CCNA2 Chapter 7

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CCNA2 Chapter 7
Distance Vector Protocols
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Dynamic Routing

• Avoids configuration of static routes
• Routers react to changes in the network
• Routers adjust their routing tables accordingly,
  without the intervention of the network
  administrator
• There are problems associated with dynamic
  distance vector routing

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Distance Vector Routing Protocols

• RIP is a distance vector routing protocol that is
  used in thousands of networks throughout the
  world
• RIP is based on open standards and is easy to
  implement makes it attractive to some network
  administrators
• RIP is a good basic protocol for networking
  students
• IGRP is another distance vector routing protocol.
  
• Unlike RIP, IGRP is a Cisco-proprietary protocol
  rather than a standards-based protocol.
• IGRP is simple to implement
• IGRP is a more complex routing protocol than RIP
  and can use many factors to determine the best
  route to a destination network.
• NOTE for our PacketTracer labs, well use EIGRP)

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Distance vector routing protocols

• Require routers to forward their entire routing
  table when passing along updates
• Routing table information is forwarded to
  neighbor routers, which continue to forward the
  information to their neighbors..
• These routing tables include information about
  the total cost of a route and the logical address
  of the first router on the path to each network
  contained in the table.
• Routers need to update the information in their
  routing tables to make good path determination
  decisions.
• Updates may be initiated when topology changes
  occur
• Changes in a network affect the decisions made by
  a router.
• A router may be taken off line for upgrades or
  repairs or an interface on a router may go down.
• If not aware of the changes that have occurred in
  a network, routers may switch packets to
  interfaces that are no longer connected to the
  best route.
• Distance vector routing protocols typically send
  out updates at certain time intervals
• Every 30 seconds for RIP..
• Every 90 seconds for IGRP

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Routing Loops
A Network converges when all the routers in the
network have the same routing information. If a
link goes down, it is possible that invalid
updates will continue to loop through out the
network. This is called the count to
infinity. RIP routing protocol counts the count
to infinity by hop count. RIPs maximum hop count
is 15. After 15 hops the packet is discarded by
RIP.
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A packet arrives at Router 1 at time t1. Router 1
has already been updated and knows that the
optimal route to the destination calls for Router
2 to be the next stop Router 1 therefore forwards
the packet to Router 2. Router 2 has not yet been
updated and believes that the optimal next hop is
Router 1. Router 2 therefore forwards the packet
back to Router 1 The packet will continue to
bounce back and forth between the two routers
until Router 2 receives its routing update or
until the packet has been switched the maximum
number of times allowed This process illustrates
the count to infinity problem - there are several
solutions to this problem
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Split Horizon

• Split Horizon
• Disables the router from sending information
  about a failed route in the routing table. This
  is done by not sending the information through
  the same interface that it learned about the
  failed route
• That is, it would prevent Router A from sending
  the updated information if received from Router B
  back to Router B

Network 171.10.0.0 is down
B
A
171.10.0.0
Get to network 171.10.0.0 via B
Is Down!
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Poison Reverse
Poison Reverse A route that is not good is
sent a poison reverse which removes the route
Network 4
Network 5
C
E
When Network 5 goes down, Router E initiates
route poisoning by entering a table entry for
Network 5 as 16, for RIP, unreachable. By this
poisoning of the route to Network 5, Router C is
not susceptible to incorrect updates about the
route to Network 5. When Router C receives a
router poisoning from Router E, it sends an
update, called a poison reverse, back to Router
E. This makes sure all routes on the segment
have received the poisoned route information.
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One Solution to Count to Infinitive

• Holddown
• Is used to prevent regular update messages from
  reinstating a route that may have gone bad
• When a router receives an update from a neighbor
  indicating that a previously accessible network
  is not working - is inaccessible, the holddown
  timer will start
• If a new update arrives from a different neighbor
  with a better metric than the original network
  entry, the holddown is removed and data is passed
• However, if an update is received from the same
  neighbor router before the holddown timer
  expires, and it has a lower metric than the
  previous route, the update is ignored and the
  holddown timer keeps ticking

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Configure RIP
210.45.20.0 net
s0
192.10.10.0 net
s1
e0
172.120.0.0 net
RouterA config t RouterA(config) router
rip RouterA(config-router) network
192.10.10.0 RouterA(config-router) network
172.120.0.0 RouterA(config-router) network
210.45.20.0 RouterA(config)int
s0 RouterA(config-if) ip rip triggered
If topology changes, this command will
triggered those updates to the next router.
Only applied to a serial interface.
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RIP Configuration Issues

