07.2 OSPF Routing Protocol

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OSPF is an open standard routing protocol which is wildly used in today’s metropolitan area network.
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Understand OSPF routing protocol features
Understand OSPF route calculation
Understand OSFP special areas.
Supporting equivalent route
OSPF is stand for open shortest path first , it is describe by RFC 2328, which is open to everyone, you can get this document freely from internet. OSPF is also a internal gateway protocol, which is running within an autonomous system .
OSPF is a link state protocol. You can, for example, think of a link as being an interface on the router. The state of the link is a description of that interface. This description would include its IP address, mask, and the type of network to which it is connected. OSPF using the SPF algorithm to calculate the best route, it is a loop free routing protocol.
OSPF support the large networks, for a good designed network, OSPF can support up to 1000 OSPF routers. OSPF support the two level
OSPF is link-state routing protocol, so when a change occurs in the network topology, route update will be generate and flooding so all the OSPF will receive the update and then run the SPF algorithm quickly to build the new routing table ,so it is fast convergence routing protocol.
OSPF supports equal-cost multi-paths, allowing multiple next hops to be recorded for the same destination .
Quiz
A: open IGP routing protocol
B: loop-free protocol
A: distance vector
B: Link state
C: Balanced hybrid
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OSPF Features (Cont .)
Dividing areas for reducing the protocol’s impact on CPU & Memory
Using of reserved multicast addresses to reduce the impact on non-OSPF-speaking devices
Supporting VLSM
Being carried with IP packet, the value of protocol field in IP header is 89
OSPF support the areas division to support the two-level hierarchy network, OSPF usually reduce the size of the SPF calculation by partitioning the network into areas, the number of routers in an area and the number of link state information that flood only within the area are small ,which means that the link state database for an area is small , consequently ,the SPF calculation is easier and takes less time ,so using the areas divisions can reducing the protocols impact on CPU % memory .
Using of reserved multicast addresses to reduce the impact on non-OSPF-speaking devices, OSPF using the 224.0.0.5 as the destination IP address for the route update send out to all OSPF enable interface .
OSPF support the variable length subnet mask (VLSM) .
OSPF is carried with IP packet, the value of protocol field in IP header is 89 . So OSPF only working the IP network, sometimes we called OSPF as IP routing protocol .
Generally speaking ,Which IP address OSPF will use as the destination IP address when OSPF router send the route update out ?
A : 224.0.0.5
B ; 224.0.0.9
C : 192.168.1.1
Choose the correct answer which support the VLSM ?
A: OSPF
A: 89
B: 86
C: 179
D: 520
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OSPF Basic Terminologies
Router ID : A 32-bit number that uniquely identifies the router in the OSPF routing domain,
Link-State Advertisement: Link-State advertisement (LSA) is an OSPF data packet containing link-state and routing information that is shared among OSPF network
Link-State Database: Link-State Database (LSDB) is the collection of LSAs ,LSDB is used in calculating the best paths through the network .
Router ID: The router ID is a 32−bit number assigned to each OSPF enabled router, which is used to uniquely identify the router within an autonomous system. The router ID calculated at boot time is the highest loopback address on the router; if no loopback interfaces are configured, the highest IP address on the router is used. Also you can manually configure the router id by yourself ,we recommend manually configure the router id yourself . Of clause ,the the manually configure router id have the highest preference .
Link-State Advertisement : OSPF is link state routing protocol ,so the routing information exchange between the OSPF route is called the link state advertisement , The state of the link is a description of that interface. This description would include its IP address, mask, and the type of network to which it is connected ,all the link state advertisement stored in a database ,which is called link state database ,or topology table .OSPF router periodically floods each LSA every 30 minutes by default .
Because the LSA describe the OSPF router and the network it attached, so the LSDB is the topology of OSPF network, OSPF using all the LSA which stored in the LSDB to calculate the best route to the destination network .
