Transcript
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CCNA2
DYNAMIC ROUTING
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DYNAMIC ROUTING
Dynamic routing protocols can help simplify
the life of a network administrator Routing Information Protocol (RIP) is a
distance vector routing protocol that is usedin thousands of networks throughout the
world. The fact that RIP is based on open standards
and is very simple to implement makes itattractive to some network administrators
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Distance VectorRouting
Routing table updates occur periodically or
when the topology in a distance vectorprotocol network changes
Routing loops can occur when inconsistent
routing tables are not updated due to slow
convergence in a changing network
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Distance VectorRouting
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Distance VectorRouting
Just before the failure of Network 1, all
routers have consistent knowledge andcorrect routing tables. The network is said to
have converged. Assume for the remainder
of this example that Router C's preferred
path to Network 1 is by way of Router B, andthe distance from Router C to Network 1 is 3.
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Distance VectorRouting
When Network 1 fails, Router E sends an update to
Router A. Router A stops routing packets to Network1, but Routers B, C, and D continue to do so
because they have not yet been informed of the
failure. When Router A sends out its update, Routers
B and D stop routing to Network 1. However, Router
C has not received an update. To Router C, Network1 is still reachable via Router B.
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Distance VectorRouting
Now Router C sends a periodic update to Router D,
indicating a path to Network 1 by way of Router B.Router D changes its routing table to reflect this
good, but incorrect, information, and propagates the
information to Router A. Router A propagates the
information to Routers B and E, and so on. Any
packet destined for Network 1 will now loop fromRouter C to B to A to D and back to again to C.
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Distance VectorRouting
The routing protocol permits the routing loop
to continue until the metric exceeds itsmaximum allowed value
When the metric value exceeds the
maximum value, Network 1 is considered
unreachable.
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Split horizon
Another possible source for a routing loop
occurs when incorrect information that hasbeen sent back to a router contradicts the
correct information that the router originally
distributed.
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Split horizon
Split-horizon attempts to avoid this situation.
If a routing update about Network 1 arrivesfrom Router A, Router B or Router D cannot
send information about Network 1 back to
Router A. Split-horizon thus reduces
incorrect routing information and reducesrouting overhead.
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Split horizon
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Split horizon
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Routepoisoning
Route poisoning is used by various distance
vector protocols in order to overcome largerouting loops and offer explicit information
when a subnet or network is not accessible.
This is usually accomplished by setting the
hop count to one more than the maximum.
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Routepoisoning
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Routepoisoning
One way to avoid inconsistent updates is
route poisoning. When Network 5 goesdown, Router E initiates route poisoning by
making a table entry for Network 5 as 16, or
unreachable.
By this poisoning of the route to Network 5,Router C is not susceptible to incorrect
updates about the route to Network 5
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Triggeredupdates
New routing tables are sent to neighboring
routers on a regular basis. For example, RIPupdates occur every 30 seconds
However a triggered update is sent
immediately in response to some change in
the routing table.
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Triggeredupdates
The router that detects a topology change
immediately sends an update message toadjacent routers that, in turn, generate
triggered updates notifying their adjacent
neighbors of the change
When a route fails, an update is sentimmediately rather than waiting on the
update timer to expire
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Holddowntimers
A state where the router cannot advertise or
accept advertisement A router is placed in holdtime when a link of
the router fails
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Holddowntimers
Holddown timers help prevent counting to
infinity but also increase convergence time.The default holddown for RIP is 180 seconds
The holddown timer can be decreased to
speed up convergence
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Holddowntimers
When a router receives an update from a neighbor
indicating that a previously accessible network isnow inaccessible, the router marks the route as
inaccessible and starts a hold down timer
If at any time before the hold down timer expires an
update is received from the same neighbor indicating
that the network is again accessible, the routermarks the network as accessible and removes the
hold down timer.
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Holddowntimers
If an update arrives from a different
neighboring router with a better metric thanoriginally recorded for the network, the routermarks the network as accessible andremoves the holddown timer.
