Distance Vector
Routing
CCNA Exploration Semester 2
Chapter 4
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1
Topics
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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
Distance vector
RIP v1
RIP v2
IGRP
EIGRP
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Exterior
Link state
OSPF
IS-IS
EGP
BGP
3
Distance vector knowledge

A distance vector protocol learns:

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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
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Network 192.168.22.0
is 2 hops away using
port fa0/1
5
Link state knowledge
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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 state
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I know all the routers and paths in
this system of networks.
7
Metrics

RIP v1 and 2

IGRP and EIGRP bandwidth, delay,
load, reliability
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hop count, maximum 15
8
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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10
Sending updates

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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
RIP v1
RIP v2
IGRP
EIGRP
Slow to converge
Easy to configure
Small networks
Little use of resources
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Link state
OSPF
IS-IS
Fast to converge
Harder to configure
Large networks
Much use of resources
12
Distance vector updates
10.1.0.0
10.2.0.0
Routers start up.
R1 adds directly
connected networks
to table.
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10.3.0.0
10.4.0.0
Network
Interface Hop
10.1.0.0
10.2.0.0
Fa0/0
S0/0/0
0
0
13
Distance vector updates
10.1.0.0
10.2.0.0
10.3.0.0
10.4.0.0
Exchange of routing
table information.
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Distance vector updates
10.1.0.0
10.2.0.0
R1 has learned
about 10.3.0.0 from
R2.
It does not know
about 10.4.0.0
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10.3.0.0
10.4.0.0
Network
Interface Hop
10.1.0.0
10.2.0.0
Fa0/0
S0/0/0
0
0
10.3.0.0
S0/0/0
1
15
Distance vector updates
10.1.0.0
10.2.0.0
10.3.0.0
10.4.0.0
Exchange of routing
table information.
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Distance vector updates
10.1.0.0
10.2.0.0
R1 has learned
about 10.4.0.0 from
R2.
R2 previously
learned about it
from R3.
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10.3.0.0
10.4.0.0
Network
Interface Hop
10.1.0.0
10.2.0.0
Fa0/0
S0/0/0
0
0
10.3.0.0
S0/0/0
1
10.4.0.0
S0/0/0
2
17
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 seconds
Invalid 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, 00:00:04, Serial0/0
R 10.4.0.0 [120/2] via 10.2.0.2, 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 10.3.0.0 refreshed
Route to 10.4.0.0 not included
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
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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 10.3.0.0 refreshed
Route to 10.4.0.0 not included
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
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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 10.3.0.0 refreshed
Route to 10.4.0.0 not included
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
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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 10.3.0.0 refreshed
Route to 10.4.0.0 not included
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
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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 10.3.0.0 refreshed
Route to 10.4.0.0 not included
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
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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 10.3.0.0 refreshed
Route to 10.4.0.0 not included
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
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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 10.3.0.0 refreshed
Route to 10.4.0.0 not included
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
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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 10.3.0.0 refreshed
Route to 10.4.0.0 not included
R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:00, Serial0/0
Route has been removed.
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RIP_JITTER
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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
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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
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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
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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
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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
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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
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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
10.1.1.0 in
3 hops
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Route to
10.1.1.0 in
4 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
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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
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
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
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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
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RIP v2
Classless, includes the
subnet mask in routing
updates, so supports
VLSM.
Sends updates as
multicasts
Authentication for
security
Supports manual route
summarization.
38
RIP v2 or EIGRP?

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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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