OSPF.pptx

Open Shortest Path First
(OSPF)
Atakan ATAK
Network Security Specialist
atakannatak16@gmail.com

Course Topics
1. Introduction
1.1. Features
1.2. Components
1.3. Header Format
2. Neighbor Adjacency
2.1. Packet Types
2.2. LSA Types
2.3. LSDB Flooding
2.4. Path Selection
2.5. Router ID
2.6. Discovery Process
2.7. DR/BDR Election
3. Network Types
4. Area Types
5.1. Reference Bandwidth
5.2. Default Route
5.3. Passive Interface
5.4. Authentication
5.5. Summarization
6. Multi Area Design
6.1. Virtual Link
6.2. Transit Capability
7. Troubleshooting Path
8. LAB
5. Configurable Features

1. Introduction
1.1. Features
INTRODUCTION / FEATURES
It is an IGP standardized by the IETF and commonly used in large Enterprise networks. OSPF is a link-state
routing protocol providing fast convergence and excellent scalability. Like all link-state protocols, OSPF is
very efficient in its use of network bandwidth. OSPF also has the following operational characteristics;
Dynamically adjust to changes in network topology
Support VLSM, CIDR and route summarization
Provides for the authentication of routing updates
Uses the cost as the route metric
Determines routing by computing a graph, abstracting the topology of the network by
using the SPF (Dijkstra) algorithm so this way a loop-free topology construction
The protocol itself minimizes the operation overhead such as the hello messages that
not contain routing information
Trigger updates for fast convergence
Load balancing with equal-cost routes for the same destination

1.2. Components
INTRODUCTION / COMPONENTS
All routing protocols share
similar components. They
all use routing protocol
messages to exchange
route information. The
messages help build data
structures, which are then
processed using a routing
algorithm.
These tables contain a list of
neighboring routers to
exchange routing
information with and are
kept and maintained in
RAM.
DATABASE TABLE INFORMATION
Neighbor Neighbor
 List of all routers that a
router communicates
bidirectional
 Vary according to each
routers
 show ip ospf neighbors
Link State Database (LSDB) Topology
 Keeps information about all
other routers in the
 Represents network
topology
 Same LSBD for all routers in
the same area
 show ip ospf database
Forwarding Routing
 An algorithm, a route list
created when the link state
is run in the database
 The routing table for each
router is unique
 show ip route

1.3. Header Format
INTRODUCTION / HEADER FORMAT
Link
Header
IP Header
OSPF Packet
Types
Link Trailer
Version Type
Packet
Length
Router
ID Area ID
Check
sum
Auth.
Type
Auth. Data
Hello
Value=1
DBD
Value=2
LSR
Value=3
LSU
Value=4
LSACK
Value=5
Unlike other routing protocols,
the OSPF will not carry the
data through the transport
protocol such as TCP and UDP
.
Rather than, the OSPF forms
the IP datagram directly,
packaging it using the
protocol number of 89 for an
IP protocol field. In the OSPF,
there are 5 different types of
packets. The format of the
common OSPF header is
shown right side.

FIELD LENGTH (Bits) DESCRIPTION
Version 8 OSPF version number. For OSPFv2, the value is 2.
Type 8 OSPF packet type.
Packet Length 16 Length of the OSPF packet with the packet header, in bytes.
Router ID 32 ID of the Router that sends the OSPF packet.
Area ID 32 ID of the area to which the Router that sends the OSPF packet belongs.
Checksum 16 Checksum of the OSPF packet that does not carry the Authentication field.
AuType 16
The values are as follows:
 0: none
 1: simple
 2: MD5
Authentication 64
This field has different meanings for different AuType values:
 0: This field is not defined.
 1: This field defines password information.
 2: This field contains the key ID, MD5 authentication data length, and
sequence number.
INTRODUCTION / HEADER FORMAT

2. Neighbor Adjacency
2.1. Packet Types
NEIGHBOR ADJACENCY / PACKET TYPES
Hello Packet
The hello message will perform 4 major functions such as;
1. It discovers the other OSPF speaking routers on the common subnets
2. Verify the bidirectional visibility between the routers
3. Check for the agreement on a particular configuration parameter
4. Monitor the neighbor's health to react suppose the neighbor fails
If any below items do not match, then the 2 routers do not form the
neighbor relationship.
1. The dead timers and OSPF hello must be equal
2. No duplicate for the RID
3. Important to pass an authentication process
4. Important to be in a same OSPF area
5. Important to be in a same primary subnet as well as same subnet mask
6. Important to be a same area types such as stub, regular, not-so-stubby area

Database Descriptor Packet (DBD)
For the link state routing protocols, it is essential that the link state database
for all the routers remain synchronized. This synchronization will begin as soon
as an adjacency formed between the neighbors. The OSPF uses the DBD -
database descriptor packets for that purpose.
The OSPF router summarizes the DBD packets and local database carry the
set of LSA belongs to the database. When the neighbor sees the LSA, which is
more recent than the own database copy, then it request the newer LSA from
a neighbor.
When 2 routers hear Hello and parameter check passes, it will not send
packets immediately which holds the LSA. Rather, each router will create and
send the database description packets, that contain the header of the each
LSA. This header includes the needed information to uniquely identify the LSA
and the revision without transmitting the body.

