EDTECH
552
(SP11)
Susan
Ferdon
Notes – Odom, Chapter 14
Routing Protocol Concepts and Configuration
Flashcards Set: http://www.flashcardmachine.com/1335840/i84t
network route
A data transmission path through one or more networks
between two end nodes.
route metrics
The cost in time and resources to send a data packet
over that route.
static route
A route that is manually configured on a router. Includes
destination IP, subnet mask, and next-hop-IP (or
outgoing interface). Route remains static/unchanged
unless reconfigured.
default route
On a router, the route that is considered to match all
packets that are not otherwise matched by some more
specific route.
dynamic route
A route that the router learns from neighboring routers.
routing
protocol
A set of messages and processes with which routers can
exchange information about routes to reach subnets in a
particular network. Examples of routing protocols include
the Enhanced Interior Gateway Routing Protocol (EIGRP),
the Open Shortest Path First (OSPF) protocol, and the
Routing Information Protocol (RIP).
RIP
Routing Information Protocol
An Interior Gateway Protocol (IGP) that uses distance
vector logic and router hop count as the metric. RIP
Version 1 (RIP-1) has become unpopular, with RIP
Version 2 (RIP-2) providing more features, including
support for VLSM.
EIGRP
Enhanced Interior Gateway Router Protocol
EIGRP is an advanced distance-vector routing protocol,
with optimizations to minimize both the routing instability
incurred after topology changes, as well as the use of
bandwidth and processing power in the router. The data
EIGRP collects is stored in three tables: Neighbor Table,
Topology Table, and Routing table. Routing information is
exchanged only upon the establishment of new neighbor
adjacencies, after which only changes are sent.
OSPF
Open Shortest Path First
An adaptive routing protocol for Internet Protocol (IP)
networks which uses a link state routing algorithm and
operates within a single autonomous system (AS). It
gathers link state information from available routers and
constructs a topology map of the network. The topology
determines the routing table presented to the Internet
Layer which makes routing decisions based solely on the
destination IP address found in IP packets.
IS-IS
Intermediate System to Intermediate System
Routing Protocol
An interior gateway protocol, designed for use within an
administrative domain or network. IS-IS is a link-state
routing protocol, operating by reliably flooding link state
information throughout a network of routers. Each IS-IS
router independently builds a database of the network's
topology, and packets (datagrams) are forwarded, based
on the computed ideal path, through the network to the
destination.
administrative
distance
In Cisco routers, a means for one router to choose
between multiple routes to reach the same subnet when
those routes were learned by different routing protocols.
The lower the administrative distance, the better the
source of the routing information.
metric
A unit of measure used by routing protocol algorithms to
determine the best route for traffic to use to reach a
particular destination.
VLSM
Variable-length subnet masking
The capability to specify a different subnet mask for the
same Class A, B, or C network number on different
subnets. VLSM can help optimize available address space.
IGP
Interior Gateway Protocol
A routing protocol that was designed and intended for use
inside a single autonomous system.
EGP
Exterior Gateway Protocol
A routing protocol that was designed and intended for use
between different autonomous systems.
AS
Autonomous System
An internetwork in the administrative control of one
organization, company, or governmental agency, inside
which that organization typically runs an Interior Gateway
Protocol (IGP).
BGP
Border Gateway Protocol
A protocol that is used to exchange routes between
routers in different autonomous systems. It is an EGP.
balanced hybrid A term that refers to a general type of routing protocol
