Lab
3
Exercise
INTRODUCTION TO OSPF FEATURES AND OPERATION
F A 2 4
T E L E C O M
S Y S T E M S
E N G I N E E R
C O U R S E
TRAINING OBJECTIVE: The Student will be familiarized with the operation and
programming of the Cisco Router using the OSPF routing protocol.
Conditions:
EQUIPMENT: The student will be given applicable reference material and a Cisco
router in an operational network and a student handbook.
REFERENCES: OSPF Design Guide; Cisco IOS IP and IP Routing Configuration Guide:
Configuring OSPF
METHOD OF INSTRUCTION: Lecture with practical exercise.
INTRODUCTION: Note: Show slide #1:
Open Shortest Path First (OSPF) is a routing protocol developed for IP networks by the
Interior Gateway Protocol (IGP) working group of the Internet Engineering Task Force
(IETF). The group was formed in 1988 to design an IGP based on the shortest path first
(SPF) algorithm for use in the Internet. OSPF was created because the Routing
Information Protocol (RIP) was, in mid-1980, increasingly unable to serve large
networks.
As indicated by the acronym, OSPF has two primary characteristics. The first is that it
is open, in that its specifications is in the public domain and was originally described in
Request for Comments (RFC) 1131. The most recent version, known as OSPF2, is
described in RFC 1583. The second principle characteristic is that it is based on the
SPF algorithm, which is sometimes referred to as the Dijkstra algorithm, named for the
person credited with its creation.
BODY:
1) Static vs Dynamic Routes (show slides #2):
 Refer to static routes class. Point out that with static routes, they must all be
maintained manually.
 All routes must be initially entered.
 When changes are made to the network (routes added or deleted), changes
must be entered manually.
 When links go down, route must be deleted from router; when link is
restored, route must be added back to router.
 Extremely man power intensive; a lot of room for error.
 Virtually impossible to maintain on very large networks.
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Static knowledge is administered manually: A network administrator enters it into the
router’s configuration. The administrator must manually update this static route entry
whenever an internetwork topology change occurs. Static knowledge can be private –
by default it is not conveyed to other routers as part of an update process. You can,
however, configure the routers to share this knowledge.
Dynamic knowledge works differently. After the network administrator enters
configuration commands to start dynamic routing, route knowledge is updated
automatically by a routing process whenever new topology information is received from
the internetwork. Changes in dynamic knowledge are exchanged between routers as
part of the update process.
2) Routing Protocols (show slide 3):
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RIP: Routing Information Protocol - distance vector type, open.
IGRP: Interior Gateway Routing Protocol - distance vector, Cisco
proprietary.
OSPF: Open Shortest Path First - link state type, open.
EIGRP: Enhanced IGRP - balanced hybrid type, Cisco proprietary.
BGP: Border Gateway Protocol - inter-autonomous system protocol, open
3) OSPF History (show slides 4):
The IETF (Internet Engineering Task Force) was looking for a fast, scalable, efficient
interior routing protocol that would replace RIP1. In 1987, work was began on OSPF,
and in 1989, OSPF V1 was finalized as RFC 1131. OSPF V2 was defined in 1991, and
the latest enhancements released in 1997. OSPF V2 is the standard that is addressed
in this section. This standard is defined in RFC 2178. OSPF is a work in progress;
features will be added and modified on an as-needed basis.
4) OSPF Related RFC’s (slides 5):
Show slide; provided for reference.
5) OSPF Feature as (slide 6):
- Open, non-proprietary
- Has no hop count limitation
- Supports VLSM
- Uses multicast addressing for updates
- Has fast convergence
- Allows for routing authentication
- Supports hierarchical routing

