Distance Vector Routing

1
Distance Vector Routing

• CCNA Exploration Semester 2
• Chapter 4

2
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

3
Routing protocols
Interior
Exterior
Distance vector
Link state
RIP v1RIP v2IGRPEIGRP
OSPFIS-IS
EGPBGP
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

5
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
6
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

7
Link state
I know all the routers and paths in this system
of networks.
8
Metrics

• RIP v1 and 2 hop count, maximum 15
• IGRP and EIGRP bandwidth, delay, load,
  reliability

9
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
11
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

12
Routing protocols
13
Distance vector updates
10.4.0.0
10.1.0.0
10.2.0.0
10.3.0.0

• 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


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Distance vector updates
10.4.0.0
10.1.0.0
10.2.0.0
10.3.0.0

• Exchange of routing table information.

15
Distance vector updates
10.4.0.0
10.1.0.0
10.2.0.0
10.3.0.0

• 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

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Distance vector updates
10.4.0.0
10.1.0.0
10.2.0.0
10.3.0.0

• Exchange of routing table information.

17
Distance vector updates
10.4.0.0
10.1.0.0
10.2.0.0
10.3.0.0

• 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
18
Update timer

• R 10.3.0.0 120/1 via 10.2.0.2, 000004,
  Serial0/0
• Show ip route gives number of seconds since last
  update.
• Routing Protocol is ripSending 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 ripSending updates every
  30 seconds, next due in 26 secondsInvalid after
  180 seconds, hold down 180, flushed after 240

Routing table contains two RIP routes
R 10.3.0.0 120/1 via 10.2.0.2, 000004,
Serial0/0 R 10.4.0.0 120/2 via 10.2.0.2,
000004, Serial0/0
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RIP timers

• Routing Protocol is ripSending updates every
  30 seconds, next due in 30 secondsInvalid after
  180 seconds, hold down 180, flushed after 240

30 seconds updateRoute to 10.3.0.0
refreshedRoute to 10.4.0.0 not included
R 10.3.0.0 120/1 via 10.2.0.2, 000000,
Serial0/0 R 10.4.0.0 120/2 via 10.2.0.2,
000030, Serial0/0
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RIP timers

• Routing Protocol is ripSending updates every
  30 seconds, next due in 30 secondsInvalid after
  180 seconds, hold down 180, flushed after 240

60 seconds updateRoute to 10.3.0.0
refreshedRoute to 10.4.0.0 not included
R 10.3.0.0 120/1 via 10.2.0.2, 000000,
Serial0/0 R 10.4.0.0 120/2 via 10.2.0.2,
000100, Serial0/0
22
RIP timers

• Routing Protocol is ripSending updates every
  30 seconds, next due in 30 secondsInvalid after
  180 seconds, hold down 180, flushed after 240

90 seconds updateRoute to 10.3.0.0
refreshedRoute to 10.4.0.0 not included
R 10.3.0.0 120/1 via 10.2.0.2, 000000,
Serial0/0 R 10.4.0.0 120/2 via 10.2.0.2,
000130, Serial0/0
23
RIP timers

• Routing Protocol is ripSending updates every
  30 seconds, next due in 30 secondsInvalid after
  180 seconds, hold down 180, flushed after 240

120 seconds updateRoute to 10.3.0.0
refreshedRoute to 10.4.0.0 not included
R 10.3.0.0 120/1 via 10.2.0.2, 000000,
Serial0/0 R 10.4.0.0 120/2 via 10.2.0.2,
000200, Serial0/0
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RIP timers

• Routing Protocol is ripSending updates every
  30 seconds, next due in 30 secondsInvalid after
  180 seconds, hold down 180, flushed after 240

150 seconds updateRoute to 10.3.0.0
refreshedRoute to 10.4.0.0 not included
R 10.3.0.0 120/1 via 10.2.0.2, 000000,
Serial0/0 R 10.4.0.0 120/2 via 10.2.0.2,
000230, Serial0/0
25
RIP timers

• Routing Protocol is ripSending updates every
  30 seconds, next due in 30 secondsInvalid after
  180 seconds, hold down 180, flushed after 240

180 seconds updateRoute to 10.3.0.0
refreshedRoute to 10.4.0.0 not included
R 10.3.0.0 120/1 via 10.2.0.2, 000000,
Serial0/0 R 10.4.0.0 120/16 via 10.2.0.2,
000300, Serial0/0
26
RIP timers

• Routing Protocol is ripSending updates every
  30 seconds, next due in 30 secondsInvalid after
  180 seconds, hold down 180, flushed after 240

210 seconds updateRoute to 10.3.0.0
refreshedRoute to 10.4.0.0 not included
R 10.3.0.0 120/1 via 10.2.0.2, 000000,
Serial0/0 R 10.4.0.0 120/16 via 10.2.0.2,
000330, Serial0/0
27
RIP timers

• Routing Protocol is ripSending updates every
  30 seconds, next due in 30 secondsInvalid after
  180 seconds, hold down 180, flushed after 240

240 seconds updateRoute to 10.3.0.0
refreshedRoute to 10.4.0.0 not included
R 10.3.0.0 120/1 via 10.2.0.2, 000000,
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

29
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

33
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.

34
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.

35
Split horizon
Route to 10.1.1.0 in 4 hops
Route to 10.1.1.0 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.

36
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.

37
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.

38
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.

39
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