Link State Request Packet (LSR)
After all the LSA header exchanged using the DD packets, each
neighboring router has the list of the LSA known by a neighbor.
Using this knowledge, the router need to request the full copy of
the each LSA, which is missing from the own LSDB. To determine
whether the neighbor has the most recent copy of the certain
LSA, the router looks at a sequence number of an LSA in the
LSDB and also compares with the sequence number of that of the
same LSA learned from a DD packet. Each LSA sequence number
is increased each time the LSA re-originated or changes. If the
router gets the LSA header with the later sequence number for
the specific LSA, that router knows that a neighbor has the most
recent LSA. The Routers will use the link state packets to request 1
or more LSA from the neighbor.
The LSR packet is one of the OSPF packets. After the DBD packet
exchange process, a router can find that it has not any up-to-date
database. An LSR packet is mainly used to request the pieces of a
neighbors database which is more up-to-date.

Link State Update Packet (LSU)
This packet implements the LSA flooding. Every LSA comprises
the metric, topology information and routing to describe the
OSPF network portion. A local router will advertise the LSA within
the LSU packet to the neighboring router. Additionally, a local
router advertises an LSU packet with the information in response
to the LSR packet.
The primary router sends the database description, one at a time
and a secondary router acknowledges the everyone and includes
in the own database descriptions. The record is compared with
the type, link state ID and advertising router. The sequence
number in a record will determines whether the record is older or
newer. The flooding of the link state advertisement is mainly
implemented by these packets. The collection of the lick state
advertisements is mostly carried by the each link state update
packet, one hop further from the origin. The packet can be
included by the several link state advertisements. Both the routers
will sit in the loading state when the LSR and LSA process
continues. After the process gets over, it will settle into the full
state that means the 2 routers must have fully exchanged their
database, results in the identical copies of a LSDB entry.

Link State Acknowledgment Packet (LSack)
The OSPF needs acknowledgement for a receipt of the each LSA.
The multiple LSA may be acknowledged in the single LSAck
packet. The LSR/LSA process will use the reliable protocol, which
has 2 options, in that link state acknowledgement is among them.
The LSU can be acknowledged by a receiver of an LSU simply
repeating a same LSU back to the sender. Alternatively, the router
will send back the LSAck packet to acknowledge the packets that
contain the acknowledged LSA header list. As s result, the LSDB
must be identical. At this point, it can independently run a Dijkstra
shortest path first algorithm to calculate the best path from own
perspective. The reliability of the flooding link state
advertisements is made by explicitly acknowledging the flooded
advertisements.

2.2. LSA Types
NEIGHBOR ADJACENCY / LSA TYPES
Type 1 – Router LSA
OSPF uses a LSDB (link state database) and fills this with LSAs (link state advertisement). Each router within the area will flood a type
1 router LSA within the area. In this LSA you will find a list with all the directly connected links of this router. Keep in mind that the
router LSA always stays within the area.
Link
Type
Description Link ID
1 P2P connection to
another router
Neighbor router ID
2 Connection to transit
network
IP address of DR
3 Connection to stub
network
IP network
4 Virtual Link Neighbor router ID
 Routers advertise their directly connected OSPF-enabled links in a type 1
 They are flooded only within the area in which they originated
 ALL OSPF speaking routers generate type 1
BUT, how do we identify a link with perspective of entire OSPF
process? So, there are two parameters which are looking;
 The IP prefix on an interface
 The link type

Type 2 – Network LSA
It is created for each multi-access network. The broadcast and non-broadcast network types require a DR/BDR. If this is the case
you will see these network LSAs being generated by the DR. In this LSA we will find all the routers that are connected to the multi-
access network, the DR and of course the prefix and subnet mask.
 Only found on NBMA networks
 Contain the router ID and IP address of the DR
 Give other routers information about multi-access networks within the same area
 They are flooded only within the area in which they originated
 This is a form of internal OSPF summarization
 Link State ID = IP Address of DR

Type 3 – Summary LSA
The name “summary” LSA is very misleading. By default OSPF is
not going to summarize anything for you.
 Describes networks that are within the OSPF AS but outside of the particular OSPF area that is receiving
the LSDB
 Summary LSA has a flooding scope of being transmitted only into the area where the network or subnet is
not found
 Generated only by ABR and describe inter area routes to various networks
 Link State ID = Destination Network
Type 1 router LSAs always stay within the area. OSPF however works with multiple areas and you probably want full connectivity
within all of the areas. Routers which are connected different areas are going to create a Type 3 summary LSA and flood it into area
0. This LSA will flood into all the other areas of our OSPF network. This way all the routers in other areas will know about the
prefixes from other areas.

Type 4 – Summary ASBR LSA
 They identify an ASBR and provide a route to it
 They are generated by an ABR only when an ASBR exists within an area
 They are flooded to other areas by ABRs
 It enables other routers to find and reach the ASBR
 Link State ID = Router ID of the ASBR
This is needed because Type 5 External LSAs are flooded to all areas and the detailed next-hop information may not be available in
those other areas. This is solved by an Area Border Router (ABR) flooding the information for the router where the type 5
originated.