• RIP uses the following techniques to reduce
  routing loops and count to infinity. In some
  cases, configuration is required
• count-to-infinity
• split horizon
• poison reverse
• holddown counters
• triggered updates
• To disable split horizon do
• RouterA(config-if) no ip split-horizon

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RIP Configuration Issues
To change RIPs update interval
do RouterA(config-router) update-timer
ltsecondsgt
To disable sending RIP updates do RouterA(config
-router) passive-interface ltinterfacegt
Command to receive either version of RIP,
do RouterA(config-if) ip rip receive version
1 RouterA(config-if) ip rip receive version
2 RouterA(config-if) ip rip receive version 1 2
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RIP Configuration Issues
Router config term
Router(config) router rip
Router(config-router)
timers basic
update
invalid
holddown
flush
Intervals between updates
route is invalid after receiving no updates in
secs
holddown time
when route is flushed from table
update 30 seconds holddown - 180
seconds Administrative Distance - 120
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RIP Configuration Issues
NOTE for RIP Its metric to determine a route to
a destination is the hop count. As a packet goes
from router to router, RIP increments a counter
called hop count.
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RIP Configuration Verification
Use the following commands to make RIP
verifications
show ip route The routing table will have R
by the routes determined by the RIP routing
protocol

• show ip protocols
• This will verify
• RIP routing is configured (which protocol is
  configured)
• Which interfaces are sending receiving RIP
  updates
• Which network it is sending information to

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Debugging Commands for RIP
Some RIP debugging commands are debug ip
rip show ip rip database show ip interface brief
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Classless Routing
NOTE Router Rip cannot handle Classless
Routing, but Rip ver2 can. A supernet route
(classless route) is a route that covers a
greater range of subnets with a single entry. An
example a supernet of 172.16.0.0/16 could be
172.16.0.0/13. However, a router by default
assumes that all subnets of a directly connected
network should be present in the routing
table. If a packet is received with an unknown
destination address within an unknown subnet of a
directly attached network, the router assumes
that the subnet does not exist, and will drop
this packet. To get around this problem, use a
global command ip classless.
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RIP Load Balancing
Load-balancing describes the ability of a router
to transmit packets to a destination IP address
over more than one path When a router learns
multiple routes to a specific network, the route
with the lowest administrative distance is
entered into the routing table To set maximum
number of parallel paths RouterA(config-router)m
aximum-paths number
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Administrative Distance
Route Source
Default Distance Connected
interface 0 Static route 1 EIGRP summary
route 5 External BGP 20 Internal EIGRP
90 External EIGRP 170 IGRP 100 OSPF 110 IS-IS 115
RIP 120 EGP 140 Internal BGP 200 Unknown 255
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Floating Static Routes
Floating static routes are static routes
configured with an administrative distance value
that is greater than that of the primary route
(or routes). Essentially, floating static routes
are fallback routes, or backup routes, that do
not appear in the routing table until another
route fails.
Example RouterA(config)ip route 200.10.10.0
255.255.255.0 192.16.10.1 30
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RIP Redistribute Static Routes
For RIP, if a static route is assigned to an
interface that is not one of the networks defined
in a network command, no dynamic routing
protocols advertise the route. Use redistribute
static command. To redistribute static default
route, must use the default-information originate
command. Example
RTA(config) ip route 0.0.0.0 0.0.0.0
s0 RTA(config) router rip RTA(config-router)
default-information originate
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IGRP

• IGRP
• must be assigned an AS (autonomous system -
  16 bit number)
• Cisco proprietary
• distance-vector
• metrics
• delay
• bandwidth (1200 bps - 10 Gbps)
• reliability (1-224) (higher the number, more
  reliable)
• load (1-244) (higher the number, more it is
  under load)
• sends updates every 90 seconds
• maximum hop count is 255 (default 100)

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IGRP

• IGRP has number of features that are designed to
  enhance its stability
• holddowns
• split horizons
• poison reverse updates

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Setting IGRP Basic Timers
Router config term
Router(config) router igrp 100
update
invalid
holddown
flush
Router(config-router) timers basic
Intervals between updates
route is invalid after receiving no updates in
secs
holddown time
when route is flushed from table
Router(config-router) timers basic 90 270
280 630 Default settings
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Configure IGRP
210.45.20.0 network address
s0
192.10.10.0 network address
s1
e0
172.120.0.0 network address
RouterA config t
RouterA(config) router igrp 101
RouterA(config-router) network
192.10.10.0 RouterA(config-router) network
172.120.0.0 RouterA(config-router) network
210.45.20.0
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Troubleshooting IGRP

• Helpful commands for troubleshooting IGRP
• show ip protocols
• show ip route
• debug ip igrp events
• debug ip igrp transactions
• ping
• traceroute