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Cisco 2 - Routers
Chapter 7
Routing Loops
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Cisco 2 - Routers
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.
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Cisco 2 - Routers
Chapter 7
A packet arrives at Router 1 at time t1. Router 1 has already been updated and so
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 so 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
Cisco 2 - Routers
Chapter 7
Split Horizons –
Disables the router from sending information about a
‘failed’ route in the routing table through the same interface
that it learned about that route from.
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
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Cisco 2 - Routers
Poison Reverse
Chapter 7
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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Cisco 2 - Routers
A Solution to Count to Infinity
Chapter 7
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 neighbor with a better metric than
the original network entry, the holddown is removed & data is
passed.
However, if an update is received from a neighbor router before
the holddown timer expires & it has a lower metric than the
previous route, the update is ignored & the holddown timer keeps
ticking.
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Configure RIP
Cisco 2 - Routers
Chapter 7
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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Cisco 2 - Routers
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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Cisco 2 - Routers
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
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
Cisco 2 - Routers
Chapter 7
Router#config term
Router(config)# router rip
Router(config-router)# timers basic Update Invalid
holdown flush
Interval between updates
route is invalid after receiving no
updates in ‘x’ secs
holddown time
when route is flushed from table
update – 30 seconds
holddown - 180 seconds
Administrative Distance - 120
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Cisco 2 - Routers
RIP Configuration Issues
Chapter 7
NOTE for RIP:
It’s metric used to determine the 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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Cisco 2 - Routers
RIP Configuration Verification
Chapter 7
Use the following commands to make RIP verifications:
show ip route
The routing table statement will be proceeded by an “R” when the
route is learned by the RIP
show ip protocols
This will verify:
• Which protocol is configured – in this case RIP
• Which interfaces are sending & receiving RIP updates
• Which network the routing protocol is sending information to
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Cisco 2 - Routers
Debugging Commands for RIP
Chapter 7
Some RIP debugging commands are:
debug ip rip
show ip rip database
show ip interface brief
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Cisco 2 - Routers
Classless Routing
Chapter 7
NOTE: Router Rip will not handle Classless Routing, but Rip
ver2 will.
A supernet route (classless route) is a route that covers a
greater range of subnets with a single entry. As 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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Cisco 2 - Routers
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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Cisco 2 - Routers
Administrative Distance
Route Source
Chapter 7
Default Distance
Connected interface
0
Static route *(conditional)
1 or 0
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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Cisco 2 - Routers
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.
As an example:
RouterA(config)#ip route 200.10.10.0 255.255.255.0 192.16.10.1 130
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Cisco 2 - Routers
RIP-Redistribute Static Routes
Chapter 7
RTA(config)# ip route 192.168.1.0 192.168.2.2
RTA(config)# router rip
RTA(config-router)#default-information originate
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Cisco 2 - Routers
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, greater the load)
• sends updates every 90 seconds
• maximum hop count is 255 (default 100)
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Cisco 2 - Routers
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 IGPR Basic Timers
Cisco 2 - Routers
Chapter 7
Router# config term
Router(config)# router igrp 100
Router(config-router)# timers basic Update Invalid
holdown flush
Interval between updates
route is invalid after receiving no
updates in ‘x’ secs
holddown time
when route is flushed from table
Router(config-router)#timers basic 90 270 280 630
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Configure IGRP
Cisco 2 - Routers
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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Cisco 2 - Routers
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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Cisco 2 - Routers
End of presentation
Chapter 7
Questions?
Post them to “Chat” for
further discussion
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Cisco 2 - Routers
Chapter 7
Module 11 Access Lists
Will Follow at 20:00
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