algorithm, the other two being distance vector and link
state. The Enhanced Interior Gateway Routing Protocol
(EIGRP) is the only routing protocol that Cisco classifies
as using a balanced hybrid algorithm.
classful routing
protocol
Does not transmit the mask information along with the
subnet number, and therefore must consider Class A, B,
and C network boundaries and perform
autosummarization at those boundaries. Does not
support VLSM.
classless
routing
protocol
An inherent characteristic of a routing protocol,
specifically that the routing protocol does send subnet
masks in its routing updates, thereby removing any need
to make assumptions about the addresses in a particular
subnet or network, making it able to support VLSM and
manual route summarization.
convergence
The time required for routing protocols to react to
changes in the network, removing bad routes and adding
new, better routes so that the current best routes are in
all the routers’ routing tables.
distance vector
The logic behind the behavior of some interior routing
protocols, such as RIP. Distance vector routing algorithms
call for each router to send its entire routing table in each
update, but only to its neighbors. Distance vector routing
algorithms can be prone to routing loops but are
computationally simpler than linkstate routing algorithms.
link-state
A classification of the underlying algorithm used in some
routing protocols. Link-state protocols build a detailed
database that lists links (subnets) and their state (up,
down), from which the best routes can then be
calculated.
routing update
A generic reference to any routing protocol’s messages in
which it sends routing information to a neighbor.
CCNA Certification for Dummies --- BEST!!
(The following is a combination of info from the textbook and Dummies
book.)
A network route is a data transmission path through one or more networks
between two end nodes. More than one route can exist. The main purpose of
a router is to find the best route to read a destination node. The best route
is calculated through route metrics: the cost in time and resources to send a
data packet over that route (Dummies, p. 593).
There are three types of network routes:
 Static routes: Defined manually on a router – they are static
(unchanged) unless you reconfigure.
o Advantages:
 Efficiency – you can leave routing protocols disabled which
saves bandwidth.
 Security – you can filter routing data using firewalls and
VPN to secure data no matter which path they travel on.
o Disadvantages:
 Maintenance – management overhead to update routes
 Accuracy – if network changes and you don’t update static
routes you will have lost or delayed data.
 Scalability – Large networks have hundreds or thousands
of alternate routes – too many to configure and maintain
statically.
 Default routes: Default routes are static routes that you define for
packets bound to a destination network that is not in any of the
routing tables on your router. A default route is a data transmission
path to that default outbound gateway in a network.
o Default routes work best when only one path exists to a part of
the network.
o Without a default route, the router will discards packets that
don’t match the routing table.
 Dynamic routes: Routes that change over time. May be due to network
topology and traffic updates, available bandwidth, and link state.
o Advantages:
 Low maintenance – routes are automatically updated, all
you have to do is configure routing protocols on your
routers.
 Accuracy – Routing protocols keep track of network
changes which means routers will send packets over the
best possible routes.
Scalability – in a large network it’s a maintenance
nightmare to define all routes statically. Routing protocols
allow routers to communicate about routes they know,
new routes they discover, and routes that become
unavailable or overloaded.
o Disadvantages:
 Overhead – consumes bandwidth because they regularly
send route update packets between routers.


