distance vector protocols

Iskra Djonova-Popova

The 4th CEENet WorkshopBratislava, Slovakia 1

Distance Vector Protocols Distance Vector Protocols (contents)(contents)

IGPs - what are they and why they are needed

Routing algorithms - the design goals

Distance vector versus Link state RIP (Routing Information Protocol)



IGP - What Are TheyIGP - What Are They

Internal Gateway Protocols Single network administration Unique routing policy Make best use of network resources



IGP - Why They are NeededIGP - Why They are Needed

A B C

D E

2

3 4

5

1

Routing algorithms would scale better

Static routes would be fine

6



Routing Algorithms - the Routing Algorithms - the Design GoalsDesign Goals

Optimally Simplicity/Low overhead Robustness/ Stability Rapid Convergence Flexibility



Routing on the Internet - the Routing on the Internet - the Two ParadigmsTwo Paradigms

Centralized approach good routes can be found but reliability is

questionable too much traffic on lines connected to the

network center Distributed approach

the failure of one part doesn’t affect the functioning of the rest of the network



Distributed Approach WinsDistributed Approach Wins

GGP - Predecessor of RIP No distinction between hosts and

routers Attempts to keep track of the load

in the network



Internet GrowsInternet Grows

Autonomous systems are introduced to support hierarchical routing

The distance vector protocols are not any more sutable

Link state protocols are developed Routers routs



Host Released from Burden Host Released from Burden of Routingof Routing

The subnet masked bits of the destination address mach those of the local address true - the destination is on the local network

(next hop’s address is the destination address)

false - the destination is remote (next hop’s address is some of the routers)



Distance Vector Protocols Distance Vector Protocols

Neighboring nodes send information in regular time intervals

Install routes directly in tables, lowest cost wins

The information sent (the distance vectors) are all routes from the table



The Routing AlgorithmThe Routing Algorithm

the shortest path tree is contained in the routing table

Calculations are based on the Bellman-Ford algorithm



The Centralized Version of the The Centralized Version of the AlgorithmAlgorithm

Cycle Node B C D E

Initial (., ) (., ) (., ) (., )

1 (1, 1) (2, 2) (3, 1) (4, 2)

A B C

D E

2

3 45

6

1 A B C

D E

2

3 4

1



The Distributed VersionThe Distributed Version

A B C

D E

1 2

3 45

6

Example of simple network with 5 nodes (routers) and 6 links (interfaces)The cost of all links is assumed to be 1

From A to Link Cost B 1 1 C 1 2 D 3 1 E 1 2

Routing table for A



AdvantagesAdvantages

simple to implement

low requirement in processing and memory at the nodes

suitable for small networks



DisadvantagesDisadvantages

Slow convergence Bouncing effect Counting to infinity problem



Slow Convergence

A B C

D E

2

3 45

6

XXX

When a link breaks the routers are supposed to reestablish the routing tables

link 1 breaks



The Bouncing Effect

link 2 breaks and A sends its routing table to B before B sends it to A

A B C

D E

3 45

6

XXX1



Counting to Infinity Problems

D E

3

A B C2

45

XXX

XXX

Links 1 and 6break.

A sends its old routing table before D sends the new routing table



Split Horizon for Preventing Two-hop Loops

Simple the information about destination routed on

the link is omitted With poisonous reverse

the corresponding distance is set to infinity if the destination is routed on the link



Triggered UpdatesTriggered Updates

A timer is associated with each entry in the routing table much longer than the period of transmission

of information Triggered updates

request nodes to send messages as soon as they notice a change in the routing table



Different Distance Vector Different Distance Vector ProtocolsProtocols

Metric they use

Structure of the addresses

Range of links they support



RIP - Routing Information RIP - Routing Information ProtocolProtocol

first used in XNS (Xerox Network Systems) designed as a component of the networking

code for the BSD release of UNIX incorporated in program “routed” (rote management

daemon)

documented in rfc 1058



RIP - CharacteristicsRIP - Characteristics

the metric is a hop-count The value of 1 to 15 is used (16 denotes infinity)

supports point-to-point links and broadcast networks

doesn't support CIDR

used only in IP networks at first the intention was to be used in variety of

networks



RIP - CharacteristicsRIP - Characteristics packets are sent every 30 seconds

or faster when necessary route is considered down if not

refreshed within 180 sec. (distance set to infinity)

two kinds of messages request response



RIP - Message FormatRIP - Message Format

command (1) version (1) must be zero(2)

address family identifier (2) must be zero(2)