OSPF is “in the public domain”. It is not owned by any one entity and can be
used by any vendor.
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Unlike RIP, which has a 15-hop count limitation (if a destination is more than 15
routers away it is deemed unreachable), OSPF has no hop count limitation.
OSPF uses metrics or cost assigned to individual links to determine the best path.
Supports Variable Length subnet masking for efficient IP address allocation.
Uses IP multi-casting for the sending of link-state updates. This ensures less
processing on routers that are not listening to OSPF packets. Also, updates are
only sent in case routing changes occur, instead of periodically. This ensures
better use of bandwidth.
OSPF has fast convergence in that it sends out routing changes instantaneously
and not just periodically.
Allows routing authentication by using different methods of password
authentication and password encryption.
OSPF allows for logical definition of networks where routers can be divided into
areas. This will limit the “explosion” of routing updates across the entire network
and ensures better usage of bandwidth. This also allows routers to be divided
into different areas of management based such factors as geographical location.
6) OSPF Hierarchical Routing (show slide 5):
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OSPF network consists of areas within an autonomous system (AS).
Areas must start with "0".
Assigned by AS network administrator and only pertain to that AS.
Autonomous Systems are assigned by InterNIC (network information
center).
There are two primary elements in the OSPF hierarchy:
Area – An area is a grouping of contiguous OSPF networks and hosts. OSPF areas are
logical subdivisions of OSPF autonomous systems. The topology of each area is
invisible to entities in other areas, and each area maintains its own topological database.
Autonomous – OSPF autonomous systems are the largest entity within an OSPF
internetwork. They consist of a collection of networks that are under a common
administration and share a common routing strategy. An autonomous system,
sometimes called a domain, is logically subdivided into multiple areas.
The hierarchical topology of OSPF has several important benefits. Because the
topology of an area is hidden from the rest of the autonomous system, routing update
traffic can be reduced through route summarization, and the topological databases and
SPF trees remain manageable and more efficient.
Within each autonomous system, a central area must be defined as area 0. All others
areas are connected off of the central, or backbone area. Area 0 is also called the
transition area because all other areas communicate through it. The OSPF backbone
also distributes routing information between OSPF areas.
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The OSPF backbone has all the properties of a normal OSPF area. Backbone routers
maintain OSPF routing information using the same procedures and algorithms as
internal routers. The backbone topology is invisible to routers in other areas, while the
topologies of individual areas are invisible to backbone routers.
7) OSPF Network Types (slide 8):
There are four network types defined for the OSPF routing protocol.
Point-to-Point: Normally found on serial connections. Neighbor relationships are formed
only with the other router on the point-to-point link. Both routers can independently
communicate with all other OSPF routers.
Broadcast Multi-access: Normally found on LAN connections. There is a potential for
many neighbor relationships since several routers can be on the same segment.
Through an election process, a Designated Router for the network is selected. The DR
communicates with all other routers regarding the LAN network.
Non-broadcast Multi-access: Routers setup in a hub spoke topology using nonbroadcast media such as Frame Relay, x.25, and ATM. Special care must be taken
when configuring this network. Neighbor relationships may have to be manually
configured.
Point-to-Multipoint: Defined as a numbered point to point interface having more than
one neighbor. Occurs when there are sub-interfaces on one end of the point-to-point
network.
8) Types of OSPF Routers (slide 9):
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Backbone Router: Has an interface to Area 0 (backbone)
Area Border Router (ABR): Attaches to multiple area, maintains separate
topological databases for each area to which they are connected, and
routes traffic destined for or arriving from other areas.
Internal Router: Has all directly connected networks belonging to the same
area. It runs a single copy of the routing algorithm.
Autonomous System Boundary Router (ASBR): Exchanges routing
information with routers belonging to other AS's.
9) OSPF Databases (slide 10):

Adjacencies Database
(1) Lists Neighbors - routers that share a common segment; normally direct
connects.
(2) Established by Hello Packets

Topology Database
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(1) Lists all possible routes
(2) Is established by the Link State Advertisements (LSA's)

Routing Table Database
(1) Lists best routes
(2) Is developed by the SPF algorithm being applied to the Topology DB
10) Establishing Neighbors (slide 11):

Read slide - what a hello packet consists of.
Routers that share a common segment become neighbors on that segment using the
Hello protocol. Hello packets are sent periodically out of each interface using IP
multicast addresses. The Hello protocol serves the primary purposes of neighbor
discovery, DR & BDR election, and link integrity verification. Two routers will become
neighbors if they agree on the following: (1) must have the same area-id and be on the
same subnet/mask; (2) they must both use the same type of authentication and
password (if any), (3) the hello and dead intervals must be the same – hello is 10 sec by
default and dead is 4 times the hello by default, (interface hello and dead intervals or
timers can be manipulated under the interface configuration using the “ip ospf”
command.), (4) must agree on the stub area flag – a bit in the hello packet that indicates
whether the interface is a stub area.
11) Establishing Neighbors (slide 12):