Type 5 – External LSA
 Generated by the ASBR
 These LSA describe routes to destinations that are external to the AS
 Flooded everywhere, with the exception of Stub Area
 Link State ID = External Network
These routes are redistributed into OSPF from other routing protocols or by the redistribution of connected or static routes.
Type 7 – Not-so-stubby area LSA
 Generated by the ASBR
 Can be summarized and converted into Type-5 LSA by the ABR for the transmission into other OSPF area
 These LSA describe routes within a NSSA
These packets are used for some special area types that do not allow external distributed routes to go through and thus block LSA
Type 5 packets from flooding through them, LSA Type 7 packets act as a mask for LSA Type 5 packets to allow them to move
through these special areas and reach the ABR that is able to translate LSA Type 7 packets back to LSA Type 5 packets.

2.3. LSDB Flooding
NEIGHBOR ADJACENCY / LSDB FLOODING
When a router receives an LSA, it checks its LSDB. If the LSA is new, the router floods the LSA out the its neighbors. After the new
LSA is added to the LSDB, the router reruns SPF algorithm. This recalculation by the SPF is essential to prevising accurate routing
tables. The SPF is responsible for calculating the routing table, and any LSA change might also cause a change in the routing table.
OSPF routers in the same area all have the same LSDB and run same SPF algorithm
with themselves as the root. The characteristics of the LSDB are as follows;
 All router belonging to the same area have the identical LSDB
 Calculating routes by using the SPF is performed separately by each router in
area
 LSA flooding is contained within the area that experienced the change
 The LSDB is comprised of LSA entries
 A router has a separate LSDB for each area to which it belongs

NEIGHBOR ADJACENCY / LSDB FLOODING
Flooding in OSPF is responsible for validating and distributing LSU to the LSDB, whenever a change or update occurs to a link.
Flooding is part of the LSDB synchronization mechanism. The goal of this mechanism is to keep the LSDBs of the routers in an OSPF
domain synchronized within time in the presence of topological changes. Also, the primary goal of flooding is to ensure that every
router receives the changed or updated LSA within the flooding scope. Flooding occurs differently between neighbors in OSPF
depending on a the following factor;
 LSA Type 1-2-3-4-7 are flooded within an area
 LSA Type 5 is flooded throughout the OSPF
domain, with exception of Stub area
 When a DR is present, only non-DRs flood to the
DR. The DR then floods to everyone as required
 When two OSPF routers have not established an
adjacency, they do not flood each other

2.4. Path Selection
NEIGHBOR ADJACENCY / PATH SELECTION
OSPF will use cost as the metric to choose the shortest path for each destination, this is true but it’s not entirely correct. OSPF will
first look at the “type of path” to make a decision and secondly look at the metric. This is the preferred path list that OSPF uses;
 Intra-Area [O]: Routes originated within an area, are known by the routers in the same area as Intra-Area routes. These
routes are flagged as O. Also called OSPF Internal routes, as they are generated by OSPF itself, when an interface is
covered with the OSPF network command.
 Inter-Area [O IA]: When a route crosses an ABR, the route is known as an OSPF Inter-Area route. These routes are flagged
as O IA. Also called OSPF Internal routes, as they are generated by OSPF itself, when an interface is covered with the
OSPF network command.
 NSSA Type 1 [N1]: When an area is configured as a NSSA, and routes
are redistributed into OSPF, the routes are known as NSSA External
Type-1. These routes are flagged as O N1.
 External Type 1 [E1]: Routes which were redistributed into OSPF, such as
connected, static, or other routing protocol, are known External Type-1.
These routes are flagged as O E1. A cost is the addition of the external
cost and the internal cost used to reach that route.
 NSSA Type 2 [N2]: The definition for type O N1 is valid for this route
type. Also, these routes are flagged as O N2.
 External Type 2 [E2]: The definition for type E1 is valid for this route type,
with one difference, which is the cost is always the external cost,
irrespective of the interior cost to reach that route. Also, these routes
are flagged as O E2.

2.5. Router ID
NEIGHBOR ADJACENCY / ROUTER ID
Each OSPF router selects a router ID (RID) that has to be
unique on your network. OSPF stores the topology of the
network in its LSDB and each router is identified with its
unique router ID , if you have duplicate router IDs then you
will run into reachability issues. Because of this, two OSPF
routers with the same router ID will not become neighbors
but you could still have duplicated router IDs in the network
with routers that are not directly connected to each other.
OSPF uses the following criteria to select the router ID;
 Manual configuration of the router ID
 Highest IP address on a loopback interface
 Highest IP address on a non-loopback interface

2.6. Discovery Process
NEIGHBOR ADJACENCY / DISCOVERY PROCESS
OSPF considers two routers that have an interface located an a
common network as “Neighbor”. When OSPF discovers its
neighbors, this is the first step of discovering the network and
building a routing table. This process begins with the router
learning the Router-ID of its neighbors via multicast “Hello
Message”.
A neighbor relationship begins when the routers exchanging
Hello packets see their own Router-ID in the other router’s
Hello packet and they agree upon the follow;
 Different Router-ID value
 Same Hello and Dead transmission intervals
 Same Area ID
 Same subnet mask – Just for multiaccess network
 Stub Area Flag
 Authentication type and password
For adjacencies to form, OSPF must first have discovered its
neighbors. Adjacencies are formed for the purpose of
exchanging routing information. NOT every neighboring
router forms an adjacency. A router’s neighbors or peers, are
those routers, which is describe below, will directly exchange
routing information.