IP address(4)

must be zero(4)

must be zero(4)

metric(4)

0 31



RIP - ProcessingRIP - Processing

When processing an incoming response a set of validation checks are performed

if each address is valid A, B or C address the network number is not 127 (loopback) or 0

(except in in the case of default address 0.0.0.0) the host part is not a “broadcast address” the metric is not larger than 16 (infinity)



RIP - ProcessingRIP - Processing

the metric associated with the destination

the address of the “next router” a “recently updated” flag several timers

Each entry in the routing tables contains:



RIP - LimitationsRIP - Limitations

Maximum hop count of 15 restricts the use of RIP in larger networks, but

prevents the count to infinity problem (endless loops)

Difference in links speed is not reflected in the hop-count metrics congested links can be still included in the best

path



RIP2 - Why Was Developed?RIP2 - Why Was Developed?

many superior IGP exists (RIP is often referred as Rest In Peace)

there are still many implementations of RIP

given that RIP will still be used, it deserves improvements

RIP 2 is documented in RFC-1287, RFC-1388 and RFC-1389



RIP2 - Message FormatRIP2 - Message Format

command (1) version (1) Routing domain(2)

address family identifier (2) Route Tag(2)

IP address(4)

Next Hop(4)

Subnet Mask(4)

metric(4)



RIP2 - The Added FieldsRIP2 - The Added Fields

routing domain used together with the next hop field

to allow multiple autonomous systems to share a single wire

route tag to flag external routes and is for use

by EGP and BGP



RIP2 - ImprovementsRIP2 - Improvements

authentication routing per subnet support of multiple metrics routing domains multicasting



AuthenticationAuthentication

specifies that first entry in a packet can be replaced by an “authentication segment”

currently the only algorithm defined is simple “password procedure”



Routing per SubnetRouting per Subnet

support CIDR

subnet mask included in the message

compatible with RIP1 because the subnet filed is ignored when cooperating with RIP1



Multiple MetricsMultiple Metrics

metric contains two components hop count throughput, measured as 10logC

ten times the decimal logarithm of the maximum data rate in Kbs

selected path with largest throughput if two paths with same throughput the one

with lower hop count is chosen



Routing DomainRouting Domain

different autonomous systems share the same wire (Ethernet or FDDI) routers don’t want to process messages

bound to “his” network “routing domain” number is the

autonomous system number



To create a routing process for RIP, use the configuration command:

router rip

no router rip

To shut down the routing process use the command:

RIP - ConfigurationRIP - Configuration



Specifying the List of Specifying the List of NetworksNetworks

Specify the list of networks with the network router Specify the list of networks with the network router configuration subcommand.configuration subcommand.

network network-numbernetwork network-number no network network-numberno network network-number

The argument network-number is a network number in dotted IP The argument network-number is a network number in dotted IP notation (of directly connected networks). Note that this number notation (of directly connected networks). Note that this number must not contain subnet information. You may specify multiple must not contain subnet information. You may specify multiple network subcommands. RIP routing updates will be sent and network subcommands. RIP routing updates will be sent and received only through interfaces on this network. The network received only through interfaces on this network. The network router subcommand is a mandatory configuration command and router subcommand is a mandatory configuration command and must be included in the configuration of each IP routing process.must be included in the configuration of each IP routing process.



Example:

The following example configuration defines RIP as the routing protocol to be used on all interfaces connected to networks 128.99.0.0 and 192.31.7.0.

router ripnetwork 128.99.0.0network 192.31.7.0

To remove a network from the list, use the no network router subcommand followed by the network address.



RIP is not alone RIP is not alone IGRPIGRP

Developed in the mid1980s by cisco Systems, Inc.

Designed to overcome the limitations of RIP

Initially worked in IP environment, but latter ported to OSI CLNP networks



IGRP - Main CharacteristicsIGRP - Main Characteristics

Distance vector protocol Uses a combination of metrics

internetwork, delay, bandwidth, reliability and load

the weighting factors are set either by administrators or default values are used



IGRP - Additional flexibilityIGRP - Additional flexibility

Wide metric ranges allow satisfactory metric setting in

internetworks with widely varying performance characteristics

Permits multipath routing dual equal-bandwidth lines may run a

single stream of traffic in round-robin fashion



IGRP - Stability FeaturesIGRP - Stability Features hold-downs split horizons poison reverse updates timers

update timer hold time period invalid timer flush timer

distance vector protocols

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