Download CCNA2 3.1-07 Distance Vector Routing Protocols

Module 7
Distance Vector
Routing Protocols
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Distance Vector Routing Updates
• Routing table updates occur periodically or when the topology
in a distance vector protocol network changes.
• Topology change updates proceed systematically from router to
router.
• Distance vector algorithms call
for each router to send its entire
routing table to each of its
adjacent neighbors.
• The routing tables include
information about the total path
cost as defined by the metrics.
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Distance Vector Routing Metric
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Convergence
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Convergence is the speed and ability of a group of
internetworking devices running a specific routing protocol to
agree on the topology of an internetwork after a change in that
topology.
A network has converged when all routers have consistent
knowledge and correct routing tables.
Routing loops can occur when inconsistent routing tables are
not updated due to slow convergence in a changing network.
If a network has not converged, the following are possible:
– Inconsistent routing tables
– Inconsistent traffic forwarding
– Routing loops
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Routing Loops
• Invalid updates will continue to loop until some process stops
the looping.
• This condition is called count to infinity.
• Packets loop continuously around the network in spite of the
fundamental fact that the destination network is down.
• These packets loop through the network because of wrong
information in the routing tables.
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Routing Loops
• To reduce routing loops and counting to infinity, RIP uses the
following techniques:
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Count-to-infinity
Split horizon
Poison reverse
Holddown counters
Triggered updates
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Routing Loops – Count to Infinity
• Distance vector routing algorithms are self-correcting, but a
routing loop problem can require a count to infinity.
• When a routing table update is received by a router in a
distance vector network, the hop count for the updated route is
incremented by one.
• Without countermeasures to stop the count to infinity process,
the distance vector metric of hop count increments each time
the packet passes through another router.
• To avoid this prolonged problem, distance vector protocols
define infinity as a specific maximum number.
• With this approach, the routing protocol permits the routing loop
to continue until the metric exceeds its maximum allowed
value.
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Routing Loops – Split-Horizon
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Another possible source for a routing loop occurs when
incorrect information that has been sent back to a router
contradicts the correct information that the router originally
distributed.
Split-horizon attempts to avoid this situation.
If a routing update about a network arrives from a router, the
router that receives the update cannot send information about
that network back to the router that originally sent the update.
Split-horizon thus reduces incorrect routing information and
reduces routing overhead.
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Routing Loops – Split-Horizon
• The split horizon rule is based on the theory that it is not useful
to send information about a route back in the direction from
which it came.
If router A
receives an
update from
router D, it would
not send that
information back
to router D.
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Routing Loops – Route Poisoning
• Route poisoning is used by various distance vector protocols in
order to overcome large routing loops and offer explicit
information when a subnet or network is not accessible.
• Route poisoning accomplishes this by setting the hop count to
one more than the maximum.
The maximum hop
count for RIP is 15.
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Routing Table Updates
• New routing tables are sent to neighboring routers on a regular
basis (RIP updates occur every 30 seconds).
• However a triggered update is sent immediately in response to
some change in the routing table.
• The router that detects a topology change immediately sends
an update message to adjacent routers that, in turn, generate
triggered updates notifying their adjacent neighbors of the
change.
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Holddown Timers
• A count to infinity problem can be avoided by using holddown
timers
• When a router receives an update from a neighbor indicating
that a previously accessible network is now inaccessible, the
router marks the route as inaccessible and starts a holddown
timer
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Holddown Timers
• If at any time before the holddown timer expires an update is
received from the same neighbor indicating that the network is
again accessible, the router marks the network as accessible
and removes the holddown timer.
• If an update arrives from a different neighboring router with a
better metric than originally recorded for the network, the router
marks the network as accessible and removes the holddown
timer.
• If at any time before the holddown timer expires an update is
received from a different neighboring router with a poorer
metric, the update is ignored.
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Holddown Timers
• Holddown timers help prevent counting to infinity but also
increase convergence time.
• The default holddown for RIP is 180 seconds.
• This will prevent any inferior route from being updated but may
also prevent a valid alternative route from being installed.
• The holddown timer can be decreased to speed up
convergence but should be done with caution.
• The ideal setting would be to set the timer just longer that the
longest possible update time for the internetwork.
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Holddown Timers
• The ideal setting would be to set the timer just longer that the
longest possible update time for the internetwork.
Set the holddown timer for just over
120 second – example 126
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Holddown Timers
• One additional item that affects convergence time, and is
configurable, is the update interval.
• The default RIP update interval in Cisco IOS is 30 seconds.
• This can be configured for longer intervals to conserve
bandwidth, or for shorter intervals to decrease convergence
time.
• To change the update internal:
– GAD(config-router)#update-timer seconds
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Routing Information Protocol (RIP)
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RIP is a distance vector routing protocol
Hop count is used as the metric for path selection
If the hop count is greater than 15, the packet will be discarded
By default, routing updates are broadcast every 30 seconds
RIP has evolved over the years from a Classful Routing
Protocol, RIP Version 1 (RIP v1), to a Classless Routing
Protocol, RIP Version 2 (RIP v2).