2.7. DR/BDR Election
NEIGHBOR ADJACENCY / DR/BDR ELECTION
The solution of managing the number of adjacencies in the multi-access
network and transferring LSAs is DR. OSPF selects a DR as aggregation and
distribution point for sent and received LSAs. In case of DR failure, a BDR is
also selected. The BDR listens passively on this exchange and maintains links
with all directors. If DR stops generating hello packets, BDR identifies itself and
assumes the DR role. Other routers without DR or BDR become DROTHER DR
is notified when a new device is added and DR forwards it to all routers. This
prevents LSA packets from consuming bandwidth.
There are two options to checking when selecting DR/BDR;
 Highest priority (0-255)
 Highest router ID
 The default priority is 1.
 A priority of 0 means you will never be elected as DR or BDR.
Routers Neighbors
4 6
10 45
20 190

3. Network Types
NETWORK TYPES
Two OSPF routers will never form a neighbor relationship and hence will never forward packets directly between each other
unless they share a common prefix.
1. Neighbor Discovery and Maintenance: Hello protocols run differently an different subnet types.
2. Database Synchronization: How does one synchronize the LSDB over the subnet? Which routers become adjacent,
and how does reliable flooding take advantage of any special properties that the subnet might provide?
3. Abstraction: In the OSPF LSDB, how does one represent the subnet and router connectivity over the subnet?
In the OSI reference model, differences subnet technologies would be called “Subnet work-Dependent convergence”
functions. The differences in the way that OSPF runs over the various subnet technologies can be grouped as follows;

4. Area Types
AREA TYPES
OSPF supports a two level hierarchical routing scheme through the use of areas. Each OSPF area is identified by a 32-bit Area ID and
consist of a collection of network segments interconnected by routers. Areas are contiguous logical segments of the network that have
been grouped together. Each area has its own LSDB, consisting of LSA Type 1-2 describing how the area’s routers and network
segments are interconnected. Routing within the area is flat, which each router knowing exactly which network segments are contained
within the area. In addition to allowing one to build much larger OSPF networks, OSPF areas provide the following functionally;
 Increased Robustness: The effects of router and/or link failures within a single area dampened external to the area.
 Routing Protection: OSPF always prefers path within an area over paths that cross area boundaries. This means that routing within an area is
protected from routing instabilities or misconfiguration in other area.
 Hidden Prefixes: It can configure prefixes so that they will not be advertised to other area
The following list provides general characteristics of an OPSF area;
 Area contain a group of contiguous hosts and networks
 Routers have a per area topological database and run the same SPF
 Each are must be connected to the backbone area known as Area 0
 Virtual links can be used to connect to Area 0 in emergencies
 Intra area routes are used for routes within to destination within the area

AREA TYPES
It is a carry a default, static, intra area, and external routes. The use of standard area is more resource intensive within an OSPF network.
The following list provides general characteristics;
 It contains a router that uses both OSPF and any other routing protocols
 A virtual link is configured across the area
 It has an ABR
 Summarize whenever and as often as possible
Standard/Normal Area
If more than one area is configured in an OSPF network, one of these areas
must be Area 0. To summarize the OSPF backbone is the part of the OSPF
network that acts as the primary path for traffic that is destined to other areas
or networks. Use the following guidelines when designing an OSPF backbone:
 It is a transit area, not a destination for traffic
 Ensure that the stability of the backbone area is maintained
 Ensure that redundancy is built into the design whenever possible
 Keep this are simple, and fewer routers are better
 Keep the BW symmetrical, so that OSPF can maintain load balancing
 Ensure that all other areas connect directly to Area 0
Backbone Area
The backbone area must be at the center of all other areas, so all areas must
be connected to the backbone. This is because OSPF expects all areas inject
routing information into the backbone, and it turns, the backbone
disseminates that routing information into other areas.

AREA TYPES
This area carries a default route and inter area routes but does not carry external routes. Stub areas are essentially dead end areas. This
reduces the routes being advertising across the network. Therefore, stub areas allow for reduction in LSA traffic and can make OSPF make
stable.
Stub area summarize all external LSAs into a default route, which provides a path to external routes for all traffic inside the stub area. The
stub ABR forwards LSAs for inter area routes but not external routes and floods them to other Area 0 routers. The stub ABR keeps the
LSDB for the stub area with this additional information and the default external route. Stub areas following functional and design
characteristic:
 The stub ABR stops the LSA Type 4-5. Therefore, no router inside a stub
area has any external routes, so ASBR cannot be internal to a stub area.
 Reduces the LSDB size and memory requirements of the routers inside a
stub area
 Routing from these areas to the outside based on a default route. Stub
area only have two type of route, such as O and O IA.
 Stub areas typically have one ABR, this is the best design. If there is
more than one ABR, accept the none optimal routing paths because you
have more than one existing point.
 All OSPF routers inside a stub area must be configured as a stub routers,
because all OSPF interfaces that belong to the area start exchanging
Hello packets with a flag that indicates that the interface is part of a
stub area (E bit).
 Backbone area cannot be a stub area
 Stub area cannot be used as a transit area for virtual links
Stub Area