Encapsulate the link with Ethernet / FDDI / Token Ring Protocol
Generally speaking ,OSPF support 4 type of the network,
Point to point network
Point to multi-point network
Non-broadcast multi-access network (NBMA)
A point to point network joins a single pair of routers, a E1 serial line configured with a link layer protocol such as point to point protocol (PPP) or high level data link control protocol is an example of a point to point network .
On point to point network, the router dynamically detects its neighboring routers by multicasting it hello packets to all SPF routers, using the address 224.0.0.5 . On point to point networks, neighboring routers become adjacent whenever they can communicate directly. No designated router and backup designated router election is performed because there can be only two routers on a point to point link ,so there is no need fro a DR or BDR .The default OSPF hello and dead intervals on point to point links are 10 seconds and 40 seconds ,respectively .
Broadcast networks, such as Ethernet, Token Ring, and FDDI, might be better defined as broadcast multi-access networks to distinguish them from NBMA networks. Broadcast networks are multi-access in that they are capable of connecting more than two devices, and they are broadcast in that all attached devices can receive a single transmitted packet. OSPF routers on broadcast networks will elect a DR and a BDR, as described in the next section, "Designated Routers and Backup Designated Routers." Hello packets are multicast with the AllSPFRouters destination address 224.0.0.5, as are all OSPF packets originated by the DR and BDR. The destination Media Access Control (MAC) identifier of the frames carrying these packets is 0100.5E00.0005. All other routers will multicast link state update and link state acknowledgment packets (described later) to the reserved class D address 224.0.0.6, known as AllDRouters. The destination MAC identifier of the frames carrying these packets is 0100.5E00.0006.
FR/ATM/X.25
FR/ATM/X.25
A point to multi-point environment is very similar to the point to point environment ,no DR or BDR is chosen, all PVCs are treated as point to point links, the only different is that all the PVCs go back to a single router.
A point to multi-point network will send the hello packet every 30 seconds and the Dead interval is four times the hello interval, 120 seconds . For all the ATM , frame-relay and x.25 network ,the default network type is non-broadcast multi-access network, so there is no default network type for the point to multi-point, you should manually configure the ATM , frame-relay ,x.25 network as a point to multi-point network yourself .
NBMA networks, such as X.25, Frame Relay, and ATM, are capable of connecting more than two routers but have no broadcast capability. A packet sent by one of the attached routers would not be received by all other attached routers. As a result, extra configuration may be necessary for routers on these networks to acquire their neighbors. OSPF routers on NBMA networks elect a DR and BDR, and all OSPF packets are unicast.
The difference between NBMA and point-to-multipoint:
In OSPF protocol, NBMA and point-to-multipoint both mean Non-Broadcast Multipoint Access networks, but NBMA must meet the requirements of a full meshed network, namely, any two points can make access of the packets to the remote port possible without forwarding. Otherwise, we will call the network a point-to-multipoint network
Quiz
A: 4
B: 5
C: 3
D: 6
What is default network type for ATM network running the OSPF routing protocol ?
A: point to multi-point
D: point to point
how often does a point to multi-point network send the hello packet to its neighbors ?
A: every 10 seconds
B: every 20 seconds
C: every 30 seconds
D: every 60 seconds
Neighbors
If two routers sharing a common data link agree on certain parameters specified in their respective hello packets, they will become neighbors.
Adjacencies may be thought of as virtual point-to-point links between neighbors. Whether the neighbors will become adjacencies depends on the type of network they are attached to.
Adjacencies
A neighbor refers to a connected router that is running an OSPF process with the adjoining interface assigned to the same area. Neighbors are found via hello packets( hello packet is discuss in the subsequence slides), no routing information is exchanged with neighbors unless adjacencies are formed!
An adjacency refers to the logical connection between a router and its corresponding designated routers and backup designated router or its point to point neighbor. The formation of this type of relationship depends heavily on the type of the network that connect the OSPF routers, on point to point connection , the two routers will form adjacency with each other without require a designated router, not all neighbors become adjacent.
