numeric value is decreased by one by every router

on the route to the destination.

– If numeric value reaches 0 then Packet is discarded

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Distance Vector Routing Protocols today

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RIP and EIGRP

• Factors used to determine whether to use RIP or EIGRP

include

– Network size

– Compatibility between models of routers

– Administrative knowledge

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RIP

• RIP:

– Supports split horizon & split horizon with poison reverse

– Capable of load balancing

– Easy to configure

– Works in a multi vendor router environment

• RIPv2 introduced the following improvements to RIPv1:

– Includes the subnet mask in the routing updates, making it a

classless routing protocol.

– Has authentication mechanism to secure routing table

updates.

– Supports variable length subnet mask (VLSM).

– Uses multicast addresses instead of broadcast.

– Supports manual route summarization.

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EIGRP

• Features:

– Triggered updates

– EIGRP hello protocol used to establish neighbor adjacencies

– Supports VLSM & route summarization

– Use of topology table to maintain all routes

– Classless distance vector routing protocol

– Cisco proprietary protocol

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Distance Vector Routing Protocols Compared

4.5.1.1

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Summary

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