AREA TYPES
The purpose of Totally Stubby areas is to limit the number of LSAs flooded into the area, to conserve bandwidth and router CPUs.
The stub area ABR will instead automatically inject a default route into the Totally Stubby area, so that those routers can reach both
inter area networks and external networks. The ABR will be the next-hop for the default route. Totally Stubby areas following
functional and design characteristic:
 The ABR advertise only a default router into the rest of the stub area.
This results in an even further reduction in the size of the LSDB and
routing table.
 TSA forwards default external route and blocks the LSA Type 3-4-5-7.
 Also share the same design criteria with Stub area.
Totally Stubby Area

AREA TYPES
Although most of the stub area restrictions, such as preventing the flooding of LSA Type-5 into the area and not allowing
configuration of virtual links through the area. The ability to import a small amount of external routing information into the NSSA
for later distribution into the rest of the OSPF routing domain.
The advent of this new type of hybrid stub area also introduced a new LSA Type-7, which is responsible for carrying external route
information. NSSA does not flood LSA Type-5 external LSAs from the core into the NSSA, on NSSA has the capability to import AS
external routes in a limited fashion within the area, which is what makes it on NSAA.
With NSSA you can extend OSPF to cover the remote connection by defining the are between the corporate router and the remote
router as on NSSA. The operation of an NSSA is rather straightforward. NSSA following functional characteristic:
 NSSA area routers will share LSA Type 1-2 to build their topology tables
 NSSA areas will also accept LSA Type-3, which contain the routes to
reach networks in all other areas
 NSSA areas will not accept LSA Type 4-5, detailing routes to external
networks
 If an ASBR exists within the NSSA area, that ASBR will generate LSA
Type-7
Not-So-Stubby Area

5. Configurable Features
5.1. Reference Bandwidth
CONFIGURABLE FEATURES / REFERENCE BANDWIDTH
OSPF uses a simple formula to calculate the OSPF cost for an interface with this formula;
cost = reference bandwidth / interface bandwidth
It uses "Cost" as the value of metric and uses a Reference Bandwidth of 100 Mbps for cost calculation. For example, in
the case of 10 Mbps Ethernet , OSPF Metric Cost value is 100 Mbps / 10 Mbps = 10.
 The default Reference Bandwidth of OSPF is 100 Mbps and the default OSPF cost
formula doesn’t differentiate between interfaces with bandwidth faster than 100
Mbps.
Interface Type Reference BW(bps) % Default BW (bps) Cost
10 Gbps 1.000.000.000 % 10.000.000.000 1
1 Gbps 1.000.000.000 % 1.000.000.000 1
100 Mbps 1.000.000.000 % 100.000.000 10
10 Mbps 1.000.000.000 % 10.000.000 100

5.2. Default Route
CONFIGURABLE FEATURES / DEFAULT ROUTE
There are a number of things. We can change the metric or metric type but
the most important thing most people forget is the always keyword.
If you use the default-information originate you can advertise a default
route in OSPF. OSPF won’t advertise a default route if you don’t already
have it in your routing table. If you add the always keyword it will advertise
the default route even if you don’t have it in the routing table. Once you
have advertised the default route it will look like this on other routers:

5.3. Passive Interface
CONFIGURABLE FEATURES / PASSIVE INTERFACE
The passive interface should be configured on interfaces that do not
have an OSPF router connected to them so that they won’t receive
any OSPF information. By silencing routing announcements on
network interfaces, we tell the router to “listen but don’t talk”. A
protocol’s routing load on the CPU can be reduced by minimizing the
number of interfaces with which it must interact.
Another reason to apply passive interface is to increase security. An
attacker could start an application that replies with OSPF hello packets
then our router will try to establish neighbor relationship. The attacker
could then advertise fake routes to misdirect traffic.
 OSPF continues to announce or advertise the interface’s connected network.
 OSPF routers stop sending OSPF Hellos on the interface.
 On the interface, OSPF no longer processes any received Hellos.
OSPF is a link-state routing protocol that creates and keeps neighbor relationships by sharing routing updates with other
OSPF routers. No routing information is exchanged and the only packets they exchange are Hello packets. Hello packets
enables dynamic neighbor discovery and preserve neighbor connections. Passive interface command is used to suppress
OSPF hello packets on a specified interface. Enabling passive interfaces in our network devices mean that:

5.4. Authentication
CONFIGURABLE FEATURES / AUTHENTICATION
All routing protocols can be protected by using authentication and OSPF is
no exception. Each OSPF packet will be authenticated if you enable any
form of authentication. In this lesson we’ll take a look at how to configure
plain text authentication for OSPF. There are two options for authentication;
 Plain text
 MD5
When neighbor authentication is configured on a router, this route checks
the identity of the source of each routing update package it receives. This
mean, with the exchange of an authentication key(sometimes known as a
password) happens in routers.