Read slide - initial exchange between routers.
Adjacency is the next step after the two routers from a neighbor relationship. Adjacent
routers go beyond the hello exchange and proceed to the database exchange. This is a
one-time swap of the entire OSPF topology database. Once completed, this is updated
with only changes occurring to the database.
12) Establishing Adjacencies and Electing the DR &BDR (slide 13):
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Only applies to a multi-access network (LAN).
Hello packets elect DR & BDR. Router with highest OSPF priority on a
segment will become the DR.
LSA's are only sent to the DR. The DR represents the multi-access
network to other networks. It is the only one that sends LSA's outside the
network.
On a multi-access segment, two routers are elected, the designated router (DR) and the
backup designated router (BDR). These routers act as the central point of contact for all
information exchange on the network. The BDR maintains the same information as the
DR and replaces it in the event it fails. Instead of each router on the network
exchanging LSA’s with every other router, they simply exchange them with the DR/BDR.
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This significantly reduces the amount of router-related traffic on the segment. Election of
the routers is done using the hello protocol. The router with the highest OSPF priority on
a segment will become the DR and the process is then repeated for the BDR. OSPF
priority must be set on an interface with a number from 0 to 255. The router with the
highest priority is elected the DR. The priority default to 1 and in case of a tie, the
highest router ID is used. A value of 0 indicates an interface that can’t be elected
DR/BDR.
13) The Link State Database: (slide 14)
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Also known as the Topology DB.
Consists of link state records including info about all its interfaces and
neighbors. It is a picture of how the router sees the network.
Link State Advertisement is a reliable (acknowledged) message.
Occurs when there are changes within the network and every 30 minutes.
Each router maintains link-state records including information about each of its interfaces
and reachable neighbors. Through flooding, each router distributes its state to all other
routers in the area/autonomous system. As a result, each router possesses an identical
database describing the area/autonomous system. All routers run the SPF algorithm in
parallel. Using the link state database, each router then constructs a tree of the shortest
paths with itself as the root. Each destination within the AS is contained within the SPF
tree.
14) Maintaining Routing Information (Flow Chart) (slide 15):

