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SEMESTER 2 Chapter 4
Distance Vector Routing Protocols
V 4.0
4.1.1
What are the three distance
vector routing protocols?
What are the key
characteristics of RIP?
What are the key
characteristics of IGRP?
What are the key
characteristics of EIGRP?
4.1.2
4.1.2.2
4.1.3
How are distance vector routes
advertised?
Does a router in a distance
vector network have knowledge
of the entire path to a
destination network?
What are the 2 things a
distance vector router knows?
What are the two reasons
sending an entire routing table
in an update inefficient?
What is the periodic update
time for RIP?
What is the periodic update
time for IGRP?
What is the address for
broadcast updates?
What is the algorithm used for?
What processes does the
routing protocol define?
RIP, IGRP, EIGRP
Hop count is used as the metric for path selection.
If the hop count for a network is greater than 15, RIP
cannot supply a route to that network.
Routing updates are broadcast or multicast every 30
seconds, by default.
Bandwidth, delay, load and reliability are used to create
a composite metric.
Routing updates are broadcast every 90 seconds, by
default.
IGRP is the predecessor of EIGRP and is now
obsolete.
It can perform unequal cost load balancing.
It uses Diffusing Update Algorithm (DUAL) to calculate
the shortest path.
There are no periodic updates as with RIP and IGRP.
Routing updates are sent only when there is a change
in the topology.
As vectors of distance and direction
No
The direction or interface in which packets should be
forwarded and
The distance or how far it is to the destination network
consume bandwidth but also consume router CPU
resources to process the updates
30 Sec
90 Sec
255.255.255.255
to calculate the best paths and then send that
information to the neighbors
Mechanism for sending and receiving routing
information.
Mechanism for calculating the best paths and installing
routes in the routing table.
Mechanism for detecting and reacting to topology
changes.
4.1.4
Define Time to Convergence.
Define Scalability.
Define Classless (Use of
VLSM) or Classful.
Define Resource Usage.
Define Implementation and
Maintenance
List the advantages for
distance vector protocols.
List the disadvantages for
distance vector protocols.
4.1.4.2
4.2.1
4.2.2.1
4.2.3.1
Definitely practice the exercise
at the right. It will help you
remember each protocol.
What are the routes that router
will initially discover after
booting?
After completing the initial
discovery what does the router
start to do?
What does the initial exchange
of routing information contain?
What does the router do with
the information that is not
contained in its routing table?
After the first exchange of
routing updates what has been
added to each routing table?
What is convergence?
Time to convergence defines how quickly the routers in
the network topology share routing information and
reach a state of consistent knowledge. The faster the
convergence, the more preferable the protocol. Routing
loops can occur when inconsistent routing tables are
not updated due to slow convergence in a changing
network.
Scalability defines how large a network can become
based on the routing protocol that is deployed. The
larger the network is, the more scalable the routing
protocol needs to be.
Classless routing protocols include the subnet mask in
the updates. This feature supports the use of Variable
Length Subnet Masking (VLSM) and better route
summarization. Classful routing protocols do not
include the subnet mask and cannot support VLSM.
Resource usage includes the requirements of a routing
protocol such as memory space, CPU utilization, and
link bandwidth utilization. Higher resource requirements
necessitate more powerful hardware to support the
routing protocol operation in addition to the packet
forwarding processes.
Implementation and maintenance describes the level of
knowledge that is required for a network administrator
to implement and maintain the network based on the
routing protocol deployed.
Simple implementation and maintenance
Low resource requirements
Slow convergence
Limited scalability
Routing loops
The directly connected networks
Exchange information
Information about their directly connected networks
It is added to the routing table
Neighbors directly connected routes
When all routers have consistent knowledge of the
network
What is slit horizon?
4.2.4
4.3.1
4.3.1.2
4.3.2
4.3.3
The amount of time it takes for
a network to converge is
directly proportional to
____________________
The speed of achieving
convergence consists of what
two things?
What are the two reasons
routers exchange routing
updates?
What is the period update time
for RIP?
What are the four reasons
listed for topology changes?
What are the three additional
timers?
Prevents information from being sent out the interface
that it was received
the size of that network
How quickly the routers propagate a change in the
topology in a routing update to its neighbors.
The speed of calculating best path routes using the
new routing information collected.