5.5. Summarization
CONFIGURABLE FEATURES / SUMMARIZATION
ABRs send summary link advertisements to describe the routes to other areas. Depending on the number of destinations, an
area can get flooded with a large number of link state records, which can utilize routing device resources. To minimize the
number of advertisements that are flooded into an area, you can configure the ABR to summarize, a range of IP addresses
and send reachability information about these addresses in a single LSA. You can summarize one or more ranges of IP
addresses, where all routes that match the specified area range are filtered at the area boundary, and the summary is
advertised in their place.
Summarization of Type-3 LSAs means we are creating a summary of all the inter area routes. This is why we call it inter area
route summarization. If you don’t use summarization (which is the default) there will be a LSA for every specific prefix. If you
have a link failure in any area then related ABR will flood a new Type-3 LSA and this change has to be propagated
throughout all our OSPF areas. Since the LSDB will change our OSPF routers they will have to re-run the SPF algorithm which
takes time and CPU power. There are a couple of things to be aware of:
 A summary route will only be advertised if you have at least one subnet that falls within the summary range
 A summary route will have the cost of the subnet with the lowest cost that falls within the summary range
 ABR that creates the summary route will create a null0 interface to prevent loops
 OSPF is a classless routing protocol so you pick any subnet mask you like for prefixes
OSPF can do summarization but it’s impossible to summarize within an area. This means we have to configure summarization
on an ABR or ASBR. OSPF can only summarize our LSA type 3 and 5. Summarization of type 3 summary LSAs means we are
creating a summary of all the inter-area routes. This is why we call it inter-area route summarization.

The obtained 10.1.0.0/22 route was summarized together with 4 different networks. In the example, the summary address matches 4
networks, although there are only 2 networks.

OSPF is a classless routing protocol, which carries subnet mask information along with route information. Therefore, OSPF supports
discontinuous subnets because the subnet masks are part of the LSDB. Network numbers in areas should be assigned contiguously
to ensure that these addresses can be summarized into a minimal number of summary addresses. Let’s consider the below
topology;
This summarization was successful because you have the following distinct and contiguous ranges
of subnets;
 128.213.96.X -- 1128.218.99.X – Area 1
 128.213.100.X --128.218.103.X – Area 2
NULL 0 is a fictitious interface that causes the router to drop into the bit bucket any
information that is destined to it. These entries are placed into the routing table to
prevent routing loops. Also, if one or more of the summarized networks are
inaccessible, the other routers within the OSPF domain are not interested this issue,
because in their routing tables there is a summarized route outlined as shown in the
area marked in red.
Only Type-3 LSAs propagate into the backbone. This is important because it prevents every router from having to rerun the SPF
algorithm. This is helps increase the networks stability and reduces unnecessary traffic.

6. Multi Area Design
MULTI AREA DESIGN
When a large OSPF area is divided into smaller areas, this is called multiarea OSPF. Multiarea OSPF is useful in larger network
deployments to reduce processing and memory overhead. Multiarea OSPF requires a hierarchical network design. The main area is
called the backbone area (Area 0) and all other areas must connect to the backbone area. Multiarea OSPF have these advantages;
 Smaller routing tables – There are fewer routing table entries as network addresses can be summarized between areas.
 Reduced frequency of SPF calculations – Localizes impact of a topology change within an area. For instance, it minimizes routing update
impact, because LSA flooding stops at the area boundary.
 Reduced link-state update overhead – Minimizes processing and memory requirements, because there are fewer routers exchanging LSAs.
 Confines network instability to single areas of the network
OSPF design rules;
 No router should be in more than three areas.
 No area should contain more than 50 routers.
 No router should have more than 60 neighbors.
 A router can be a DR or BDR for more than one network segment, but be careful that the router is not overworked by doing so.
 Do not run more than one OSPF process on an ABR.

6.1. Virtual Link
MULTI AREA DESIGN / VIRTUAL LINK
The backbone should never be intentionally partitioned, but if partitioning occurs consider using a virtual link to temporarily repair
the backbone area. Virtual links are logical connections that are vaguely analogous to a tunnel. The two backbone routers establish
a virtual adjacency so that LSAs and other OSPF packets are exchanged as if no other internal OSPF router were involved. A virtual
link can connect on ABR to the backbone, even though the virtual link is not directly connected.
Some of the characteristics and suggested uses for virtual links;
 In order to avoiding routing loops, transit area should have full
knowledge of routing information giving by LSAs
 Other thing Stub areas have only one point of entry, think about
the only one way out or in. So, if you create a virtual link by using
Stub area you create another way out Stub area. In this situation,
we can receiving Type-3-5 LSAs through the virtual link, ending
whole concept of Stub area and the usefulness of Stub default
route.
 Stability is determined by the stability of the area that the virtual transit
 Can only be configured on ABRs
 Assist in solving network connectivity problems
 Can assist in providing logical redundancy
 OSPF treats two routers joined by a virtual link as if they were connected by
an P2P network
 Cannot run across stub areas