Lead class through flow chart
(1) Router receives LSA/LSU (update).
(2) Determines if LSA is already in DB
(3) If no, added to database, flooded to network, and then runs SPF to
come up with new routing table. END
(4) If yes, is it the same sequence number, if yes, then ignore.
(5) If sequence number is different, is it newer, if no then send back to
source with newer information.
(6) If sequence number is newer, send LSA to DR and add to database,
flood network, and run SPF.
LSA’s are handled in a very efficient manner between the source router (attached to the
link) and the nearest neighboring router. The incoming LSA is checked against existing
entries in the topological database. Each database entry has a sequence number (also
called a version number), and only the largest number (indicating the most recent
record) is kept. If the entries are identical, then there is no need to forward the LSA to
other routers. If the incoming LSA is different from the topological database, then the
database is updated and the LSA is forwarded through the network until all databases
are synchronized. Associating version numbers with LSA’s contributes to the efficiency
of link-state routing technology.
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15) Types of Link State Packets/LSA’s (slide 16 & 17): explain slides
together; slide 16 depicts location of LSA in network, slide 17 defines each
type of LSA.
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
Cover diagram and point out each type
(1) Router
(2) Network
(3) Summary
(4) External
Cover how OSPF routes show up in the routing table
(1) "O" - OSPF derived intra-area (router LSA)
(2) "IA" - Inter-Area (Summary LSA)
(3) "E1" - Type 1 External Route
(4) "E2" - Type 2 External Route
External Routes (Type 5) fall into two categories, type 1 and type 2. The difference
between the two is the way the cost is being calculated. A type 2 route is only the
external cost; the internal is not added. A type 1 is the external plus the internal cost to
reach a specific destination. Type 2 is the default.
16) Routing Table (slide 18):
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It is developed by running the SPF algorithm on the LSA database.
Preferred routes placed into table; all possible routes still stored in LSA
database.
Discuss routing table
- cover codes listed at top
- gateway set or not set; gateway is where packet is sent if router does
not where to send it.
- lists classful address (example has class B), number of subnets, &
number of different masks.
- Connected: lists address of distant interface directly connected, mask
(/32) and interface connected on local router.
- OSPF: lists network learned via OSPF; lists distance & metric
(110/455), learned via what distant address, time route has been in
table, and learned via which router interface.
- BGP: lists network learned, via which address, & amount of time route
has been in table.
Administrative distance is the first factor used to determine which routes are placed into
the table. If routes have the same distance, the cost or metrics is then used.
17) Distance and Metrics (slide 19):
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Point out that the two numbers at the end of the routing table entry in
parentheses are the distance and metric.
18) Administrative Distance (slide 20):
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Administrative distance is a rating of the trustworthiness of a routing
information source.
 The higher the value, the lower the trust rating.
 A number from 0 - 255
 Can be manually manipulated.
Administrative distance is a rating of the trustworthiness of a routing information source,
such as an individual router or a group of routers. Distance is an integer from 0 to 255.
In general: the higher the value, the lower the trust rating. A distance of 255 means the
routing information source cannot be trusted at all and should be ignored. Specifying
distance values enables the router to discriminate between sources of routing
information. The router always picks the route whose routing protocol has the lowest
distance.
19) Administrative Distance Defaults (slide 21):
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Read Slide
Administrative distance can be manually configured on the router to give certain routing
protocols preference over others. Under the desired routing protocol configuration, use
the “distance” command.
Metrics (slide 22):
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Called cost in OSPF
Used to determine best path to a destination when multiple paths exist.
Can be used to load share if routing protocol supports it.
The cost (also called metric) of an interface in OSPF is an indication of the overhead
required to send packets across a certain interface. The cost of an interface is inversely
proportional to the bandwidth of that interface. A higher bandwidth indicates a lower
cost. The default formula used to calculate the cost is {cost=108 / bandwidth in bps}. If
no bandwidth statement is used, serial interfaces default to 1.544 mbs (T1) and Ethernet
defaults to 10 mbs. The bandwidth statement has no actual affect on data transfer rate.
It is simply used to calculate the cost of the link. The cost of an interface can be set
manually which will override the bandwidth statement. Under the interface use the
command “ip ospf cost”. Manipulating the cost of links can make them more or less
preferential for use by the router. It is recommended cost be manipulated using the
bandwidth statement.
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20) OSPF Basic Configuration Commands (slide 23):
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Enable an OSPF routing process (turn on OSPF):
(1) At the router (config)# prompt, type router ospf 1.
(2) Router prompt should read router(config-router)#.
(3) The number 1 is the indicates the OSPF process ID. It is arbitrary.
Select interfaces which will run OSPF:
(1) At the router(config-router)# prompt, type the network address,
wildcard-mask, and area ID. Example - network 148.43.200.1 0.0.0.0
area 0.
(2) This will start an OSPF routing process on the interface which is part of
the network selected.
Use the router OSPF command to define an OSPF routing process. The process-id is
an internally used identification number. A unique value is assigned for each OSPF
routing process within a single router. The OSPF process-id does not have to match
process-ids on other routers. It is possible to run multiple OSPF processes on the same
router, but it is not recommended because it creates multiple databases, which add extra
overhead to the router.
The network command defines which interfaces will run OSPF. The command also
assigns an interface to a certain area. The network command uses a “wildcard” mask,
which is essentially the inverse of a traditional mask. The mask in the network command
can be used as a shortcut for assigning a list of interfaces to the same area with one
configuration line.
21) Passive Interface (slide 24):
As stated above, the network command is used to define which interfaces will run OSPF.
In addition to this, these will be the network addresses advertised to other routers.
