ccna exp2 - chapter04 - distance vector routing protocols
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Chapter 4 - Distance Vector Routing ProtocolsRouting Protocols and Concepts
CCNA Exploration version 4.0
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Objectives
Identify the characteristics of distance vector routingprotocols.
Describe the network discovery process of distancevector 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) 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 acomposite 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 theshortest path.
Routing updates are sent only when there is a change in thetopology.
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Distance Vector Routing Protocols
Distance Vector Technology The Meaning of Distance Vector:
A router using distance vector routing protocolsknows 2 things:
Distance to final destination
Vector or direction traffic should be directed
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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
Distance Vector Routing Protocols
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At the core of the distance vector protocol is the algorithm. The
algorithm is used to calculate the best paths and then send thatinformation to the neighbors.
Routing Protocol Algorithm:
- Defined as a procedure for accomplishing a certain task.
Distance Vector Routing Protocols
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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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Distance Vector Routing Protocols Comparison
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Network Discovery
Router initial start up After a cold start and before the exchange of routing
information, the routers initially discover their owndirectly connected networks and subnet masks.
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Network Discovery
Initial Exchange of Routing Information If a routing protocol is configured then- Routers will exchange routing information
Routing updates received from otherrouters
Router checks update for new information If there is new information:
o Metric is updated
o New information is stored in
routing table Refer to 4.2.2.1
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Exchange of RoutingInformation
Router convergence isreached when
- All routing tables in the
network contain the samenetwork information
Routers continue to exchangerouting information
-If no new information is foundthen Convergence is reached
Refer to 4.2.3.1.
Network Discovery
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Network Discovery
Convergence must be reachedbefore a network is consideredcompletely operable
Speed of achievingconvergence consists of 2
interdependent categories Speed of broadcasting
routing information
Speed of calculating routes
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Routing Table Maintenance
Periodic Updates : RIPv1 & RIPv2
These are time intervals in which a router sends out itsentire 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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Routing Table Maintenance
RIP uses 4 timers Update timer 30s.
Invalid timer 180s.
Holddown timer 180s.
Flush timer 240s.
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Routing Table Maintenance
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 routersthat need the information are updated.
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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Random Jitter Synchronized updates
A condition where multiple routers on multi access LAN segmentstransmit routing updates at the same time.
Problems with synchronized updates
Bandwidth consumption. Packet collision.
Solution to problems with synchronized updates
- Used of random variable called RIP_JITTER, 0% to 15% of thespecified update interval (25 to 30 seconds for the default 30-second
interval).
Routing Table Maintenance
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Routing Loops
Definitions & Implications Routing loops are
A condition in which apacket is continuouslytransmitted within a series of
routers without everreaching its destination.
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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
timely manner.
Routing Loops
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Routing Loop Issues
Routing Loop Example
Assume for the remainder of this example that Router Cs preferred path to
network 1 is by way of Router B. Router Cs routing table has a distance of 3 to network 1 via Router B.
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Routing Loop Issues
Network 1 Fails
Router E sends an update to Router A.
Router A stops routing packets to network 1.
But Routers B, C, and D continue to do so because they have not yet beeninformed about the failure.
Router A sends out its update.
Routers B and D stop routing to network1, (via Router A).
However, Router C is still not updated.
To router C, network 1 is still reachable via router B.
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Routing Loop Issues
Router C sends a periodic update to Router D
Router C sends a periodic update to Router D indicating a path to network 1(by way) of via Router B. (4 hops).
Router Ds Routing Table information for Network 1
Current path to Network 1 = Unreachable (down) Information from Router C: Network 1 : 4 hops by way of Router C
Normally, RouterD ignores this routing information because it usually has abetter route, 2 hops, via Router A, but this route is now down.
Router D changes its routing table to reflect this (good) better, but incorrectinformation, Network 1 by way of Router C (4 hops)
Router D propagates the information to Router A.
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Routing Loop Issues
Routers A changes its routing table
Router A adds new route to its routing table, get to Network 1 by way of RouterD (5 hops).
Propagates the information to Routers B and E.
Router B (and Router E) change their routing tables
Router B now believes it can get to Network 1 by way of Router A (6 hops).
Wow! I was about to tell Router C that Network 1 was down via Router B, butnow I have new information!
Propagates the incorrect information to Router C.
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Routing Loop Issues
Router C changes its routing table
Router C still believesit can get to Network 1 by way of Router B (7 hops).
Of course now it believes it is 7 hops instead of 3.
Propagates the newer but still incorrect information to Router D.
Here we go again!
Data packets destined for Network 1 get caught in a routing loop, from RoutersA to D to C to B to A to D etc.
As routing updates continue between the routers, the hop count gets greater
to infinity? (Not quite we will see in a moment.)
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Prevent routing loops
Several remedies to have been added to distance-vectoralgorithms to help prevent routing loops including:
Defining a maximum metric
Hold-down timers Split horizon
Route poisoning or poison reverse
Triggered updates
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Defining a Maximum
Problem: Count to infinity
Solution: Defining a Maximum
Distance vector routing algorithms are self-correcting, but a routing loopproblem can require a count to infinity.
To avoid this prolonged problem, distance vector protocols define infinity as aspecific maximum number.
This number refers to a routing metric which may simply be the hop count. When the metric value exceeds the maximum value, and as each router
receives this maximum metric, the network is then considered unreachable.
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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- This means that routers will leave a route marked asunreachable in that state for a period of time that is longenough for updates to propagate the routing tables with themost current information.
Routing Loops
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Holddown timers
Holddown timers work in the following way:
A router receives an update from a neighbor indicating thata network that previously was accessible is now no longeraccessible.
Refer to 4.4.4.1
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Routing Loops
The Split Horizon Rule is used to prevent routing loops Split Horizon rule:
A router should not advertise a network throughthe interface from which the update came.
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Routing Loops
Split horizon with poisonreverse: The rule statesthat once a router learnsan unreachable routethrough an interface,
advertise it asunreachable back throughthe same interface
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Routing Loops
IP & TTL Purpose of the TTL field
The TTL field is found in an IP header and is usedto prevent packets from endlessly traveling on anetwork
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Routing Loops
How the TTL field works TTL field contains a numeric value
The numeric value is decreased by one by every routeron the route to the destination.
If numeric value reaches 0 then Packet is discarded
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Routing Protocols Today
Factors used to determine whether to use RIP or EIGRPinclude
Network size
Compatibility between models of routers
Administrative knowledge
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Routing Protocols Today
RIP
Features of RIP:
Supports split horizon & split horizon with poisonreverse
Capable of load balancing Easy to configure
Works in a multi vendor router environment
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Routing Protocols Today
EIGRP Features of EIGRP:
Triggered updates.
EIGRP hello protocol used to establish neighboradjacencies.
Supports VLSM & route summarization. Use of topology table to maintain all routes.
Classless distance vector routing protocol.
Cisco proprietary protocol.
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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 linkand are configured to use the same protocol
The network discovery process for D.V. routingprotocol
Directly connected routes are placed in routingtable 1st
If a routing protocol is configured then Routers will exchange routing information
Convergence is reached when all network routershave the same network information
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Summary
D.V. routing protocols maintains routing tables by RIP sending out periodic updates
RIP using 4 different timers to ensure information isaccurate and convergence is achieved in a timelymanner
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 includedefining maximum hop count, holddown timers, splithorizon, route poisoning and triggered updates
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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 performanceincludes:
Excess use of bandwidth
CPU resources may be strained
Network convergence is degraded
Routing updates may be lost or not processed
S
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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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