Connected and Static Routes
Connected Routes
 A router adds routes to its routing table for the subnets connected to
each of the router’s interfaces - the router must have an IP address
and mask configured on the interface (statically with the ip address command
or dynamically using Dynamic Host Configuration Protocol [DHCP]) and both
interface status codes must be “up.”
 In Example 14-1, the output of the show ip route command confirms
that Albuquerque indeed added a route to all three subnets to its
routing table.
 The output begins with a single-letter code legend, with “C” meaning
“connected.”
 The output lists the mask in prefix notation by default.
 In cases when one mask is used throughout a single classful network—
in other words, static-length subnet masking (SLSM) is used—the
show ip route command output lists the mask on a heading line above
the subnets of that classful network.
Static Routes
http://www.petri.co.il/csc_how_to_static_routes_cisco_ios.htm
http://ciscotests.org/ccna.php?part=6
 Static routes are best for LANs – maintenance issues make static
routes practically impossible for WAN/large networks. If a route
changes, that change would have to be manually configured on each
individual router.
 Configure static routes using the ip route command in global
configuration mode. Syntax:
ip route dest-ip subnet {next-hop-ip | interface}
o Dest-ip: IP address of the destination network. You are
registering a static route to the destination network.
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



o Subnet: This is the subnet mask of the destination network;
defines which part of address is network and which is host.
o {next-hop | interface}: IP gateway (router) through which you
reach the destination network. Specify the IP address of the next
hop or the outbound interface through which the router can
reach the destination. Interface is used when it’s a point-to-point
serial link.
o {} means you must specify something, | (pipe) means choose
one or the other of the options.
To remove a static route use the usual no prefix with ip route
command: no ip route dest-ip [subnet] {next-hop-ip | interface}.
You configure default routes using the same ip route command in
global configuration mode. However, the IP address and subnet mask
are 0.0.0.0 – which means “match all packets” - since the network is
unknown.
You can see all routes using show ip route command. To see only
statically configured routes, use show ip route static.
Telnet to a router and use ping commands to see which routes a
router is able to connect to (Text, p. 444).
When you troubleshoot this internetwork, you can use the extended
ping command to act like you issued a ping from a computer on that
subnet, without having to call a user and ask to enter a ping command
for you on the PC.
Routing Protocol Overview
Routing protocols help routers learn routes by having each router advertise
the routes it knows. Each router begins by knowing only connected routes.
Then, each router sends messages, defined by the routing protocol, that list
the routes. When a router hears a routing update message from another
router, the router hearing the update learns about the subnets and adds
routes to its routing table (Text, p. 448).
Routing protocols exchange network, routes, and metric information
between routers to help find optimal routes as fast as possible. Routers use
the information provided by routing protocols to build their routing tables for
each routed protocol to keep track of networks, paths to networks, and
metrics associated with each route (Dummies, p. 598).
The most widely used routing protocols are:
 Routing Information Protocol (RIP)
 Enhanced Interior Gateway Routing Protocol (EIGRP)
 Open Shortest Path First (OSPF)
Routing Decision Criteria
 Routers pick different network routes depending on various criteria.
 Some routes may be deemed faster by different routing protocols.
 Routers keep separate routing tables for each protocol.
 A route that is best now may not be best in a few minutes, depending
on various criteria like traffic, available bandwidth, and link state.
 Routing tables keep track of networks, paths to networks, and metrics
associated with each route.
 Routers consider two aspects when deciding which network routes are
best at a given moment:
o Administrative Distance – How reliable is the information source
that provided the data about the network route?
o Routing Protocol Metrics – What are the costs associated with
each network route?
Administrative Distance (AD)
 Routers learn about networks using various methods:
o Directly connected – AD = 0. Router learns about the network
firsthand, because it connects to it.
o Static route – AD = 1. The router does not “see” the network,
but it’s been informed about its existence by a fairly reliable
source (the static route).
o Connected indirectly – The router heard about it from another
router (EIGRP, SOPF, RIP).
 Routers prefer sources with lower AD numbers.
 If the same routing protocol finds two different routes to the same
destination and the AD is the same, other metrics are considered to
decide which route to use.
Routing Protocol Metrics (Dummies, p. 600)
 Each routing protocol calculates the efficiency (the cost) of a route
differently.
 Whenever routing protocols contradict each other, the one with the
lowest AD is preferred.
 If the same routing protocol finds two (or more) routes to the same
destination, specific decision criteria are used:
o Hop count – the number of routers that need to be transversed
(prefer few hops because there is delay at every hop). RIP uses
hop count metric to choose network routes.
o Bandwidth – prefer routes with larger bandwidth; very likely
packets will arrive faster. EIGRP uses bandwidth metric to
choose network routes.
o Delay – Total delay calculated on processing delay, queuing
delay, transmission delay and propagation delay (EIGRP).
o Reliability – Percentage of time the route is available (EIGRP)
o Load – Bandwidth consumed by current traffic on a given route;
the difference between total bandwidth and available bandwidth
of the route (EIGRP).
o Maximum Transmission Unit (MTU) – The size, in bytes, of each
data packet. The higher the MTU, the more data can be
transferred at once (EIGRP).
o Cost – Calculated based on the bandwidth of a network route;
108/bandwidth (OSPF).
Routing Methods (Dummies, p. 602)
Routing protocols use different methods to exchange the info that helps
routers build their routing tables.
 Distance vector routing:
o Build routing tables based on route distance.
o Exchange and combine their routing table with their neighbors
(called convergence).
o Neighbor routers trust each other’s route information, and they
relay the combined information farther.
o Routing tables are combined and relayed to all routers in the
network.
o Because DV routing protocols combine the routing tables of all
routers and propagate them to all neighbors, the convergence
process can be very long in larger networks.
o Distance vector routing can cause routing loops so various
features are used to avoid routing loops:
 Maximum hop count – never takes route that exceeds
certain number of hops (looping).
 Split horizon – prevents route from be advertised back to
its advertiser.
 Route poisoning – changes hop count for route that
become unreachable, which disables a route quickly.
 Poison reverse – breaks the split horizon rule ensuring that
all neighbors receive “route down” message as quickly as
possible.
 Hold-down timer – prevents router from accepting updates
about a router for a certain amount of time if that was
reported as down.
 Triggered update – allows routers to update each other as
soon as a change occurs, rather than waiting for scheduled
update to be exchanged.
o Best suited for access or distribution layer routers.