6.2. Transit Capability
MULTI AREA DESIGN / TRANSIT CAPABILITY
(Chapter 16.3 in the OSPFv2 RFC 2328)
This is a special property of a non-backbone area that allow this area to transport traffic for other areas. Per the OPSF definition, a transit
area is the area that has a virtual link connecting two or more ABRs attached to this area. Thus, having a virtual link provisioned across
the area is the necessary thing to make the area transit. In fact, it a just an alternate definition of a transit area. The idea of a virtual link is
to extend area 0 across non-backbone area. There are two main situations when you may want to do this;
Virtual links are only used to flood specific LSAs, which are Type 1-2-3 found in area 0. Type-5 LSAs are not flood across the virtual links, because Type 1-2-3-4
LSAs have the flooding scope of a single area. Thus, if you have a virtual link connecting two ABRs you cannot floods LSAs across the transit area, since this area
is different from area 0. However, Type-5 LSA have the flooding scope of OSPF AS domain, and thus they are flooded across the area anyways (unless it’s Stub
area). So, there is no need to duplicate information across the virtual link, obviously, a Stub area cannot be a transit area due to this reasons. The main ideas are;
1. Due to design considerations, where you have an area not directly connected to the backbone area. This could be a result of two networks
merging together.
2. Using a non-backbone area to reach destinations in other areas. The main idea of OSPF inter area routing is that all areas should be
communicating across the backbone. The backbone area is used to exchange information the routing in a distance vector manner, requiring
the star topology to avoid routing loops. For the RFC, the router is only considered on ABR, if it has an interface in area 0 and ignores Type-
3 LSAs delivered across the non-backbone areas. This is ensure the simple “Loop Free” star topology,
1. TransitCapability is the flag that tells you an area carries traffic that neither originates nor terminates in the area itself.
 This only happens when you are using Virtual-Links to connect isolated areas to the backbone or to reconnect a partitioned backbone.
2. The additional checks are done after Inter- and Intra-Area routes have been calculated and they look only at backbone prefixes that
 Native to Area 0
 Inter-area summaries (come from other areas via Area 0)
3. If there is a better path to reach such prefixes than the one through the Virtual-Link ABR, then use it.
4. If there's any summarization configured on the ABR, ignore it when originating summary-LSAs into the transit area to prevent loops.

7. Troubleshooting Path
TROUBLESHOOTING PATH
As shown in the illustration, in general, OSPF problems are related to one of the
following aspects;
 Adjacencies of neighbors
 Missing routes
 Route selection
When troubleshooting neighbors, verify if the router established adjacencies with
neighboring routers using the show ip ospf neighbors command .
 If there are no adjacencies, routers cannot exchange routes. Check if the
interfaces work and are enabled for OSPF using the show ip interface brief and
show ip ospf interface commands .
 Now, if the interfaces work and are enabled for OSPF, make sure that the
interfaces on both routers are configured for the same OSPF area and are not
configured as passive interfaces.
 If the adjacency between the two routers is established, verify that there are
OSPF routes in the routing table using the show ip route ospf command.
 If there are no OSPF routes, verify that there are no other routing protocols
with lower administrative distances running on the network. Verify if all
required networks are advertised in OSPF. Also check if there is an access list
configured on a router that could filter incoming or outgoing routing updates.
If all the required routes are in the routing table but the route the traffic takes is
incorrect, verify the OSPF cost of the interfaces on the route.

8. LAB
LAB / SUMMARIZATIONS / EXAMPLE-1
OSPF is a classless routing protocol, which carries subnet mask
information along with route information. Therefore, OSPF
supports multiple subnet masks for the same major network,
which is known as VLSM. OSPF supports discontinuous subnets
because the subnet masks are part of the LSDB. Network
numbers in areas should be assigned contiguously to ensure
that these addresses can be summarized into a minimal number
of summary addresses. This summarization was successful
because you have the following distinct and contiguous ranges
of subnets;
As can be seen in the output above, the information of Type-3
LSA transmitted via R2 is reflected in the routing table. NULL 0
is a fictitious interface that causes the router to drop into the bit
bucket any information that is destined to it. These entries are
placed into the routing table to prevent routing loops. Also, if
one or more of the summarized networks are inaccessible, the
other routers within the OSPF domain are not interested this
issue, because in their routing tables there is a summarized
route outlined as shown in the area marked in red above.
Only Type-3 LSAs propagate into the backbone. This is
important because it prevents every router from having to
rerun the SPF algorithm. This is helps increase the networks
stability and reduces unnecessary traffic.
▪ 128.213.96.X to 128.218.99.X – Area 1
▪ 128.213.100.X to 128.218.103.X – Area 2
8.1. Summarizations

LAB / PREFIX SUPPRESSION / EXAMPLE-1
In large OSPF networks, a lot of space is wasted in the LSDB and routing tables because of prefixes on
transit links. OSPF prefix suppression is a feature to get rid of these unnecessary prefixes.
The routing table of R1 still shows the
entry for the loopback IP however it has
removed the prefix of the transit link
10.10.23.0/30 from the routing table. To
test it, let’s re-configured the link
between R2 and R3 as P2P network.
The colored entries describe the Transit
links, which need to be processes for the
SPF algorithm but really do not need in the
routing tables of endpoint routers. Thus,
this is the moment where it is very
important to realize that an SPF tree could
consist entirely of unnumbered links and
still function as before.
8.2. Prefix Suppression
NOTE: The source IP must be of the loopback as the routers
have no information for the transit IPs to send the echo replies.