= 13
DR
BDR
Multiaccess networks present two problems for OSPF, relating to the flooding of LSAs
The formation of an adjacency between every attached router would create many unnecessary LSAs. If n is the number of routers on a multiaccess network, there would be n(n- 1)/2 adjacencies . Each router would flood n- 1 LSAs for its adjacent neighbors, plus one LSA for the network, resulting in n 2 LSAs originating from the network.
Flooding on the network itself would be chaotic. A router would flood an LSA to all its adjacent neighbors, which in turn would flood it to all their adjacent neighbors, creating many copies of the same LSA on the same network.
to solve this problem, OSPF designates a router, Designated Router, DR in short, to be responsible for packet flooding. All the routers transmit their information through the routes to the DR only, then the DR transmits the route information to other routers in this stub network. Two routers other than DR (DROther) no longer establish an adjacency relationship and no longer exchange any route information. In this way, only adjacency relations of N in number will be needed to be established among the routers in the same stub network, and the route change every time will be forwarded for N times.
BDR stand for the backup designated Router .
Once the DR and BDR are selected, any router to the network established adjacencies with the DR and BDR only
Each OSPF interface (multi-access network only) possesses a configurable router priority , the default value is 1 , if you don’t want a router interface to participate in the DR/BDR election, set the router priority to 0 in the interface configuration view .
Link State Request Packet
Link State Update Packet
Link State Acknowledgment Packet
There are five type of OSPF packets which are used by OSPF routers to exchange the information.
Hello packets are OSPF packet type 1. These packets are sent periodically on all interfaces in order to establish and maintain neighbor relationships. In addition, Hello Packets are multicast on those physical networks having a multicast or broadcast capability, enabling dynamic discovery of neighboring routers.
Database Description packets are OSPF packet type 2. These packets are exchanged when an adjacency is being initialized. They describe the contents of the link-state database.
Link State Request packets are OSPF packet type 3. After exchanging Database Description packets with a neighboring router, a router may find that parts of its link-state database are out-of-date. The Link State Request packet is used to request the pieces of the neighbor's database that are more up-to-date. Multiple Link State Request packets may need to be used.
Link State Update packets are OSPF packet type 4. These packets implement the flooding of LSAs. Each Link State Update packet carries a collection of LSAs one hop further from their origin. Several LSAs may be included in a single packet.
Link State Acknowledgment Packets are OSPF packet type 5. To make the looding of LSAs reliable, flooded LSAs are explicitly acknowledged. This acknowledgment is accomplished through the sending and receiving of Link State Acknowledgment packets. Multiple LSAs can be acknowledged in a single Link State Acknowledgment packet
Packet Data
As we know that for most protocol ,it will cover the protocol header and the protocol body, it is the same to OSPF protocol .before we begin discuss the detail information inside the OSPF packet , we should have a basic understanding of the OSPF header:
OSPF header contain the following information :
Version : The OSPF version number. In today’s network, all the OSPF router will running the OSPF version 2 or above ,but now we just talking about the OSPF version 2.
Type: The OSPF packet types are as follows
Type Description
Packet length
The length of the OSPF protocol packet in bytes. This length includes the standard OSPF header.
Router ID
Area ID
A 32 bit number identifying the area that this packet belongs to. All OSPF packets are associated with a single area. Mosttravel a single hop only. Packets travelling over a virtual link are labelled with the backbone Area ID of 0.0.0.0.
Checksum
The standard checksum of the entire contents of the packet, starting with the OSPF packet header but excluding the 64-bit authentication field. This checksum is calculated as the 16-bit one's complement of the one's complement sum of all the 16-bit words in the packet, excepting the authentication field. If the packet's length is not an integral number of 16-bit words, the packet is padded with a byte ofzero before checksumming. The checksum is considered to be part of the packet authentication procedure; for some authentication types the checksum calculation is omitted.
AuType
Identifies the authentication procedure to be used for the packet.
