1. Distance Vector Routing
16/05/2019
Distance Vector Routing
CCNA Exploration Semester 2
Chapter 4
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Topics
Characteristics of distance vector routing protocols
Distance vector routing protocols in use today
How they discover routes
How they maintain routing tables
Routing loops
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Routing protocols
Interior
Exterior
Distance vector
Link state
RIP v1 RIP v2 IGRP EIGRP
OSPF IS-IS
EGP BGP
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4. Distance vector knowledge
A distance vector protocol learns:
The distance to a network, measured in hops or in some other way
The direction of the network: which port should be used to reach it
It puts the routes in the routing table
It does not know any more details of the route or the other routers along the way
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5. Distance vector Network 192.168.48.0 is 3 hops away using port fa0/0
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Link state knowledge
A link state routing protocol finds out about all the routers in the system and the networks they link to.
It builds up a complete picture of the topology
It can then work out the best path to any network
It puts these best paths in the routing table
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7. Link state
I know all the routers and paths in this system of networks.
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Metrics
RIP v1 and 2 hop count, maximum 15
IGRP and EIGRP bandwidth, delay, load, reliability
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Distance vector
Exchange complete routing tables with immediate neighbours
Do this at regular intervals
Adjust the metric, e.g. add 1 to the hop count, or add number based on bandwidth and delay of link.
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Adjust the metric
is 2 hops away
is 3 hops away
Send update
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Sending updates
RIP v1 Whole routing table Broadcast every 30 sec
RIP v2 Whole routing table Multicast every 30 sec
IGRP Whole routing table Broadcast every 90 sec
EIGRP Initial learning process then small updates when topology changes
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Routing protocols
Distance vector
Link state
RIP v1 RIP v2 IGRP
OSPF IS-IS
EIGRP
Slow to converge Easy to configure Small networks Little use of resources
Fast to converge Harder to configure Large networks Much use of resources
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13. Distance vector updates
Routers start up.
R1 adds directly connected networks to table.
Network
Interface
Hop
Fa0/0
S0/0/0
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14. Distance vector updates
Exchange of routing table information.
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15. Distance vector updates
R1 has learned about from R2.
It does not know about
Network
Interface
Hop
Fa0/0
S0/0/0 1
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16. Distance vector updates
Exchange of routing table information.
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17. Distance vector updates
R1 has learned about from R2.
R2 previously learned about it from R3.
Network
Interface
Hop
Fa0/0
S0/0/0 1 2
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Update timer
R [120/1] via , 00:00:04, Serial0/0
Show ip route gives number of seconds since last update.
Routing Protocol is “rip” Sending updates every 30 seconds, next due in 3 seconds
Show ip protocols says when next update is due.
Update timer default is 30 seconds
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RIP timers
Routing Protocol is “rip” Sending updates every 30 seconds, next due in 26 seconds Invalid after 180 seconds, hold down 180, flushed after 240
Routing table contains two RIP routes
R [120/1] via , 00:00:04, Serial0/0 R [120/2] via , 00:00:04, Serial0/0
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RIP timers
Routing Protocol is “rip” Sending updates every 30 seconds, next due in 30 seconds Invalid after 180 seconds, hold down 180, flushed after 240
30 seconds – update Route to refreshed Route to not included
R [120/1] via , 00:00:00, Serial0/0 R [120/2] via , 00:00:30, Serial0/0
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RIP timers
Routing Protocol is “rip” Sending updates every 30 seconds, next due in 30 seconds Invalid after 180 seconds, hold down 180, flushed after 240
60 seconds – update Route to refreshed Route to not included
R [120/1] via , 00:00:00, Serial0/0 R [120/2] via , 00:01:00, Serial0/0
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RIP timers
Routing Protocol is “rip” Sending updates every 30 seconds, next due in 30 seconds Invalid after 180 seconds, hold down 180, flushed after 240
90 seconds – update Route to refreshed Route to not included
R [120/1] via , 00:00:00, Serial0/0 R [120/2] via , 00:01:30, Serial0/0
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RIP timers
Routing Protocol is “rip” Sending updates every 30 seconds, next due in 30 seconds Invalid after 180 seconds, hold down 180, flushed after 240