LAB / VIRTUAL LINK / EXAMPLE-1
Let's reference the topology right side to practice the theoretical information
mentioned before about the Virtual Link and Transit Capability. After performing all
configurations and disabling “Transit Capability” feature, let's view the database
information of 100.100.100.0/24 prefix on R4.
8.3. Virtual Link
 100.100.100.1/32 – R6 – Cost 40
 100.100.0.0/16 – R2 – Cost 20
 100.100.0.0/16 – R3 – Cost 20
As it can be seen in the outputs, we can
observe that it has learned the prefixes as
follows from 3 different routers;

LAB / VIRTUAL LINK / EXAMPLE-1
R4's routing table is going to have both option installed, but it will use the longest-match, which is 100.100.100.1/32 via R6.
From the R6’s perspective, 100.100.100.1 prefix is a backbone prefix, and it finds two equal routes of this prefix. One of them is R4 and the
other is R5. Therefore, R6 sends packets to R4 or R5 and they send the packets back to R6 on the most specific route they have. This is why
we facing the routing loop if we have no transit capability feature. Also some situation occurs on R7, you can see clearly in below;

LAB / FORWARDING ADDRESS / EXAMPLE-1
First thing first, R3 and R4 do not include a part of R5 interface on the OSPF process. Therefore, the
LSA Type-5 generated by both routers have the forwarding address set to 0.0.0.0. In this example, R1
or R2 can be used to view the external LSAs. To view external LSAs, issue the below output;
The forwarding addresses for the Type-5 generated by both R3 and
R4 are set to 0.0.0.0. In this case, the LSA to be installed in the R1
routing table is determined by comparing the metrics to the ASBRs
generating the LSAs. You can see the metrics that R1 has for the
ASBRs on above. Therefore, R1 chooses the LSA generated by ASBR
3.3.3.3, which is R3, to place in its routing table that is shown below;
8.4. Forwarding Address

After that, R3 has changed to include
network 192.168.1.0/29 in area 0 of the
OSPF process. The result of the
configuration change is that the Type-5 LSA
generated by R3 now has the forwarding
address set to the IP address of R5 which is
point of next hop for static route, as shown
in the database output taken from R1 right
side;
LSA to be installed in the R1 routing table is
determined by comparing the R1 metric to
the ASBR R4 that generated the LSA with a
forwarding address of 0.0.0.0 to the R1
metric to reach the forwarding address of
192.168.1.5, which was set for the LSA
generated by the ASBR R3. In the above
output, the metric to R4 is 120. This is
compared to the R1 metric to reach the
forwarding address of 192.168.1.5, which
can be seen using the show ip route
192.168.1.5 command. The output of this
command is below;
So, the metric to reach the ASBR R4, which is
74, is compared to the metric to reach the
forwarding address of 192.168.1.5 generated
by R3, which is . Therefore, the LSA installed
in the routing table is the LSA generated by
R4, as shown in the R1 output below;
When the metric of the redistributed route from multiple ASBRs are equal as illustrated in the document,
the forwarding address changes the behavior of the Type-5 LSA path selection. When a router receives two
Type-5 LSAs to the same destination with the forwarding addresses set on both LSAs, the router makes a
comparison based on the metric to the forwarding addresses. The LSA with a forwarding address that offers
the smaller metric is placed into the routing table. If the metric of the redistributed routes are different, the
routers prefer the route with the lowest metric and not the lowest metric to the forwarding address.

In this scenarios where we have multiple ABRs converting/translating same network
from Type-7 to Type-5 LSA, then OSPF LSDB need to act on only one of them. The
other must withdraw its LSA. This is one of many loop avoidance techniques adapted
in OSPF. So, who sets the forwarding address ABRs or ASBR, of course answer is NSSA
ASBR which is R4.
We can see R3 converts Type-7 to
Type-5, because it has higher
Router-ID to compared R2 value,
and retains the forwarding address
value 4.4.4.4. Lets show the related
output below;
As all the path costs are default, in R1 we can
see two equal cost paths available for the
destination of 55.55.55.55, because R1 gets
the external LSA for 55.55.55.55 as forwarding
address set to 4.4.4.4. Now R1 checks its LSDB
to reach 4.4.4.4 and finds two equal cost
paths. Hence it installs both to reach
55.55.55.55. Lets show the related output
below;

When a router is forced to pick a forwarding address for a Type-7 LSA, preference is should be given first to the router’s internal addresses.
If internal addresses are not available, preference should be given to the router's active OSPF Stub network addresses. These choices avoid
the possible extra hop that may happen when a transit network’s address is used. When the interface whose IP address is the LSA’s
forwarding address transitions to a Down State, which case this is the R4. The router must select a new forwarding address for the LSA and
then re-originate it. IF one is not available the LSA should be flushed.
Now in this example the internal address is
the loopback address 4.4.4.4. If this interface
goes down, that force the router to choose
another interface as forwarding address how
that affects the route selection process.
According to the above output, we encounter
a new question, which is why the R4 pick
10.34.34.4 against 10.24.24.4? This selection
process will examine the ordered in below;
 Loopback IP address
 Non-loopback IP address that is connected
to a transit Stub network
 Non-loopback IP address that is connected
to a non-transit Stub network
Also now you see R1 does not multi pathing and only sends the traffic towards the R3, but
what changed the routing decision of R1? We neither changed any path cost nor anything in
the redistribution to influence the path cost, because the forwarding address is set to
10.34.34.4, R1 has the shortest path via R3 (20) against via R2 (30).
I want to mentioned one more thing, which is
Type-5 LSA is generated by R3 Loopback IP
address but the traffic with related to external
destination will follow via R4 interface. You
can clearly see this situation on right side
output;

OSPF.pptx

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OSPF.pptx