There may be cases where we want to advertise a network to other routers but do not
necessarily want routing updates being sent from an interface. One case is an Ethernet
interface with only hosts connected to it. The passive-interface command will keep
updates from being sent from the interface even though there is a network statement
relating to the address of the interface. Another instance where this command may be
used is when interfacing to the Tactical Packet Network (TPN).
OSPF Network Diagram (slide 25):
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At this time a network will be established running the OSPF routing protocol.
Reference network diagram #1.
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Have students perform the following
(1) configure loopback interface (s)
- config t, int loopback X, ip address xxx.xxx.xxx.xxx (a loopback address
must be established for each area)
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(2) configure interfaces (ethernet and serial)
- config t, int sX, ip unnumbered loopback X 255.255.255.255, encapsulation
ppp, clock rate 250000 (if needed), bw 256, no shut. (serial interfaces in
different areas must reference different loopback addresses.
- config t, int eX, ip address xxx.xxx.xxx.xxx 255.255.255.240, no shut.
(3) configure OSPF
- config t, router OSPF 1
(4) Put in network statements under OSPF
- serial: network xxx.xxx.xxx.xxx 0.0.0.0 area X (statements must be made
for each area/loopback address)
- ether: network xxx.xxx.xxx.xxx 255.255.255.240 area X
Have students perform the following:
(1) sho ip route: verify that each router sees all the networks being advertised.
(2) Ping various address
(3) Traceroute to various addresses
22) Passive Interface (slide 26):
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Reference network diagram #2.
Have students remove all entries from the router using the "no".
Establish a physical ethernet connection between routers 2 & 4.
Network #2 is basically the same as 1 but with all routers being in area 1 except
the ethernet link between routers 2 & 4 being in area 2.
Point of this configuration is to show that traffic will not leave area 1 and travel
down area 2 even if it is the shortest path.
(1) configure loopback interface (s)
- config t, int loopback X, ip address xxx.xxx.xxx.xxx (a loopback address
must be established for each area)
(2) configure interfaces (ethernet and serial)
- config t, int sX, ip unnumbered loopback X 255.255.255.255, encapsulation
ppp, clock rate 250000 (if needed), bw 256, no shut. (serial interfaces in
different areas must reference different loopback addresses.
- config t, int eX, ip address xxx.xxx.xxx.xxx 255.255.255.240, no shut.
(3) configure OSPF
- config t, router OSPF 1
(4)
(5) Put in network statements under OSPF
- serial: network xxx.xxx.xxx.xxx 0.0.0.0 area X (statements must be
made for each area/loopback address)
23) Passive Interface (slide 27):
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Cover each of the show commands.
Students have examples and explanations of each.
Allow time for them to use the commands on the network.
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The show IP protocol command provides information about all IP routing protocols
configured. The routing protocol and process are identified along with information
concerning routing filters, redistribution, and summarization. Routing network
statements can be verified along with routing information sources. This is displayed
using the sources router ID’s, the distance of the protocol, and when the last update was
received.
The show IP OSPF command can be used to verify your OSPF configuration and the
overall configuration of the areas within the router. The router ID and process ID can be
verified here. Information concerning frequency of updates and other timers are
provided. Information is provided for each individual area the router is connected.
The show IP OSPF neighbor command contains the following information:
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Neighbor ID: router ID
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Priority: used in the election of a DR (1 is default), normally manipulated
on NBMA networks
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State: Init – first hello sent
2wy – neighbor discovered but adjacency not built
Full – adjacency built, databases exchanged
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Drother – not a DR or BDR, unique to broadcast multi-access
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DR – designated router
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BDR – backup designated router
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Dead Time – dead-interval timer (defaults to 40 sec), amount of time left
before neighbor is
declared dead
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Address – lists the link IP identifier or neighbors interface IP
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Interface – the router interface connected to the neighbor
The show IP OSPF interface command provides an inventory of all the interfaces in your
router and their status with respect to OSPF. The cost assigned to each interface
along with the type of OSPF network it belongs to can be verified here.
The show IP OSPF database command is used to view the OSPF link-state (topology)
database. Each LSA gets an entry into this database and is organized by area and the
type of LSA. The database contains six columns:
1) Link ID – will either be the router ID (LSA type 1 &4), the destination network
number (LSA type
3 & 5), or IP of the interface of the DR (LSA type 2).
2) ADV Router – router ID of advertising router.
3) Age – age of LSA in seconds.
4) Seq# - sequence number of LSA, used to determine if LSA updates are newer,
older, or duplicates.
5) Checksum – used for error detection.
6) Link count – the number of interfaces or links in an area, only available on Router
Link States;
OSPF adds a “stub link” for each point-to-point interface.
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24) Route Summarization (slide 28):
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Consolidation of multiple routes into one single advertisement.
Directly affects the amount of bandwidth, CPU, and memory resources consumed
by the OSPF process.
Two types:
(1) Interarea - Done on an ABR and applies to routes within an autonomous
system.
(2) External - Specific to external routes that are injected into the OSPF
redistribution (another AS).
Summarizing is the consolidation of multiple routes into one single advertisement.
Proper summarization requires contiguous addressing.
Route summarization directly affects the amount of bandwidth, CPU, and memory
resources consumed by the OSPF process. With summarization, if a network link fails,
the topology change will not be propagated into the backbone (and other areas by way
of the backbone). As such, flooding outside the area will not occur, so routers outside of
the area with the topology change will not have to run the SPF algorithm (also called the
Dijkstra algorithm after the computer scientist who invented it). Running the SPF
algorithm is a CPU-intensive activity.
There are two types of summarization:

Inter-area route summarization—Inter-area route summarization is done on
ABRs and applies to routes from within the autonomous system. It does not
apply to external routes injected into OSPF via redistribution. In order to take
advantage of summarization, network numbers in areas should be assigned in
a contiguous way so as to be able to consolidate these addresses into one
range. This graphic illustrates inter-area summarization.

External route summarization—External route summarization is specific to
external routes that are injected into OSPF via redistribution. Here again, it is
important to ensure that external address ranges that are being summarized
are contiguous. Summarization overlapping ranges from two different routers
could cause packets to be sent to the wrong destination.
25) Route Summarization (slide 29):
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Slide shows how router B (ABR) consolidates the routes being advertised to
router C.
Route summarization minimizes the number of entries in the routing table and database
in the receiving routers. Summarization is done on ABRs and applies to routes within
the autonomous system. Although summarization could be configured between any two
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areas, it is better to summarize in the direction of the backbone. This way, the backbone
receives all the aggregate addresses and in turn injects them, already summarized, into
other areas. In order to take advantage of summarization, network numbers in areas
should be assigned in a contiguous way to be able to group these addresses into one
range. Summary routes are advertised with a mask. The mask specifies the range of
addresses to be summarized into one route. Because the mask 255.255.240.0 does not
use the low-order four bits of the third octet, subnets 131.108.4.0 and 131.108.8.0
cannot be summarized using this mask. Neither can subnet 131.108.12.0 because it
creates an invalid zero subnet (discussed on next slide). Even so, route summarization
can represent the remaining four subnets with one advertisement.
26) Configuring Route Summarization (slide 30):
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Cover commands for route summarization.
Types of Areas (slide 31):
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Backbone – interconnects all areas, accepts all LSA’s
Stub – does not accept external (E1/E2) LSA’s
Totally Stub – does not accept external (E1/E2) or summary (IA) LSA’s
NSSA (Not so Stubby Area) – allows external routes to go through the area but does
not accept or process them.
27) StubAreas (slide 32):
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Hide external routes, reduce database
Consolidate external links---0.0.0.0
OSPF allows certain areas to be configured as stub areas. Configuring a stub area
reduces the size of the topological database inside an area and as a result reduces the
memory requirements of routers inside that area. External networks, such as those
redistributed from other protocols into OSPF, are not allowed to be flooded into a stub
area. Routing from these areas to the outside world is based on a a default route
(0.0.0.0). This allows routers within the stub to reduce the size of their routing tables
because of single default route replaces the many external routes. If your network has
no external routes, there is no need to configure a stub area.
28) Stub Area Restrictions (slide 33):
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
Single exit point
ASBR cannot be internal to stub
All OSPF routers within the area must be configured as stub routers. This is so
they will become neighbors and exchange info.
An area could be qualified as a stub when there is a single exit point from that area or if
routing to outside of the area does not have to take an optimal path. The latter
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description is just an indication that a stub area with multiple exit points will have one or
more ABRs injecting a default into that area. Routing to the outside world could take a
sub-optimal path in reaching the destination by going out of the area via an exit point
that is farther to the destination than other exit points.
Other stub area restrictions are that a stub area cannot be used as a transit area for
virtual links. Also, an ASBR cannot be internal to a stub area. These restrictions are
made because a stub area is mainly configured not to carry external routes, and any of
the situations described cause external links to be injected in that area. The backbone,
of course, cannot be configured as a stub.
29) Totally Stub Area (slide 34):


Block external and summary routes
Know only intra-area and default routes
A totally stubby area is a stub area that blocks external routes and summary routes
(interarea routes) from going into the area. This way, intra-area routes and the default of
0.0.0.0 are the only routes known to the stub area. ABRs inject the default summary link
of 0.0.0.0 into the totally stubby area. Each router picks the closest ABR as a gateway
to everything outside the area. The totally stubby area is a Cisco-specific feature.
30) Configuring Stub & Totally Stub Areas (slide 35):