Authentication
Neighbor
All the OSPF packet share the common OSPF header ,and now les’s t discuss the different of the OSPF packet .
The first packet is the OSPF hello packet, the following field include:
Network Mask. The subnet mask associated with the interface. If subnetting is not used, it will be set to the appropriate hexadecimal value for each class of IP address.
Hello Interval. The number of seconds between when the router transmits hello packets
Router Priority. This is the where the router’s priority would be annotated if this feature is used, otherwise the default is 1.
Router Dead Interval. Number of seconds since the last hello packet was received before declaring a silent router as no longer reachable
Designated Router. The network’s designated router (if present) IP address. This field defaults to 0.0.0.0 when a designated router is not present, like on-demand circuits.
Backup Designated Router. The network’s backup designated router (if present) IP address. This field defaults to 0.0.0.0 when a designated router is not present, like on-demand circuits.
Neighbor. Contains the router IDs of each router that has sent a valid hello packet. This field can have multiple entries.
LSA Headers
Interface MTU
The size in bytes of the largest IP datagram that can be sent out the associated interface, without fragmentation. Interface MTU should be set to 0 in Database Description packets sent over virtual links.
Options
I-bit
The Init bit. When set to 1, this packet is the first in the sequence of Database Description Packets.
M-bit
The More bit. When set to 1, it indicates that more Database Description Packets are to follow.
MS-bit
The Master/Slave bit. When set to 1, it indicates that the router is the master during the Database Exchange process. Otherwise, the router is the slave.
DD sequence number: Used to sequence the collection of Database Description Packets. The initial value (indicated by the Init bit being set) should be unique. The DD sequence number then increments until the complete database description has been sent. The rest of the packet consists of a (possibly partial) list of the link-state database's pieces. Each LSA in the database is described by its LSA header.
LSA header
Advertising Router
Link state request packet is used to request to most recent LSA :
Each LSA requested is specified by its LS type, Link State ID, and Advertising Router. This uniquely identifies the LSA, but not its instance. Link State Request packets are understood to be requests for the most recent instance (whatever that might be).
LSAs
Numbers of LSAs: The number of LSAs included in this update packet
LSAs : LSA instances
The link state update packet is the only packet which contain the detail LSA information ! All the other packet will contain only the LSA heraders, without the LSA body information.
LSA Headers
Link State Acknowledgment Packets are OSPF packet type 5, which is used to make the flooding of LSAs reliable, flooded LSAs are explicitly acknowledged.
A: open IGP routing protocol
B: loop-free protocol
A: distance vector
B: Link state
C: Balanced hybrid
Generally speaking ,Which IP address OSPF will use as the destination IP address when OSPF router send the route update out ?
A : 224.0.0.5
B ; 224.0.0.9
C : 192.168.1.1
Choose the correct answer which support the VLSM ?
A: OSPF
A: 89
B: 86
C: 179
D: 520
Which of the following packet are used by OSPF router to exchange the routing information ?
A: link state advertisement (LSA)
B: link state packet (LSP)
C: link information packet
D: routing table
Which of the following is the correct order for selecting the router id by the ospf router
1: manually configured router id
2: highest physical IP address
3: highest logical IP address
A: 1-2-3
B: 1-3-2
C: 3-2-1
D: 2-3-1
A: 4
B: 5
C: 3
D: 6
What is default network type for ATM network running the OSPF routing protocol ?
A: point to multi-point
D: point to point
how often does a point to multi-point network send the hello packet to its neighbors ?
A: every 10 seconds
B: every 20 seconds
C: every 30 seconds
D: every 60 seconds
2.1 SPF Basic Operation
2.3 OSPF SPF Inter-area Claculation And As External Route Calculation
Neighbor and Adjacency initialization
Neighbor and Adjacency initialization
LSA flooding
SPF calculation
OSPF routers send hello packets out all interface participating in the OSPF process. If the router and the router on the other sides of the connection agree on the parameters set forth in the hello packet, the routers will form neighbor relationships .