120 seconds – update Route to refreshed Route to not included
R [120/1] via , 00:00:00, Serial0/0 R [120/2] via , 00:02:00, Serial0/0
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RIP timers
Routing Protocol is “rip” Sending updates every 30 seconds, next due in 30 seconds Invalid after 180 seconds, hold down 180, flushed after 240
150 seconds – update Route to refreshed Route to not included
R [120/1] via , 00:00:00, Serial0/0 R [120/2] via , 00:02:30, Serial0/0
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RIP timers
Routing Protocol is “rip” Sending updates every 30 seconds, next due in 30 seconds Invalid after 180 seconds, hold down 180, flushed after 240
180 seconds – update Route to refreshed Route to not included
R [120/1] via , 00:00:00, Serial0/0 R [120/16] via , 00:03:00, Serial0/0
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RIP timers
Routing Protocol is “rip” Sending updates every 30 seconds, next due in 30 seconds Invalid after 180 seconds, hold down 180, flushed after 240
210 seconds – update Route to refreshed Route to not included
R [120/1] via , 00:00:00, Serial0/0 R [120/16] via , 00:03:30, Serial0/0
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RIP timers
Routing Protocol is “rip” Sending updates every 30 seconds, next due in 30 seconds Invalid after 180 seconds, hold down 180, flushed after 240
240 seconds – update Route to refreshed Route to not included
R [120/1] via , 00:00:00, Serial0/0
Route has been removed.
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RIP_JITTER
RIP updates can become synchronised
This is a problem if routers are linked by hubs because the updates will collide
RIP_JITTER is a random variable that makes updates vary a little from the default 30 seconds
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Triggered updates
These are to speed up convergence
Interface goes up/down, route added/removed
Router detects change, sends update to neighbour at once without waiting for timer
Neighbour passes on update at once.
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EIGRP
Does not send regular updates
Does not send its whole routing table
Sends only information about changes
Sends only to routers that need the information
Non-periodic, partial, bounded.
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Routing loop
A packet is sent from router to router in a loop until it is eventually dropped when its TTL field drops to 0
Caused by incorrect or out of date information in routing tables
Very bad for network – uses up bandwidth and processing power in routers
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32. Avoiding routing loops
Defining a maximum metric to prevent count to infinity
Holddown timers
Split horizon
Route poisoning or poison reverse
Triggered updates
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Maximum metric
Routers exchanging wrong information can report higher and higher values of the metric.
RIP sets a maximum metric.
The hop count can go up to 15.
If it reaches 16 then the route is regarded as unreachable.
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Holddown timers
Router receives update saying that a network is down.
Router marks the network as possibly down and starts holddown timer.
Update with a better metric for that network arrives: network is reinstated and holddown timer removed.
Update with the same or worse metric for that network arrives: update is ignored.
Timer runs out : network removed from table.
Packets still forwarded to network while timer runs.
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Split horizon
Route to in 4 hops
Route to in 3 hops
Router receives information about a route through an interface.
It will not send out information about the same route through that interface.
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Route poisoning
A router detects that a route has gone down.
It marks that route as unreachable in its routing table. (16 hops for RIP)
It sends out updates that show the route as unreachable.
Neighbour routers pass on these “poison” updates.
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Poison reverse
This is an exception to split horizon.
If a router receives an update marking a route as unreachable then it will send this information back to the router that sent it.
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RIP v1 and RIP v2
RIP v1
Classful, does not send subnet mask in updates so does not support VLSM
Sends updates as broadcasts
No authentication
No manual route summarisation
RIP v2
Classless, includes the subnet mask in routing updates, so supports VLSM.
Sends updates as multicasts
Authentication for security
Supports manual route summarization.
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RIP v2 or EIGRP?
RIP runs on any make of router, EIGRP only on Cisco routers.
EIGRP is suitable for large networks
EIGRP uses a more efficient metric and may choose faster routes.
EIGRP converges faster than RIP
EIGRP uses less bandwidth but it needs more processing power and RAM
RIP is simpler to configure
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The End
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