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18 Apr 2023 S Ward Abingdon and Witney College 1
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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RIP v1RIP v2IGRPEIGRP
Routing protocols
Interior Exterior
Distance vector Link state
OSPFIS-IS
EGPBGP
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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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Distance vector
Network 192.168.48.0 is 3 hops away using
port fa0/0
Network 192.168.22.0 is 2 hops away using
port fa0/1
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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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Link stateI 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
192.168.13.0 is 2 hops
away
192.168.13.0 is 3 hops
away
Send update
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Sending updates
RIP v1 Whole routing tableBroadcast every 30 sec
RIP v2 Whole routing tableMulticast every 30 sec
IGRP Whole routing tableBroadcast every 90 sec
EIGRP Initial learning process then small updates when topology changes
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RIP v1RIP v2IGRP
Routing protocols
Distance vector Link state
OSPFIS-IS
EIGRP
Slow to convergeEasy to configureSmall networksLittle use of resources
Fast to convergeHarder to configureLarge networksMuch use of resources
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Distance vector updates
Routers start up.
R1 adds directly connected networks to table.
Network Interface Hop
10.1.0.0 Fa0/0 0
10.2.0.0 S0/0/0 0
10.1.0.0 10.2.0.0 10.3.0.0 10.4.0.0
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Distance vector updates
Exchange of routing table information.
10.1.0.0 10.2.0.0 10.3.0.0 10.4.0.0
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Distance vector updates
R1 has learned about 10.3.0.0 from R2.
It does not know about 10.4.0.0
Network Interface Hop
10.1.0.0 Fa0/0 0
10.2.0.0 S0/0/0 0
10.3.0.0 S0/0/0 1
10.1.0.0 10.2.0.0 10.3.0.0 10.4.0.0
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Distance vector updates
Exchange of routing table information.
10.1.0.0 10.2.0.0 10.3.0.0 10.4.0.0
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Distance vector updates
R1 has learned about 10.4.0.0 from R2.
R2 previously learned about it from R3.
Network Interface Hop
10.1.0.0 Fa0/0 0
10.2.0.0 S0/0/0 0
10.3.0.0 S0/0/0 1
10.4.0.0 S0/0/0 2
10.1.0.0 10.2.0.0 10.3.0.0 10.4.0.0
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Update timer
R 10.3.0.0 [120/1] via 10.2.0.2, 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 secondsInvalid after 180 seconds, hold down 180, flushed after 240
R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:04, Serial0/0 R 10.4.0.0 [120/2] via 10.2.0.2, 00:00:04, Serial0/0
Routing table contains two RIP routes
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RIP timers
Routing Protocol is “rip”Sending updates every 30 seconds, next due in 30 secondsInvalid after 180 seconds, hold down 180, flushed after 240
R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:00, Serial0/0 R 10.4.0.0 [120/2] via 10.2.0.2, 00:00:30, Serial0/0
30 seconds – updateRoute to 10.3.0.0 refreshedRoute to 10.4.0.0 not included
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RIP timers
Routing Protocol is “rip”Sending updates every 30 seconds, next due in 30 secondsInvalid after 180 seconds, hold down 180, flushed after 240
R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:00, Serial0/0 R 10.4.0.0 [120/2] via 10.2.0.2, 00:01:00, Serial0/0
60 seconds – updateRoute to 10.3.0.0 refreshedRoute to 10.4.0.0 not included
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RIP timers
Routing Protocol is “rip”Sending updates every 30 seconds, next due in 30 secondsInvalid after 180 seconds, hold down 180, flushed after 240
R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:00, Serial0/0 R 10.4.0.0 [120/2] via 10.2.0.2, 00:01:30, Serial0/0
90 seconds – updateRoute to 10.3.0.0 refreshedRoute to 10.4.0.0 not included
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RIP timers
Routing Protocol is “rip”Sending updates every 30 seconds, next due in 30 secondsInvalid after 180 seconds, hold down 180, flushed after 240
R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:00, Serial0/0 R 10.4.0.0 [120/2] via 10.2.0.2, 00:02:00, Serial0/0
120 seconds – updateRoute to 10.3.0.0 refreshedRoute to 10.4.0.0 not included
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RIP timers
Routing Protocol is “rip”Sending updates every 30 seconds, next due in 30 secondsInvalid after 180 seconds, hold down 180, flushed after 240
R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:00, Serial0/0 R 10.4.0.0 [120/2] via 10.2.0.2, 00:02:30, Serial0/0
150 seconds – updateRoute to 10.3.0.0 refreshedRoute to 10.4.0.0 not included
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RIP timers
Routing Protocol is “rip”Sending updates every 30 seconds, next due in 30 secondsInvalid after 180 seconds, hold down 180, flushed after 240
R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:00, Serial0/0 R 10.4.0.0 [120/16] via 10.2.0.2, 00:03:00, Serial0/0
180 seconds – updateRoute to 10.3.0.0 refreshedRoute to 10.4.0.0 not included
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RIP timers
Routing Protocol is “rip”Sending updates every 30 seconds, next due in 30 secondsInvalid after 180 seconds, hold down 180, flushed after 240
R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:00, Serial0/0 R 10.4.0.0 [120/16] via 10.2.0.2, 00:03:30, Serial0/0
210 seconds – updateRoute to 10.3.0.0 refreshedRoute to 10.4.0.0 not included
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RIP timers
Routing Protocol is “rip”Sending updates every 30 seconds, next due in 30 secondsInvalid after 180 seconds, hold down 180, flushed after 240
R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:00, Serial0/0
240 seconds – updateRoute to 10.3.0.0 refreshedRoute to 10.4.0.0 not included
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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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
Router receives information about a route through an interface.
It will not send out information about the same route through that interface.
Route to 10.1.1.0 in
3 hops
Route to 10.1.1.0 in
4 hops
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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