Transcript Distance Vector Routing Protocols

Distance Vector Routing
Protocols and
RIP V.1
Routing Protocols and Concepts – Chapter 4, 5
Editted by Nugroho Agus H., M.Si.
Version 4.0
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Routing Protocol Algorithm:
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Routing Protocol Characteristics
– Criteria used to compare routing protocols includes
• Time to convergence
• Scalability
• Resource usage
• Implementation & maintenance
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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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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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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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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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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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Characteristics of Distance Vector routing protocols:
– Periodic updates
– Neighbors
– Broadcast updates
– Entire routing table is included with routing update
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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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Distance Vector Routing Protocols
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Routing Table Maintenance
 Periodic Updates: RIPv1 & RIPv2
– These are time intervals in which a router sends out its entire
routing table
 RIP uses 4 timers
– Update timer
– Invalid timer
– Holddown timer
– Flush timer
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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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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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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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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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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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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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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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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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 Routing Information Protocol (RIP)
– A distance vector protocol that has 2 versions
• RIPv1 - a classful routing protocol
• RIPv2 - a classless routing protocol
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RIPv1
 RIP Characteristics
– A classful, Distance Vector (DV) routing protocol
– Metric = hop count
– Routes with a hop count > 15 are unreachable
– Updates are broadcast every 30 seconds
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RIPv1
 RIP Operation
– RIP uses 2 message types:
• Request message
– This is sent out on startup by each RIP enabled
interface
– Requests all RIP enabled neighbors to send routing
table
• Response message
– Message sent to requesting router containing routing
table
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RIPv1
 IP addresses initially
divided into classes
– Class A
– Class B
– Class C
 RIP is a classful routing
protocol
– Does not send subnet
masks in routing
updates
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RIPv1
 Administrative Distance
– RIP’s default administrative distance is 120
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Basic RIPv1 Configuration
 A typical topology
suitable for use by
RIPv1 includes:
– Three router set up
– No PCs attached to
LANs
– Use of 5 different IP
subnets
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Basic RIPv1 Configuration
 Router RIP Command
– To enable RIP enter:
• Router rip at the global configuration prompt
• Prompt will look like R1(config-router)#
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Basic RIPv1 Configuration
 Specifying Networks
– Use the network
command to:
• Enable RIP on all
interfaces that
belong to this
network
• Advertise this
network in RIP
updates sent to
other routers every
30 seconds
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Verification and Troubleshooting
 Show ip Route
 To verify and troubleshoot
routing
– Use the following commands:
• show ip route
• show ip protocols
• debug ip rip
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Verification and Troubleshooting
 show ip protocols command
– Displays routing protocol configured on router
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Verification and Troubleshooting
 Debug ip rip command
– Used to display RIP routing updates as they are
happening
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Verification and Troubleshooting
 Passive interface command
– Used to prevent a router from sending updates through
an interface
– Example:
• Router(config-router)#passive-interface interface-type interface-number
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Passive interfaces
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Automatic Summarization
 Modified Topology
 The original scenario has been
modified such that:
– Three classful networks are used:
• 172.30.0.0/16
• 192.168.4.0/24
• 192.168.5.0/24
– The 172.30.0.0/16 network is
subnetted into three subnets:
• 172.30.1.0/24
• 172.30.2.0/24
• 172.30.3.0/24
– The following devices are part of the
172.30.0.0/16 classful network address:
• All interfaces on R1
• S0/0/0 and Fa0/0 on R2
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Configuration Details
– To remove the RIP
routing process use the
following command
• No router rip
– To check the
configuration use the
following command
• Show run
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Boundary Routers
– RIP automatically summarizes classful networks
– Boundary routers summarize RIP subnets from one
major network to another
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Automatic Summarization
 Advantages of automatic
summarization:
– The size of routing
updates is reduced
– Single routes are used
to represent multiple
routes which results in
faster lookup in the
routing table
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Automatic Summarization
 Disadvantage of
Automatic
Summarization:
– Does not support
discontiguous
networks
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Automatic Summarization
 Discontiguous
Topologies do not
converge with RIPv1
 A router will only
advertise major
network addresses
out interfaces that do
not belong to the
advertised route
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Default Route and RIPv1
 Modified Topology: Scenario C
 Default routes
– Packets that are not defined specifically in a routing
table will go to the specified interface for the default
route
– Example: Customer routers use default routes to
connect to an ISP router
– Command used to configure a default route is ip
route 0.0.0.0 0.0.0.0 s0/0/1
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Default Route and RIPv1
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Default Route and RIPv1
 Propagating the Default Route in RIPv1
 Default-information originate command
– This command is used to specify that the router is to originate
default information, by propagating the static default route in
RIP update
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Summary
 RIP characteristics include:
– Classful, distance vector routing protocol
– Metric is Hop Count
– Does not support VLSM or discontiguous subnets
– Updates every 30 seconds
 Rip messages are encapsulated in a UDP segment
with source and destination ports of 520
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Summary: Commands used by RIP
Command
Command’s purpose
Rtr(config)#router rip
Enables RIP routing process
Rtr(config-router)#network …. …. … …
Associates a network with a RIP routing process
Rtr#debug ip rip
used to view real time RIP routing updates
Rtr(config-router)#passive-interface fa0/0
Prevent RIP updates from going out an interface
Rtr(config-router)#default-information originate
Used by RIP to propagate default routes
Rtr#show ip protocols
Used to display timers used by RIP
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