If at any time before the holddown timerexpires an update is received from a differentneighboring router with a poorer metric, theupdate is ignored.
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RIP routingprocess
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). RIP v2
enhancements include:
Ability to carry additional packet routing information. Authentication mechanism to secure table updates.
Supports variable length subnet masking (VLSM).
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RIP routingprocess
RIP prevents routing loops from continuingindefinitely by implementing a limit on the number of
hops allowed in a path from the source to adestination
When a router receives a routing update thatcontains a new or changed entry, the metric value isincreased by 1
RIP implements split horizon and holddownmechanisms to prevent incorrect routing informationfrom being propagated.
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RIP routingprocess
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RIP routingprocess
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RIP routingprocess
router running RIP can be configured to send
a triggered update when the networktopology changes using the ipriptriggeredcommand
This command is issued only on serialinterfaces at the router(config-if)#
These updates, called triggered updates, aresent independently of the regularlyscheduled updates that RIP routers forward
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RIP routingprocess
IP Classless command
If a router receives packets for a subnet not in therouting table of the router and the ip classlesscommand is disable the packet will be discarded
classless command is enabled by default inCisco IOSSoftware Release 11.3 and later.
To disable this feature, use the no form of thiscommand.
Is a global configuration command
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RIP routingprocess
Convergence is when all routers in the same
internetwork have the same routinginformation.
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RIP routingprocess
There are several commands that can be
used to verify that RIP is properly configured. Two of the most common are the showip
route command and the showipprotocols
command
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RIP routingprocess
Most of the RIP configuration errors involve
an incorrect network statement,discontiguous subnets, or split horizons. One
highly effective command for finding RIP
update issues is the debugiprip command
The debugiprip command displays RIProuting updates as they are sent and
received
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RIP routingprocess
Using the passiveinterface command can
prevent routers from sending routing updatesthrough a router interface
Keeping routing update messages from
being sent through a router interface
prevents other systems on that network fromlearning about routes dynamically.
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RIP routingprocess
For RIP and IGRP, the passiveinterface
command stops the router from sending
updates to a particular neighbor, but the router
continues to listen and use routing updates
from that neighbor
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RIP routingprocess
Load-balancing describes the ability of a
router to transmit packets to a destination IPaddress over more than one path
The paths are derived either statically or with
dynamic protocols, such as RIP, EIGRP,
OSPF, and IGRP.
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RIP routingprocess
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IGRP
IGRP is a distance vector routing protocol
developed by Cisco. IGRP sends routing updates at 90 second
intervals
By default, the IGRP routing protocol uses
bandwidth and delay as metrics
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IGRP
Additionally, IGRP can be configured to use
a combination of variables to determine acomposite metric. Those variables include:
Bandwidth
Delay
Load Reliability
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IGRP
The composite metric of IGRP is more
accurate than the hop count metric that RIPuses when choosing a path to a destination.
The path that has the smallest metric value
is the best route.
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IGRP
The metrics that IGRP uses are:
Bandwidth The lowest bandwidth value in the path
Delay The cumulative interface delay along the path
Reliability The reliability on the link towards the destinationas determined by the exchange of keepalives
Load The load on a link towards the destination based on bits
per second MTU The Maximum Transmission Unit value of the path.
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IGRP
By default, only bandwidth and delay are
considered. The other parameters areconsidered only if enabled via configuration.
A link with a higher bandwidth will have a
lower metric, and a route with a lower
cumulative delay will have a lower metric.
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IGRP
IGRP advertises three types of routes:
Interior System
Exterior
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IGRP
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IGRP
IGRP has a number of features that are
designed to enhance its stability, such as: Holddowns
Split horizons
Poison reverse updates
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IGRP
To configure the IGRP routing process, use
the routerigrp configuration command. Toshut down an IGRP routing process, use the
no form of this command.
RouterA(config)#routerigrp as-number
RouterA(config)#norouterigrp as-number
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IGRP
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IGRP
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