Cover commands.
31) Stub Area Configuration Example (slide 36):
In this example, area 2 is defined as the stub area. No external routes from the
external autonomous system will be forwarded into the stub.
The last line in each configuration, area 2 stub, defines the stub area. The area stub
default-cost
has not been configured on R3, so this router will advertise 0.0.0.0 (the default route)
with a default cost metric of 1 plus any internal costs.
Each router in the stub must be configured with the area stub command.
The only routes that will appear in R4’s routing table are intra-area routes
(designated with an O in
the routing table), the default route, and interarea rotes (both designated with an IA in
the routing
table; the default route will also be denoted with an asterisk).
Notice that both R3 and R4 are configured with the area stub command. The area
stub command determines whether the routers in the stub exchange hello messages
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and become neighbors. This command must be included in all routers in the stub if
they are to exchange routing information.
32) Totally Stub Area Configuration Example (slide 37):
In this example, the keyword no-summary has been added to the area stub
command on R3. This keyword causes summary routes (interarea) to also be
blocked from the stub. Each router in the stub picks the closest ABR as a gateway to
everything outside the area. The only routes that will appear in R4’s routing table are
intra-area routes (designated with an O in the routing table) and the default route. No
interarea routes (designated with an IA in the routing table) will be included. With the
area stub default-cost command, R3 adds 20 to the internal cost when it injects the
default route into the stub area. It is only necessary to configure the no-summary
keyword on the stub border routers. This is because the area is already configured as
a stub.
33) Virtual Links (slide 38):



Backbone center of communication
Virtual links provide path to backbone
Avoid configuring virtual links if possible
OSPF has certain restrictions when multiple areas are configured. One area must be
defined as area 0. Area 0 is also called the backbone because all communication must
go through it. In addition, all areas should be physically connected to area 0. All other
areas must be logically connected to area 0. This is because all other areas inject
routing information into area 0, which in turn disseminates that information to other
areas. In special cases where a new area is added after the OSPF network has been
designed and Configured, it is not always possible to provide that new area with direct
access to the backbone. In These cases, a virtual link will have to be defined to provide
the needed connectivity to the backbone. The virtual link provides the disconnected
area a logical path to the backbone. The virtual link must be established between two
routers that share a common area, and one of these routers must be connected to the
backbone.
Virtual Links continued (slide 39):

Link discontiguous backbone
–Merged networks
–Redundancy
Virtual links serve two purposes:

Linking an area that does not have a physical connection to the backbone.
15

Patching the backbone in case discontinuity of area 0 occurs.
This slide illustrates the second purpose. Discontinuity of the backbone might occur if,
for example, two companies, each running OSPF, are trying to merge the two separate
networks into one with a common area 0. The alternative would be to redesign the entire
OSPF network and create a unified backbone. Another reason for creating a virtual link
is to add redundancy in cases where a router failure causes the backbone to be split
into two. In the graphic, the disconnected area 0s are linked via a virtual link through the
common area 3. If a common area does not already exist, one can be created to
become the transit area.
34) Configuring Virtual Links (slide 40):

Cover commands.
- The router ID’s must be used when configuring virtual links. Telnet to the
router to verify ID.
35) Configuring Virtual Links Example (slide 41):
In this example, area 3 does not have a direct physical connection to the backbone (area
0). This is
an OSPF requirement because the backbone is a collection point for LSAs.s forward
summary
LSAs to the backbone, which in turn forwards the traffic to all areas. All interarea traffic
transits the
backbone.
To provide connectivity to the backbone, a virtual link must be configured between R2
and R1. Area
1 will be the transit area and R1 will be the entry point into area 0. R2 will have a logical
connection
to the backbone through the transit area.
Both sides of the virtual link must be configured.

R2: area 1 virtual-link 192.168.10.5—With this command, area 1 is defined to be the
transit
area and the router ID of the other side of the virtual link is configured.

