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Chapter_4_Distance Vector Routing Protocols

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Nguồn: ATHENA
Người gửi: Nguyễn Việt Vương
Ngày gửi: 18h:45' 21-09-2016
Dung lượng: 1.2 MB
Số lượt tải: 1
Số lượt thích: 0 người
Distance Vector Routing Protocols
www.Athena.Edu.Vn
Routing Protocols and Concepts – Chapter 4
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.
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)
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
Distance Vector Routing Protocols
Characteristics of Distance Vector routing protocols:
Periodic updates
Neighbors
Broadcast updates
Entire routing table is included with routing update
Distance Vector Routing Protocols
Routing Protocol Algorithm:
Defined as a procedure for accomplishing a certain task
Distance Vector Routing Protocols
Routing Protocol Characteristics
Criteria used to compare routing protocols includes
Time to convergence
Scalability
Resource usage
Implementation & maintenance
Distance Vector Routing Protocols
Network Discovery
Router initial start up (Cold Starts)
Initial network discovery
Directly connected networks are initially placed in routing table
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
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
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
Routing Table Maintenance
Periodic Updates: RIPv1 & RIPv2
These are time intervals in which a router sends out its entire routing table
Routing Table Maintenance
RIP uses 4 timers
Update timer
Invalid timer
Holddown timer
Flush timer
Routing Table Maintenance
Bounded Updates: EIGRP
EIRPG routing updates are:
Partial updates
Triggered by topology changes
Bounded
Non periodic
Routing Table Maintenance
Triggered Updates
Conditions in which triggered updates are sent
Interface changes state
Route becomes unreachable
Route is placed in routing table
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
Use of random variable called RIP_JITTER
Routing Loops
Routing loops are
A condition in which a packet is continuously transmitted within a series of routers without ever reaching its destination.
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
Routing Loops
Count to Infinity
This is a routing loop whereby packets bounce infinitely around a network
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
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
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
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
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
Routing Protocols Today
Factors used to determine whether to use RIP or EIGRP include
Network size
Compatibility between models of routers
Administrative knowledge
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
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
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
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

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
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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