1. Routing Loops

2. After 15 hops the packet is discarded.
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.

3. A packet arrives at Router 1 at time t1
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:

4. Split Horizon
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 is down B A
Get to network via B

5. 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.

6. A Solution to Count to Infinity
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.

7. RouterA(config)# router rip
Configure RIP
net s0
net s1 e0
net
RouterA# config t
RouterA(config)# router rip
RouterA(config-router)# network
RouterA(config-router)# network
RouterA(config-router)# network
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.

8. 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

9. RIP Configuration Issues
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

10. RIP Configuration Issues
Router#config term
Router(config)# router rip
Router(config-router)#
timers basic
Update Invalid
Interval between updates
holdown
route is invalid after receiving no updates in ‘x’ secs
holddown time
flush
when route is flushed from table
update – 30 seconds
holddown seconds
Administrative Distance - 120

11. RIP Configuration Issues
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.

12. RIP Configuration Verification
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

13. Debugging Commands for RIP
Some RIP debugging commands are:
debug ip rip
show ip rip database
show ip interface brief

14. 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 /16 could be /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.

15. To set maximum number of parallel paths:
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)#maximum-paths [number]

16. Administrative Distance
Route Source Default Distance
Connected interface 0
Static route *(conditional) 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

17. 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.
As an example:
RouterA(config)#ip route

18. RIP-Redistribute Static Routes
RTA(config)# ip route
RTA(config)# router rip
RTA(config-router)#default-information originate

19. must be assigned an “AS” (autonomous system # - 16 bit number)
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, greater the load)
sends updates every 90 seconds
maximum hop count is 255 (default 100)

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

21. Setting IGPR Basic Timers
Router# config term
Router(config)# router igrp 100
Router(config-router)#
timers basic
Update Invalid
Interval between updates
holdown
route is invalid after receiving no updates in ‘x’ secs
holddown time
flush
when route is flushed from table
Router(config-router)#timers basic

22. RouterA(config)#router igrp 101
Configure IGRP
network address s0
network address s1 e0
network address
RouterA#config t
RouterA(config)#router igrp 101
RouterA(config-router)#network
RouterA(config-router)#network
RouterA(config-router)#network

23. Helpful commands for troubleshooting IGRP: show ip protocols
show ip route
debug ip igrp events
debug ip igrp transactions
ping
traceroute

24. Post them to “Chat” for further discussion
End of presentation
Questions?
Post them to “Chat” for further discussion

25. Module 11 Access Lists
Will Follow at 20:00