R1: area 1 virtual-link 192.168.20.123—With this command, area 1 is defined to be
the transit
area and the router ID of the other side of the virtual link is configured.
Stub Area Restrictions Chart (slide42):

Read chart
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36) Using NSSA (slide43):
Not-so-stubby areas (NSSAs) are an extension of OSPF stub areas. Like stub areas,
they prevent the flooding of AS-external link-state advertisements (LSAs) into NSSAs,
relying instead on default routing to external destinations. As a result, NSSAs (like stub
areas) must be placed at the edge of an OSPF routing domain. NSSAs are more flexible
than stub areas in that an NSSA can import external routes into the OSPF routing
domain, thereby providing transit service to small routing domains that are not part of the
OSPF routing domain.
37) Configuring NSSA (slide44):
To define an NSSA stub area, use the OSPF router configuration command area X
nssa. To define an NSSA totally stub area, use the OSPF router configuration
command area X nssa no-summary.
38) Route Summarization PE (slide45):
1) Configure routers for IP unnumbered network or TFTP config files if available.
2) Router 7 add six sequential loopback interfaces with IP addresses for each.
- Loopback 21 – 26
- IP 150.150.150.1 – 6
- add network statements under OSPF
3) All routers do sho ip route; loopback addresses from router 7 should be in
routing table
4) Routers 1,3,4, & 6 are ABR’s. Summarize loopback addresses there.
5) Routers 2 & 5 do sho ip route; shows two summarized routes, one from each
ABR.
6) One of the ABR’s in each area change the bandwidth on a serial interface.
7) Routers 2 & 5 should only show one summarized route now.
39) Total Stub Area PE (slide46):
1) Configure routers for IP unnumbered network or TFTP config files if available.
2) Everyone telnet to router 2 or 5 and examine routing table; note “IA” routes.
3) Routers 2 & 5 configure for stub area.
4) Routers 1,3,4,& 6 configure for totally stub area.
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5) Router 2 & 5 should have two default routes, one from each ABR.
6) On one of the serial interfaces on routers 2 & 5, change the bandwidth.
7) Routers 2 & 5 should only show one default route now.
40) Virtual Link PE (slide47 & 48):
Reference slide 47 network diagram. Do not disconnect cables; just turn off
interfaces not used in diagram.
1) Configure routers as per diagram or TFTP config files if available.
2) Install ethernet link between routers 2 & 5.
3) Review all router routing tables; area 1 & 0 should not see routes in area 2 and
vise versa.
4) Configure a virtual link between routers 1 & 5.
5) All routes should have connectivity to the entire network.
Grading:
40 points: Print out final router configurations and routing tables for one each
routers 1, 3, 4, or 6; router 2 or 5; and router 7.
60 points: Completed questions (typed)
Total: 100 points
OSPF Questions
1. Which of the following are the benefits of OSPF routing over RIP v1? [Choose
all that apply and explain your answer].
A.
B.
C.
D.
No hop count limitation
Faster convergence
Best path selection
Support VLSM
15 pts
2. The OSPF path cost in Cisco routers is calculated using which parameters?
Explain
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A. Bandwidth, Number of Hops
B. Bandwidth only
C. Ticks
D. Bandwidth, MTU, Reliability, Delay, and Load.
5 pts
3. Match the following in the context of an OSPF area. Explain your choice:
1. LSU A. This packet is sent by slave router if the DDP has more up-to-date
link-state entry.
2. LSA B. This packet is sent when a router notices a change in a link-state
3. LSR C. These are contained in LSPs and have the information about
neighbors and path costs.
A. 1->A; 2->B; 3->C
B. 1->B; 2->A; 3->C
C. 1->B; 2->C; 3->A
D. 1->C; 2->A; 3->B
15 pts
4. Up to how many equal-cost route entries are maintained in the OSPF routing
table? Explain
A.
B.
C.
D.
2
4
6
16
10 pts
5. Match the following with regard to OSPF operation: Explain your choice
A. DDP
1. A router that resides within an area.
B. Hello Packet
2. This includes summary information about link-state entries
C. Internal router 3. This packet includes information that enables routers to
establish themselves as neighbors.
A.
B.
C.
D.
A->3; B->2; C->1
A->2; B->3; C->1
A->1; B->2; C->3
A->3; B->1; C->2
5 pts
6. Which of the following statements is true in multiple OSPF areas environment?
A. ASBR is responsible for redistribution (import) of routing information in
an OSPF network.
B. ABRs have all interfaces in the same area.
C. Internal routers are responsible for routing information redistribution
(import).
D. In a multi-area OSPF network, Area 0 may or may not be present.
10 pts
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