Distance Vector Protocols contents

1
Distance Vector Protocols (contents)

• IGPs - what are they and why they are needed
• Routing algorithms - the design goals
• Distance vector versus Link state
• RIP (Routing Information Protocol)

2
IGP - What Are They

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

3
IGP - Why They are Needed
1
5
6

• Static routes would be fine

• Routing algorithms would scale better

4
Routing Algorithms - the Design Goals

• Optimally
• Simplicity/Low overhead
• Robustness/ Stability
• Rapid Convergence
• Flexibility

5
Routing on the Internet - the Two 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 doesnt affect the
  functioning of the rest of the network

6
Distributed Approach Wins

• GGP - Predecessor of RIP
• No distinction between hosts and routers
• Attempts to keep track of the load in the network

7
Internet 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

8
Host Released from Burden of Routing

• The subnet masked bits of the destination
  address mach those of the local address
• true - the destination is on the local network
  (next hops address is the destination address)
• false - the destination is remote (next hops
  address is some of the routers)

9
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

10
The Routing Algorithm

• the shortest path tree is contained in the
  routing table
• Calculations are based on the Bellman-Ford
  algorithm

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The Centralized Version of the Algorithm
1
2
A
B
C
4
3
5
6
D
E
Cycle Node B C D E Initial (.,
) (., ) (., ) (., ) 1 (1, 1) (2,
2) (3, 1) (4, 2)
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The Distributed Version
Routing table for A
2
1
A
B
C
From A to Link Cost B 1
1 C 1 2 D
3 1 E 1
2
4
3
5
6
D
E
Example of simple network with 5 nodes (routers)
and 6 links (interfaces) The cost of all links
is assumed to be 1
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Advantages

• simple to implement
• low requirement in processing and memory at the
  nodes
• suitable for small networks

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Disadvantages

• Slow convergence
• Bouncing effect
• Counting to infinity problem

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Slow Convergence
link 1 breaks
When a link breaks the routers are supposed to
reestablish the routing tables
16
The Bouncing Effect
link 2 breaks and A sends its routing table to
B before B sends it to A
17
Counting to Infinity Problems
A sends its old routing table before D sends the
new routing table
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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

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Triggered 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

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Different Distance Vector Protocols

• Metric they use
• Structure of the addresses
• Range of links they support

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RIP - Routing Information Protocol

• 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

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RIP - 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

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RIP - 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

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RIP - Message Format
0
31
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)
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RIP - 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)

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RIP - Processing
Each entry in the routing tables contains

• the metric associated with the destination
• the address of the next router
• a recently updated flag
• several timers

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RIP - 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

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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

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RIP2 - Message Format
command (1) version (1)
Routing domain(2)
address family identifier (2)
Route Tag(2)
IP address(4)
Subnet Mask(4)
Next Hop(4)
metric(4)
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RIP2 - 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

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RIP2 - Improvements

• authentication
• routing per subnet
• support of multiple metrics
• routing domains
• multicasting

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Authentication

• specifies that first entry in a packet can be
  replaced by an authentication segment
• currently the only algorithm defined is simple
  password procedure

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Routing per Subnet

• support CIDR
• subnet mask included in the message
• compatible with RIP1 because the subnet filed is
  ignored when cooperating with RIP1

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Multiple 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

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Routing Domain

• different autonomous systems share the same wire
  (Ethernet or FDDI)
• routers dont want to process messages bound to
  his network
• routing domain number is the autonomous system
  number

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RIP - Configuration
To create a routing process for RIP, use the
configuration command
router rip
To shut down the routing process use the command
no router rip
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Specifying the List of Networks
Specify the list of networks with the network
router configuration subcommand.
network network-number
no network network-number
The argument network-number is a network number
in dotted IP notation (of directly connected
networks). Note that this number must not contain
subnet information. You may specify multiple
network subcommands. RIP routing updates will be
sent and received only through interfaces on this
network. The network router subcommand is a
mandatory configuration command and must be
included in the configuration of each IP routing
process.
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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 rip
network 128.99.0.0
network 192.31.7.0
To remove a network from the list, use the no
network router subcommand followed by the network
address.
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RIP is not alone IGRP

• 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

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IGRP - 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

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IGRP - 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

42
IGRP - Stability Features

• hold-downs
• split horizons
• poison reverse updates
• timers
• update timer
• hold time period
• invalid timer
• flush timer