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Routing Information Protocol (RIP)
• RIP prevents routing loops from continuing indefinitely by
implementing a limit on the number of hops allowed in a path
from the source to a destination.
• The maximum number of hops in a path is 15.
• When a router receives a routing update that contains a new or
changed entry, the metric value is increased by 1 to account for
itself as a hop in the path.
• If this causes the metric to be incremented beyond 15, it is
considered to be infinity and the network destination is
considered unreachable.
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Routing Information Protocol (RIP)
• RIP sends routing-update messages at regular intervals.
• When a router receives a routing update that includes changes
to an entry, it updates its routing table to reflect the new route.
• The received metric value for the path is increased by 1, and
the source interface of the update is indicated as the next hop
in the routing table.
• RIP routers maintain only the best route to a destination but can
maintain multiple equal-cost paths to the destination.
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Configuring RIP
• The router rip command enables RIP as the routing
protocol.
• The network command is then used to tell the router on which
interfaces to run RIP.
• The routing process then associates specific interfaces with the
network addresses and begins sending and receiving RIP
updates on these interfaces.
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Configuring RIP
• Router(config)#router rip
– Enables the RIP routing process
• Router(config-router)#network network-number
– Associates a network with the RIP routing process
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Configuring RIP
How would a user configure RIP on the GAD router?
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ip classless Command
• In order for the Cisco IOS software to forward packets to the
best supernet route possible, use the ip classless global
configuration command.
• If the router receives a packet for a subnet that does not have a
default route and ip classless is enabled, it will forward the
packet to the subnet via a supernet route.
• A supernet route is a route that covers a greater range of
subnets with a single entry.
• For example, an enterprise uses the entire subnet 10.10.0.0
/16, then a supernet route for 10.10.10.0 /24 would be
10.10.0.0 /16.
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ip classless Command
• The ip classless command is enabled by default in Cisco
IOS Software Release 11.3 and later.
• To disable this feature, use the no form of this command.
– no ip classless
• When this feature is disabled any packets received that are
destined for a subnet that numerically falls within the router’s
subnetwork addressing scheme will be discarded.
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passive-interface command
• Another issue with routing protocols is the unwanted
advertisement of routing updates out a particular interface.
• When a network command is issued for a given network, RIP
will immediately begin sending advertisements out all interfaces
within the specified network address range.
• To control the set of interfaces that will exchange routing
updates, the network administrator can disable the sending of
routing updates on specified interfaces by configuring the
passive-interface command.
– GAD(config-router)#passive-interface e0
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Verifying RIP
• There are several commands that can be used to verify that
RIP is properly configured.
• Two of the most common methods are:
– show ip route command
– show ip protocols command.
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show ip protocols
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show ip route
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Troubleshooting RIP
• One highly effective command for finding RIP update issues is
the debug ip rip command.
• The debug ip rip command displays RIP routing updates as
they are sent and received.
Shows activity as
it occurs
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Troubleshooting RIP
• Other commands to troubleshoot RIP:
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show ip rip database
show ip protocols {summary}
show ip route
debug ip rip {events}
show ip interface brief
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Load Balancing with RIP
• Load balancing is a concept that allows a router to take
advantage of multiple best paths to a given destination.
• RIP is capable of load balancing over as many as six equalcost paths, with four paths being default.
• RIP performs what is referred to as “round robin” load
balancing. This means that RIP takes turns forwarding packets
over the parallel paths.
– The router will start with an interface pointer to the interface
connected to router 1.
– Then the interface pointer cycles through the interfaces and routes
in a deterministic fashion such as 1-2-3-4-1-2-3-4-1.
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Load Balancing with RIP
• Because the metric for RIP is hop count, no regard is given
to the speed of the links.
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Load Balancing with RIP
• Because the metric for RIP is hop count, no regard is given
to the speed of the links.
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Load Balancing
• Load balancing is a concept that allows a router to take
advantage of multiple best paths to a given destination.
• The paths are derived either statically or with dynamic
protocols, such as RIP, EIGRP, OSPF, and IGRP.
• When a router learns multiple routes to a specific network, the
route with the lowest administrative distance is installed in the
routing table.
• Sometimes the router must select a route from among many,
learned via the same routing process with the same
administrative distance.
• The router chooses the path with the lowest cost or metric to
the destination.
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Load Balancing
• Each routing process calculates its cost differently and the
costs may need to be manually configured in order to achieve
load balancing.
• RIP networks must have the same hop count to load balance,
whereas IGRP uses bandwidth to determine how to load
balance.
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Administrative Distance
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Load Balancing
• When routing IP, the Cisco IOS offers two methods of load
balancing, per-packet and per-destination load balancing.