To exchange routing information with their neighbors
and to maintain up-to-date routing information in the
routing table
30 seconds
Failure of a link
Introduction of a new link
Failure of a router
Change of link parameters
Invalid
Flush
Holddown
It is marked invalid
If an update has not been
received to refresh an existing
route after 180 seconds, what
happens to that route in the
routing table?
What happens to the route after It is removed from the routing table. Flushed
240 seconds have elapsed?
What does a holddown timer
Keeps a route in the routing table, marked as
do?
unreachable, for at least 180 seconds so all other
routers receive that information
What are the two commands
Show ip route
that will show the timers?
Show ip protocols
Define bounded update.
Updates that are sent only to the routers that need the
information instead of sending information to all routers
What are characteristic of
Non-periodic because they are not sent out on a
EIGRP updates?
regular basis.
Partial updates sent only when there is a change in
topology that influences routing information.
Bounded, meaning the propagation of partial updates
are automatically bounded so that only those routers
that need the information are updated.
What is a triggered update?
a routing table update that is sent immediately in
response to a routing change
What are the three reasons to
An interface changes state (up or down)
send a triggered update?
A route has entered (or exited) the "unreachable" state
A route is installed in the routing table
What are the two problems with Packets containing the update message can be
triggered updates?
4.3.4
4.4.1
What is it called when all the
routers send updates at the
same time on a network with a
hub at the center?
It this a problem on a switched
network?
What is a routing loop?
What are some reasons for
routing loops?
4.4.2
4.4.3
4.4.4
dropped or corrupted by some link in the network.
The triggered updates do not happen instantaneously.
It is possible that a router that has not yet received the
triggered update will issue a regular update at just the
wrong time, causing the bad route to be reinserted in a
neighbor that had already received the triggered
update.
Synchronized updates
No
a packet is continuously transmitted within a series of
routers without ever reaching its intended destination
network
Incorrectly configured static routes
Incorrectly configured route redistribution (redistribution
is a process of handing the routing information from
one routing protocol to another routing protocol and is
discussed in CCNP-level courses)
Inconsistent routing tables not being updated due to
slow convergence in a changing network
Incorrectly configured or installed discard routes
TTL time-to-live
What mechanism is built into IP
to overcome routing loops?
What conditions can be created Link bandwidth will be used for traffic looping back
because of routing loops?
and forth between the routers in a loop.
A router's CPU will be strained due to looping
packets.
A router's CPU will be burdened with useless
packet forwarding that will negatively impact the
convergence of the network.
Routing updates may get lost or not be processed
in a timely manner. These conditions would
introduce additional routing loops, making the
situation even worse.
Packets may get lost in "black holes."
What are some of the
Defining a maximum metric to prevent count to infinity
mechanisms used to avoid
Holddown timers
routing loops?
Split horizon
Route poisoning or poison reverse
Triggered updates
What is count to infinity?
a condition that exists when inaccurate routing updates
increase the metric value to "infinity" for a network that
is no longer reachable
What is infinity defined by?
Maximum metric value
What is infinity for RIP?
16
What is it called when a route
flapping
goes up, then down, then up
4.4.5
4.4.6
4.4.6.2
4.4.7
4.5.1
etc.?
What are holddown timers used to prevent regular update messages from
for?
inappropriately reinstating a route that may have gone
bad.
Describe the process of how a
1. A router receives an update from a neighbor
holddown timer works.
indicating that a network that previously was accessible
is now no longer accessible.
2. The router marks the network as possibly down and
starts the holddown timer.
3. If an update with a better metric for that network is
received from any neighboring router during the
holddown period, the network is reinstated and the
holddown timer is removed.
4. If an update from any other neighbor is received
during the holddown period with the same or worse
metric for that network, that update is ignored. Thus,
more time is allowed for the information about the
change to be propagated.
5. Routers still forward packets to destination networks
that are marked as possibly down. This allows the
router to overcome any issues associated with
intermittent connectivity. If the destination network truly
is unavailable and the packets are forwarded, black
hole routing is created and lasts until the holddown
timer expires.
