© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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Chapter 1 1
Distance Vector Routing
Protocols
Routing Protocols and Concepts – Chapter 4
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Chapter 1 2© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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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Distance Vector Routing Protocols
 Examples of Distance Vector routing protocols:
Routing Information Protocol (RIP)
Interior Gateway Routing Protocol (IGRP)
Enhanced Interior Gateway Routing Protocol
(EIGRP)
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Chapter 1 4© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Distance Vector Routing Protocols
 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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Chapter 1 5© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Distance Vector Routing Protocols
Characteristics of Distance Vector routing protocols:
 Periodic updates
 Neighbors
 Broadcast updates
 Entire routing table is included with routing update
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Distance Vector Routing Protocols
 Routing Protocol Algorithm:
-Defined as a procedure for accomplishing a certain task
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Chapter 1 7© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Distance Vector Routing Protocols
Routing Protocol Characteristics
–Criteria used to compare routing protocols includes
-Time to convergence
-Scalability
-Resource usage
-Implementation & maintenance
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Chapter 1 8© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Distance Vector Routing Protocols
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Network Discovery
 Router initial start up (Cold Starts)
-Initial network discovery
Directly connected networks are initially placed in
routing table
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Chapter 1 10© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Network Discovery
 Initial Exchange of Routing Information
–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
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Chapter 1 11© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Network Discovery
 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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Chapter 1 12© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Network Discovery
 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
-Speed of calculating routes
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Chapter 1 13© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Table Maintenance
 Periodic Updates: RIPv1 & RIPv2
These are time intervals in which a router sends
out its entire routing table.
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Routing Table Maintenance
 RIP uses 4 timers
-Update timer
-Invalid timer
-Holddown timer
-Flush timer
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Routing Table Maintenance
 Bounded Updates: EIGRP
 EIRPG routing updates are
-Partial updates
-Triggered by topology changes
-Bounded
-Non periodic
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Routing Table Maintenance
 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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Routing Table Maintenance
 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
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Chapter 1 18© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Loops
 Routing loops are
A condition in
which a packet is
continuously
transmitted within
a series of routers
without ever
reaching its
destination.
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Chapter 1 19© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Loops
 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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Routing Loops
 Count to Infinity
This is a routing loop whereby packets bounce
infinitely around a network.
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Routing Loops
 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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Chapter 1 22© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Loops
 Preventing loops with holddown timers
-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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Chapter 1 23© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Loops
 The Split Horizon Rule is used to prevent routing
loops
 Split Horizon rule:
A router should not advertise a network through the
interface from which the update came.
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Routing Loops
 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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Chapter 1 25© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Loops
 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
 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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Chapter 1 26© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Protocols Today
 Factors used to determine whether to use RIP or EIGRP
include
-Network size
-Compatibility between models of routers
-Administrative knowledge
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Chapter 1 27© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Protocols Today
 RIP
Features of RIP:
-Supports split horizon & split horizon with
poison reverse
-Capable of load balancing
-Easy to configure
-Works in a multi vendor router environment
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Chapter 1 28© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Protocols Today
 EIGRP
Features of EIGRP:
-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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Chapter 1 29© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Summary
 Characteristics of Distance Vector routing
protocols
–Periodic updates
–RIP routing updates include the entire routing table
–Neighbors are defined as routers that share a link and are
configured to use the same protocol
 The network discovery process for D.V. routing
protocol
–Directly connected routes are placed in routing table 1st
–If a routing protocol is configured then
•Routers will exchange routing information
–Convergence is reached when all network routers have the
same network information
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Chapter 1 30© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Summary
 D.V. routing protocols maintains routing tables by
–RIP sending out periodic updates
–RIP using 4 different timers to ensure information is accurate
and convergence is achieved in a timely manner
–EIGRP sending out triggered updates
 D.V. routing protocols may be prone to routing loops
– routing loops are a condition in which packets continuously
traverse a network
–Mechanisms used to minimize routing loops include defining
maximum hop count, holddown timers, split horizon, route
poisoning and triggered updates
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Chapter 1 31© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Summary
 Conditions that can lead to routing loops include
–Incorrectly configured static routes
–Incorrectly configured route redistribution
–Slow convergence
–Incorrectly configured discard routes
 How routing loops can impact network performance
includes:
–Excess use of bandwidth
–CPU resources may be strained
–Network convergence is degraded
–Routing updates may be lost or not processed
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Chapter 1 32© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Summary
 Routing Information Protocol (RIP)
A distance vector protocol that has 2 versions
RIPv1 – a classful routing protocol
RIPv2 - a classless routing protocol
 Enhanced Interior Gateway Routing Protocol
(EIGRP)
–A distance vector routing protocols that has some features of
link state routing protocols
–A Cisco proprietary routing protocol
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Chapter 1 33© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public