• If process switching is enabled, the router will alternate paths
on a per-packet basis.
• If fast switching is enabled, only one of the alternate routes will
be cached for the destination address, so all packets in the
packet stream bound for a specific host will take the same path.
• Packets bound for a different host on the same network may
use an alternate route, traffic is load balanced on a perdestination basis.
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Static Routes
• Static routes are user-defined routes that force packets moving
between a source and a destination to take a specific path.
• They are useful for specifying a “gateway of last resort”,
commonly referred to as a default route.
• A static route can be defined as less desirable than a
dynamically learned route, as long as the AD of the static route
is higher than that of the dynamic route.
• The syntax for configuring a static route is:
– ip route destination mask {interface / nexthop}
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Static Routes
• A static route has been configured on the GAD router to take
the place of the RIP route in the event that the RIP routing
process fails.
• This is referred to as a floating static route.
• The floating static route was configured by defining an AD on
the static route (130) greater than the default AD of RIP (120).
GAD(config)#ip route 172.16.0.0 255.255.0.0 192.168.14.2 130
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IGRP
• IGRP is a distance vector Interior Gateway Protocol (IGP).
• Cisco created this routing protocol to overcome the problems
associated with RIP.
• IGRP converges faster than RIP
• Routers using distance vector protocols must send all or a
portion of their routing table in a routing update message at
regular intervals to each of their neighboring routers.
• As routing information spreads throughout the network, routers
perform the following functions:
– Identify new destinations
– Learn of failures
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IGRP
• IGRP is a distance vector routing protocol developed by Cisco
(it is a Cisco proprietary routing protocol).
• IGRP sends routing updates at 90 second intervals,
advertising networks for a particular autonomous system.
• By default, the IGRP routing protocol uses bandwidth and delay
as metrics.
• Additionally, IGRP can be configured to use a combination of
variables to determine a composite metric.
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Bandwidth
Delay
Load
Reliability
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A composite metric is more accurate than the hop
count metric that RIP uses when choosing a path
to a destination.
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IGRP
• The show ip
protocols
command displays
parameters, filters,
and network
information
concerning the
routing protocols in
use on the router.
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Default is 100, max. is 255
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IGRP
?
• Given the following information from the show ip protocols
command, when would the next update interval be expected?
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IGRP
IGRP has a set of timers to enhance its performance and
functionality:
• Update Timer: These specify how frequently routing-update
messages should be sent. The default is 90 seconds.
• Invalid Timers: These specify how long a router should wait
before declaring a route invalid if it does not receive a specific
update about it. The default is three times the update period.
• Hold-down Timers: The holddown timer specifies the amount of
time for which information about poorer routes is ignored.The
default is three times the update timer period plus 10 seconds.
• Route Flush Timer:These indicate how much time should pass
before a route should be flushed from the routing table. The
default is seven times the routing period.
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IGRP
If default settings are used, by looking
at the update interval you can tell what
routing protocol is being used.
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IGRP
• The show ip
route command
shows the metric
values in brackets.
• The “I” verifies that
IGRP is configured.
Administrative distance / Composite Metric
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IGRP Routes
• IGRP advertises three types of routes:
– Interior – routes between subnets of a network attached to a router
interface (within an autonomous system).
– System – routes to networks within an autonomous system and
does not include subnets
– Exterior – exterior routes are routes to networks outside the
autonomous system
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IGRP
• IGRP has a number of features that are designed to enhance
its stability, such as:
– Holddowns - used to prevent regular update messages from
inappropriately reinstating a route that may not be up.
– Split horizons - derived from the premise that it is usually not
useful to send information about a route back in the direction from
which it came.
– Poison reverse updates – used to defeat larger routing loops,
increases the hop count to one more than the maximum hop
count.
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Configuring IGRP
• To configure the IGRP routing process, use the router igrp
configuration command. To shut down an IGRP routing
process, use the no form of this command.
– Router(config)#router igrp as-number
• The Autonomous System number is one that identifies the
IGRP process. It is also used to tag the routing information.
• To specify a list of networks for IGRP routing processes, use
the network router configuration command. To remove a
network, use the no form of this command.
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Configuring IGRP
RouterA
RouterB
192.168.2.32/24
192.168.1.32/27
192.168.1.64/27
192.168.3.32/24
RouterA(config)#router igrp 100
RouterA(config-router)#network 192.168.1.0
RouterA(config-router)#network 192.168.2.0
• What commands would be needed to configure RouterB for
IGRP with the autonomous system number 100?
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IGRP
• IGRP is showing its age, it lacks support for variable length
subnet masks (VLSM).
• Rather than develop an IGRP version 2 to correct this problem,
Cisco has built upon IGRP's legacy of success with Enhanced
IGRP.
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For more information on IGRP, check out
the following links.
http://www.cisco.com/warp/public/103/5.html
or
http://www.cisco.com/warp/public/103/5.pdf
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