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Chapter 7
CCNA2 Chapter 7
Distance Vector Protocols
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Dynamic Routing
Chapter 7
• 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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Chapter 7
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, we’ll use EIGRP)
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Chapter 7
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
Chapter 7
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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Chapter 7
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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Chapter 7
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
Get to network 171.10.0.0 via B
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171.10.0.0
Is Down!
Page 7
Poison Reverse
Chapter 7
Poison Reverse –
A route that is not ‘good’ is sent a poison reverse which removes the
route
Network 4
C
Network 5
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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Chapter 7
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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Chapter 7
Configure RIP
210.45.20.0 net
s0
s1
192.10.10.0 net
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
If topology changes, this
command will ‘triggered’ those
updates to the next router. Only
applied to a serial interface.
RouterA(config-router)# network 210.45.20.0
RouterA(config)#int s0
RouterA(config-if)# ip rip triggered
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RIP Configuration Issues
Chapter 7
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
Chapter 7
To change RIP’s update interval do:
RouterA(config-router)# update-timer <seconds>
To disable sending RIP updates do:
RouterA(config-router)# passive-interface <interface>
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
Chapter 7
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
Chapter 7
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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Chapter 7
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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Chapter 7
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
Chapter 7
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
Chapter 7
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)#maximum-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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Chapter 7
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Floating Static Routes
Chapter 7
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
Chapter 7
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
Chapter 7
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
Chapter 7
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
Chapter 7
Router# config term
Router(config)# router igrp 100
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
Router(config-router)# timers basic 90 270 280 630 [Default settings]
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Configure IGRP
Chapter 7
210.45.20.0 network address
s0
s1
192.10.10.0 network address
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
Chapter 7
Helpful commands for troubleshooting IGRP:
• show ip protocols
• show ip route
• debug ip igrp events
• debug ip igrp transactions
• ping
• traceroute
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