SEMESTER 2 Chapter 4
Distance Vector Routing Protocols
What are the three distance
vector routing protocols?
RIP, IGRP, EIGRP
What are the key
characteristics of RIP?
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.
What are the key
characteristics of IGRP?
Bandwidth, delay, load and reliability are used to create
Routing updates are broadcast every 90 seconds, by
IGRP is the predecessor of EIGRP and is now
What are the key
characteristics of EIGRP?
It can perform unequal cost load balancing.
It uses Diffusing Update Algorith
m (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.
How are distance vector routes
As vectors of distance and direction
s a router in a distance
vector network have knowledge
of the entire path to a
What are the 2 things a
distance vector router knows?
The direction or interface in which packets should be
The distance or how far it is
to the destination network
What are the two reasons
sending an entire routing table
in an update inefficient?
consume bandwidth but also consume router CPU
resources to process the updates
What is the periodic update
time for RIP?
is the periodic update
time for IGRP?
What is the address for
What is the algorithm used for?
to calculate the best paths and then send that
information to the neighbors
What processes does the
Mechanism for sending and receiving routing
Mechanism for calculating the best paths and installing
routes in the routing table.
Mechanism for detecting and reacting to topology
Define Time to Convergence.
e 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 ta
not updated due to slow convergence in a changing
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
eeds to be.
Classless (Use of
VLSM) or Classful
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
include the subnet mask and cannot support VLSM.
Define Resource Usage.
Resource usage includes the requirements of a routing
protocol such as memory space, CPU utilization, and
link bandwidth utilization. Higher resource requirements
more powerful hardware to support the
routing protocol operation in addition to the packet
Implementation and maintenance describes the level of
knowledge that is required for a network administ
to implement and maintain the network based on the
routing protocol deployed.
List the advantages for
distance vector protocols.
Simple implementation and maintenance
Low resource requirements
List the disadvantages for
distance vector protocols.
Definitely practice the exercise
at the right. It will help you
remember each protocol.
What are the routes that router
will initially discover after
The directly connected netw
After completing the initial
discovery what does the router
start to do?
What does the initial exchange
of routing information contain?
Information about their directly connected networks
What does the router do with
the information that is not
contained in its routing table?
It is added to the routing table
After the first exchange of
routing updates what has been
added to each routing table?
Neighbors directly connected routes
What is convergence?
routers have consistent knowledge of the
What is slit horizon?
Prevents information from being sent out the interface
that it was received
The amount of time it takes for
a network to converge is
directly proportional to
the size of that network
The speed of achieving
convergence consists of what
w 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
What are the two reasons
routers exchange routing
o exchange routing information with their neighbors
and to maintain up
date routing information in the
What is the period update time
What are the four reasons
listed for topology changes?
Failure of a link
Introduction of a new link
Failure of a router
Change of link parameters
What are the three additional
If an update has not been
ceived to refresh an existing
route after 180 seconds
happens to that route in the
It is marked invalid
What happens to the route after
240 seconds have elapsed?
It is removed from the routing table. Flushed
What does a holddown ti
Keeps a route in the routing table, marked as
unreachable, for at least 180 seconds so all other
routers receive that information
What are the two commands
that will show the timers?
Show ip route
Show ip protocols
Define bounded update.
pdates that are sent only to the routers that need the
information instead of sending information to all routers
What are characteristic of
periodic because they are not sent out on a
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 tabl
e update that is sent immediately in
response to a routing change
What are the three reasons to
send a triggered update?
An interface changes state (up or down)
A route has entered (or exited) the "unreachable" state
A route is installed in the routing t
What are the two problems with
Packets containing the update message can be
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 recei
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
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
What is a routing loop?
a packet is continuously transmitted within a series of
routers without ever reaching its intended destination
What are some reasons for
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
Inconsistent routing tables not being updated due to
slow convergence in a changing network
Incorrectly configured or installed discard routes
What mechanism is built into IP
to overcome routing loops?
itions can be created
because of routing loops?
Link bandwidth will be used for traffic looping back
and forth between the routers in a loop.
A router's CPU will be strained due to looping
A router's CPU will be burdened with useless
ding 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
What are some of the
mechanisms used to avoid
Defining a maximum metric to prevent count to infinity
Route poisoning or poison reverse
What is count to infinity?
a condition t
hat 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?
What is it called when a route
up, then down, then up
What are holddown timers used
to prevent regular update messages from
inappropriately reinstating a route that may have gone
Describe the process of how a
holddown timer works.
1. A router receives an up
date from a neighbor
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
m 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
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
connectivity. If the destination network truly
is unavailable and the packets are forwarded, black
hole routing is created and lasts until the holddown
What is the split horizon rule?
a router should not advertise a network through t
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,
signate any networks that were learned on that
interface as unreachable
What is time
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
TTL reaches 0?
The packet is discarded
What are the factors that affect
the distance vector protocol
Size of the network
Compatibility between models of routers
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 rou
making it a classless routing protocol.
Has authentication mechanism to secure routing table
Supports variable length subnet mask (VLSM).
Uses multicast addresses instead of broadcast.
Supports manual route summarization.
the features of
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 prot
Support for VLSM and manual route summarization.
These allow EIGRP to create hierarchically structured
What are the advan
Although routes are propagated in a distance vector
manner, the metric is based on minimum band
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 fa
ils. Because it is a
local procedure, the switchover to the backup route is
immediate and does not involve the action in any other
Bounded updates mean that EIGRP uses less
bandwidth, especially in large networks with many
multiple network layer protocols
through Protocol Dependent Modules, which include
support for IP, IPX, and AppleTalk.