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Distance Vector Routing. CCNA Exploration Semester 2 Chapter 4. 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. Routing protocols. Interior. Exterior.

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distance vector routing

Distance Vector Routing

CCNA Exploration Semester 2

Chapter 4

topics
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
routing protocols
Routing protocols

Interior

Exterior

Distance vector

Link state

RIP v1RIP v2IGRPEIGRP

OSPFIS-IS

EGPBGP

distance vector knowledge
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
distance vector
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

link state knowledge
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
link state
Link state

I know all the routers and paths in this system of networks.

metrics
Metrics
  • RIP v1 and 2 hop count, maximum 15
  • IGRP and EIGRP bandwidth, delay, load, reliability
distance vector1
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.
adjust the metric

Send update

Adjust the metric

192.168.13.0 is 2 hops away

192.168.13.0 is 3 hops away

sending updates
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
routing protocols1

Link state

Slow to convergeEasy to configureSmall networksLittle use of resources

Fast to convergeHarder to configureLarge networksMuch use of resources

Distance vector

EIGRP

RIP v1RIP v2IGRP

OSPFIS-IS

Routing protocols
distance vector updates
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.

distance vector updates1
Distance vector updates

10.4.0.0

10.1.0.0

10.2.0.0

10.3.0.0

Exchange of routing table information.

distance vector updates2
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

distance vector updates3
Distance vector updates

10.4.0.0

10.1.0.0

10.2.0.0

10.3.0.0

Exchange of routing table information.

distance vector updates4
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.

update timer
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
rip timers
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

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

rip timers1
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

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, 00:00:00, Serial0/0 R 10.4.0.0 [120/2] via 10.2.0.2, 00:00:30, Serial0/0

rip timers2
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

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, 00:00:00, Serial0/0 R 10.4.0.0 [120/2] via 10.2.0.2, 00:01:00, Serial0/0

rip timers3
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

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, 00:00:00, Serial0/0 R 10.4.0.0 [120/2] via 10.2.0.2, 00:01:30, Serial0/0

rip timers4
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

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, 00:00:00, Serial0/0 R 10.4.0.0 [120/2] via 10.2.0.2, 00:02:00, Serial0/0

rip timers5
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

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, 00:00:00, Serial0/0 R 10.4.0.0 [120/2] via 10.2.0.2, 00:02:30, Serial0/0

rip timers6
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

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, 00:00:00, Serial0/0 R 10.4.0.0 [120/16] via 10.2.0.2, 00:03:00, Serial0/0

rip timers7
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

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, 00:00:00, Serial0/0 R 10.4.0.0 [120/16] via 10.2.0.2, 00:03:30, Serial0/0

rip timers8
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

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, 00:00:00, Serial0/0

Route has been removed.

rip jitter
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
triggered updates
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.
eigrp
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.
routing loop
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
avoiding routing loops
Avoiding routing loops
  • Defining a maximum metric to prevent count to infinity
  • Holddown timers
  • Split horizon
  • Route poisoning or poison reverse
  • Triggered updates
maximum metric
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.
holddown timers
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.
split horizon
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.
route poisoning
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.
poison reverse
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.
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.

RIP v1 and RIP v2
rip v2 or eigrp
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