What is the split horizon rule?
a router should not advertise a network through the
interface from which the update came
What is route poisoning?
to mark the route as unreachable in a routing update
that is sent to other routers
What is split horizon with
when sending updates out a specific interface,
poison reverse?
designate any networks that were learned on that
interface as unreachable
What is time-to-live (TTL)?
an 8-bit field in the IP header that limits the number of
hops a packet can traverse through the network before
it is discarded
What happens when a packet’s The packet is discarded
TTL reaches 0?
What are the factors that affect Size of the network
the distance vector protocol
Compatibility between models of routers
you choose?
Administrative knowledge required
What are the features of RIP?
Supports split horizon and split horizon with poison
reverse to prevents loops.
Is capable of load balancing up to six equal cost paths .
The default is four equal cost paths.
What features were introduced Includes the subnet mask in the routing updates,
with RIPv2?
making it a classless routing protocol.
Has authentication mechanism to secure routing table
updates.
Supports variable length subnet mask (VLSM).
Uses multicast addresses instead of broadcast.
What are the features of
EIGRP?
What are the advantages of
EIGRP?
Supports manual route summarization.
Triggered updates (EIGRP has no periodic updates).
Use of a topology table to maintain all the routes
received from neighbors (not only the best paths).
Establishment of adjacencies with neighboring routers
using the EIGRP hello protocol.
Support for VLSM and manual route summarization.
These allow EIGRP to create hierarchically structured
large networks.
Although routes are propagated in a distance vector
manner, the metric is based on minimum bandwidth
and cumulative delay of the path rather than hop count.
Fast convergence due to Diffusing Update Algorithm
(DUAL) route calculation. DUAL allows the insertion of
backup routes into the EIGRP topology table, which are
used in case the primary route fails. Because it is a
local procedure, the switchover to the backup route is
immediate and does not involve the action in any other
routers.
Bounded updates mean that EIGRP uses less
bandwidth, especially in large networks with many
routes.
EIGRP supports multiple network layer protocols
through Protocol Dependent Modules, which include
support for IP, IPX, and AppleTalk.
* What are the examples of distance routing protocol?
* What are the characteristics of EIGRP?
* What are the characteristics of RIP?
* What are the advantages of EIGRP over RIP?
* What is the “routing loop”? * What could cause the “routing loop”?
* How does RIP v1 send its routing table update? How does RIP v2 send its routing
table update? How often does it send the update? What is included in the update?
* RIP Triggered Updates
– Routing table update that is sent immediately to adjacent routers in
response to a routing change
– The receiving routers, in turn, generate triggered updates that notify
their neighbors of the change.
* What 3 event will trigger the “triggered update” to take place in distance vector routing
protocol?
– Interface changes state
– Route becomes unreachable
– Route is placed in routing table
* What does the 1st thing router do regarding to the network discovery after the boot-up
(or the reload or the cold start)?
[Hint: not the route exchange]
* What is Hold-down timer and what does it prevent from happing to the routing table?
* What is the definition of infinity in RIP?
* What is the definition of RIP_JITTER?
* What is the definition of “split horizon” and how does it work?
* What is the definition of “route poisoning” and how does it work?
* What is the purpose of TTL? How does it work to prevent loop on a network?
 Purpose of the TTL field
– The TTL field is found in an IP header and is used to prevent packets from
endlessly traveling on a network
 How the TTL field works
 TTL field contains a numeric value. The numeric value is decreased by
one by every router on the route to the destination.
 If numeric value reaches 0 then Packet is discarded.
* What are the purposes of the following timers and how long is each timer?
- Update timer?
- Invalid timer?
- Holddown timer?
- Flush timer?
* What is the definition of “networks are converged”? What are the consequences if the
distance vector networks cannot get converged?
* Will the router forward any packet if the routing tables are not converged?
* What are the consequences if the router forwards the packet when the
networks are not converged?
* What happen when one of the networks in RouterC goes down?
- What is routerC’s response of losing a network?
- What is routerB’s response of RouterC losing a network?
- What is routerA’s response of RouterC losing a network?
* The following is the hint for the above question (- split horizon):
* Which route will router uses to send packet from PC1 to PC2 if RIP is the routing
protocol? (Uses the picture below)
Which route will router uses to send packet from PC1 to PC2 if EIGRP is the routing
protocol? (Uses the picture below)
- Make sure you understand the concept but not just memorize the question.
You will get a different picture at the chapter quiz.
* What is the metric for RIP in R1 to the network 192.168.8.0 /24?