o RIP and IGRP (replaced by EIGRP) are distance vector routing
protocols.
Link-state routing:
o Build their routing tables independently based on route updates
they receive from their neighbors.
o Do not merge the routing tables of neighbor routers.
o Enable routers to have clear image of their neighbors, network
topology, and routes to neighbors and beyond.
o OSPF uses link-state routing.
Hybrid routing:
o Have both distance vector and link-state characteristics.
o Like distance vector protocols …
 hybrid routing protocols use distance to evaluate quality of
routes.
 hybrid routing protocols send route updates that contain
the whole routing table
o Like link-state protocols …
 hybrid routing protocols use other metrics in addition to
distance to evaluate the quality of routes.
 hybrid routing protocols only exchange “hello” messages
initially so convergence time is faster than distance vector
protocols.
 hybrid routing protocols send updates only when routes
change.
o Hybrid routing protocols are well-suited for core layer,
distribution layer, and even access layer routers.
o EIGRP is considered a hybrid routing protocol.
RIP-2 Basic Concepts (p. 449)
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

Routers using RIP-2 advertise a small amount of simple information
about each subnet to their neighbors. Their neighbors in turn advertise
the information to their neighbors, and so on, until all routers have
learned the information.
RIP routers send periodic routing updates about every 30 seconds by
default. When something changes, the routers will react and converge
to use the then-best working routes.
Figure 14-3 (p. 449) shows RIP-2 advertising subnet number, mask,
and metric to its neighbors.
Comparing and Contrasting IP Routing Protocols (p. 450)

Important considerations:
o Is it a public standards (defined in RFCs) or Cisco proprietary?
o Does the routing protocol support variable-length subnet
masking (VLSM)?
Interior and Exterior Routing Protocols (p. 451)
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


Two types:
o Interior Gateway Protocol (IGP): A routing protocol that was
designed and intended for use inside a single autonomous
system.
o Exterior Gateway Protocol (EGP): A routing protocol that was
designed and intended for use between different autonomous
systems.
An autonomous system is an internetwork under the administrative
control of a single organization.
Each autonomous system can be assigned a number, called an
autonomous system number (ASN). Like public IP addresses, the
Internet Corporation for Assigned Network Numbers (ICANN) controls
the worldwide rights to assign ASNs.
By assigning each autonomous organization an ASN, Border Gateway
Protocol (BGP is an EGP) can ensure that packets do not loop around
the global Internet by making sure that packets do not pass through
the same autonomous system twice.
Metrics give an objective number to the “goodness” of each route. The
lower the metric, the better the route. Figure 14-5 compares two
routes – RIP/hop count and EIRGP/bandwidth.
Autosummarization and Manual Summarization (p. 454)

Routers generally perform routing (forwarding) more quickly with
smaller routing tables, and less quickly with larger routing tables.
Route summarization helps shorten the routing table while retaining all
the needed routes in the network.

Manual summarization gives the network engineer a great deal of
control and flexibility, allowing the engineer to choose what summary
routes to advertise, instead of just being able to summarize with a
classful network.
Classless and Classful Routing Protocols (p. 454)




Classful routing protocol:
o must consider the Class A, B, or C network number that a subnet
resides in when performing some of its tasks.
o does NOT support VLSM.
o does NOT send subnet mask in routing updates.
o does NOT support manual route summarization.
Classless routing protocols do not need to consider class rules.
o DOES support VLSM.
o DOES send subnet mask in routing updates.
o DOES support manual route summarization.
The processes used by routing protocols to recognize the changes, to
figure out the now-best routes to each subnet, and to change all the
routers’ routing tables, is called convergence. Some routing protocols
converge more quickly than others.
Later-defined IGPs typically support some kind of authentication as a
means of mitigating possible DoS attacks.
Configuring and Verifying RIP-2 (p. 456)
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



Three-step process:
o Step 1 - Use the router rip configuration command to move into
RIP configuration mode.
o Step 2 - Use the version 2 RIP subcommand to tell the router to
use RIP Version 2 exclusively.
o Step 3 - Use one or more network net-number RIP
subcommands to enable RIP on the correct interfaces.
Each RIP network command enables RIP on a set of interfaces.
The RIP network command only uses a classful network number as its
one parameter.
For any of the router’s interface IP addresses in that entire classful
network, the router does the following three things:
o The router multicasts routing updates to a reserved IP multicast
IP address, 224.0.0.9.
o The router listens for incoming updates on that same interface.
o The router advertises about the subnet connected to the
interface.
Sample RIP configuration on page 457.


RIP configuration does not provide a way to enable RIP on only some
interfaces so the network must be configured then use the passiveinterface type-number RIP subcommand to stop sending RIP updates
out that interface.
IOS includes three primary show commands to confirm how well RIP2 is working (detailed example begins on p. 459).
Of particular importance for real-life troubleshooting and for the exam, focus
on both the version information and the routing information sources. If you
forget to configure the version 2 command on one router, that router will
send only RIP-1 updates by default, and the column labeled “Send” would
list a 1 instead of a 2. The other routers, only listening for Version 2
updates, could not learn routes from this router.
(Text goes over Administrative Distance, see notes from Dummies book,
above.)
Examining RIP Messages with debug (p. 464)



The best way to understand whether RIP is doing its job is to use the
debug ip rip command. This command enables a debug option that
tells the router to generate log messages each time the router sends
and receives a RIP update.
Example 14-9 shows the output generated by the debug ip rip
command on the Albuquerque router, based on Figure 14-1. Note that
to see these messages, the user needs to be connected to the console
of the router, or use the terminal monitor privileged mode EXEC
command if using Telnet or SSH to connect to the router.
A close examination of the number of subnets in each routing update
(Example 14-9) shows that the routers do not advertise all routes in
the updates. The reason has to do with the theory behind RIP,


specifically a feature called split horizon. This loop-avoidance feature
limits which subnets are advertised in each update to help avoid some
forwarding loops.
Before using the debug command, look at the router’s CPU utilization
with the show process command. On routers with a higher CPU
utilization, generally above 30 to 40 percent, be very cautious when
enabling debug options, as this may drive the CPU to the point of
impacting packet forwarding.
To make the router generate time stamps, you need to configure the
service timestamps global configuration command.
“Do I Know This Already” Quiz, Chapter 14 - pp. 436 - 438
TOPIC
Connected and Static
Routes
Routing Protocol Overview
Q#
1
2
3
4
1st Try
B, C
A, C, D
A
E, F
5
B
6
B, D, E
A, D, G,
I
F
D, E, F
B, C, F
7
Configuring and Verifying
RIP-2
8
9
10
2nd Try
Answer
A, C
A
A, B
B, D, E,
F
D, E, F
A, D, E,
H
A
B
B, C
Q1: Which of the following must be true for a static route to be installed in a
router’s IP routing table?
a. The outgoing interface associated with the route must be in an “up
and up” state.
b. The router must receive a routing update from a neighboring router.
c. The ip route command must be added to the configuration.
d. The outgoing interface’s ip address command must use the special
keyword.
Answer: A, C
Explanation: Typo – I really meant A. B is not correct since configuring static
routes do not rely on learning routes from neighbors (that would be dynamic
routing).
Q2: Which of the following commands correctly configures a static route?
a. ip route 10.1.3.0 255.255.255.0 10.1.130.253
b. ip route 10.1.3.0 serial 0
c. ip route 10.1.3.0 /24 10.1.130.253
d. ip route 10.1.3.0 /24 serial 0
Answer: A
Explanation: Commands for static route include destination IP, subnet mask,
and next-hop-ip | interface. B is not correct because there is no subnet
mask. C and D are not correct because it must be dotted decimal format.
Q3: Which of the following routing protocols are considered to use distance
vector logic?
a. RIP
b. IGRP
c. EIGRP
d. OSPF
Answer: A and B
Explanation: I forgot about IGRP. The Dummies books says that isn’t used
anymore – it was replaced by EIGRP.
Q5: Which of the following routing protocols support VLSM?
a. RIP
b. RIP-2
c. IGRP
d. EIGRP
e. OSPF
f. Integrated IS-IS
Answer: B, D, E and F
Explanation: A is not correct – RIP does not send subnet mask so variablelength subnet mask would not be supported. C is not correct – IGRP is also
old, not supported. Basically, new ones support VLSM old ones don’t.
Q6: Which of the following routing protocols are considered to be capable of
converging quickly?
a. RIP
b. RIP-2
c. IGRP
d. EIGRP
e. OSPF
f. Integrated IS-IS
Answer: D, E and F
Explanation: I said B, D and F. B is not correct – both RIP and RIP-2 send
the whole routing table so convergence won’t be quick. Hybrid and link-state
don’t merge tables, so convergence is faster – that would be EIGRP, OSPF
and Integrated IS-IS.
Q7: Router1 has interfaces with addresses 9.1.1.1 and 10.1.1.1. Router2,
connected to Router1 over a serial link, has interfaces with addresses
10.1.1.2 and 11.1.1.2. Which of the following commands would be part
of a complete RIP Version 2 configuration on Router2, with which
Router2 advertises out all interfaces, and about all routes?
a. router rip
b. router rip 3
c. network 9.0.0.0
d. version 2
e. network 10.0.0.0
f. network 10.1.1.1
g. network 10.1.1.2
h. network 11.0.0.0
i. network 11.1.1.2
Answer: A, D, E, H
Explanation: I said A, D, G, I. A is correct because we are configuring with
RIP routing protocol. D is correct because we are using RIP-2, not RIP. Other
answers were wrong because I misread the question – I thought we were
configuring Router 1, so network numbers for that router (answers G and I)
would not be used. We are configuring Router 2, so static routes to Router 1
addresses would be configured. **Is H a mistake? 11.0.0.0 is on Router 2.
Q8: Which of the following network commands, following a router rip
command, would cause RIP to send updates out two interfaces whose
IP addresses are 10.1.2.1 and 10.1.1.1, mask 255.255.255.0?
a. network 10.0.0.0
b. network 10.1.1.0 10.1.2.0
c. network 10.1.1.1. 10.1.2.1
d. network 10.1.0.0 255.255.0.0
e. network 10
f. You cannot do this with only one network command.
Answer: A
Explanation: I said F because I thought you had to do the thing where you
configure RIP for the whole router then disable some of the interfaces. A is
correct because that is the address for the network that the two interfaces
are part of. B and C are not correct because you can’t put two IP addresses
in the same command plus using the IP address for the entire network
covers both bases with one command. Answer E is not correct – “10” is not a
valid address.
Q9: What command(s) list(s) information identifying the neighboring routers
that are sending routing information to a particular router?
a. show ip
b. show ip protocol
c. show ip routing-protocols
d. show ip route
e. show ip route neighbor
f. show ip route received
Answer: B
Explanation: I said D, E and F. Per Table 14-5, show ip protocol “lists
information about the RIP configuration, plus the IP addresses of
neighboring RIP routers from which the local router has learned routes.” D,
E, and F are not correct – show ip route command shows learned routes
but not IP addresses from which those routes were learned. I haven’t found
any documentation that shows that E or F use correct syntax.
Q10: Review the snippet from a show ip route command on a router:
R 10.1.2.0 [120/1] via 10.1.128.252, 00:00:13, Serial0/0/1
Which of the following statements are true regarding this output?
a. The administrative distance is 1.
b. The administrative distance is 120.
c. The metric is 1.
d. The metric is not listed.
e. The router added this route to the routing table 13 seconds ago.
f. The router must wait 13 seconds before advertising this route again.
Answer: B and C
Explanation: I said B, C and F. F is not correct - the time listed is the
amount of time since the router last heard about this route.