Some of the neighbors will adjacencies, Forming adjacencies is dependent upon the type of network the hello packet is being sent across and the type of routers exchanging the hello packets.
The routers will send link state advertisement (LSAs) , which contain description of the router’s links and the state of each link to the adjacent router .
The routers receive the LSAs will then record the information into their link state database and forward the LSAs on to their respective neighbors . This allows all routers participating in the OSPF process to have the same view of the network, although from their own perspective .
After learning all LSAs, each router will run the SPF algorithm to learn the shortest path to all the known destination , each router uses this information to create its SPF tree. The information contained in the SPF tree is then used to populate the routing table .
Neighbor And Adjacency Initialization
In a broadcast network, DR and BDR ,DR and DRother, BDR and DRother will form the neighbor and adjacency relationship, but DRother and DRother only form neighbor relationship
In point to point /NBMA / point to multi-point network ,if two OSPF router form the neighbor relationship, they will form the adjacency relationship
In a broadcast network, DR is represent the multi-access network and its attached routers to the rest of the network , DR manage the flooding process on the multi-access network ,so in the broadcast network, DR and BDR ,DR and DRother, BDR and DRother will form the neighbor and adjacency relationship, but DRother and DRother only form neighbor relationship .
In point to point / NBMA / point to multi-point network ,if two OSPF router form the neighbor relationship, they will form the adjacency relationship.
So you should know that in a broadcast network such as Ethernet , two DRother router will never exchange routing information even they are form the neighbor relationship .
Quiz :
Which of the following statement is true ,choose all apply ( )
A: DRother will form the adjacent with the DR and BDR
B: DRother will form the adjacent with the DR only.
C: DRother will form the adjacent with the DRother
D: DRother will never form the adjacent with the DRother
Down
Attempt
Init
2-way
ExStart
Exchange
Loading
Full
An OSPF router will transition a neighbor through several states before the neighbor is considered fully adjacent.
Down:The initial state of the neighbor state machine means that during the past DeadInterval, no HELLO packets were received from the remote router.
Attempt: Adaptable only to NBMA type interfaces. When in this state, hello packets are periodically transmitted to those neighbors configured manually.
Init: This state indicates that the HELLO packets have been received from the neighbors, but in the neighbors listed in the packets, "my" Router ID is not included (The opposite has not yet received "my” HELLO packets).
2-Way: This state indicates that the two routers have received the HELLO packets from each other and they have set up an adjacency relation. With respect to the broadcast networks and NBMA type networks, two routers in the interface state of DROther will stay in this state, while other state machines will further be converted to higher levels.
ExStart:In this state, a router determines the master/slave relation in the transmission by exchanging DD packets between it and its neighbors (The DD packets do not have any substantial contents but some identification signs). To set up the master/slave relation is to ensure an orderly transmission in the follow-up DD packet exchange.
Exchange: The routers use the DD packets to describe the local LSDB and send it to the neighbors.
Loading: The routers transmit the LSR packets to the neighbors to apply for the DD packets from the opposite party.
Full: In this status, this router already has all the LSAs in the neighbor router's LSDB, namely, this router has already established an adjacency status with its neighbors.
Note:
The state in gray color means a stable state, while the state in other colors are existent only in a short while (generally not longer than a few minutes) during the exchange.
The state of this router may differ from that of the remote router. For instance, when the state of this router is Full, it may be Loading in the opposite router.
DD (Seq = x,I = 1, M = 1, MS = 1)
DD (Seq = y,I = 1, M = 1, MS = 1)
DD (Seq = y,I = 0, M = 1, MS = 0)
DD (Seq = y+1,I = 0, M = 1, MS = 1)
DD (Seq = y+1,I = 0, M = 1, MS = 0)
DD (Seq = y+n,I = 0, M = 0, MS = 1)
DD (Seq = y+n,I = 0, M = 0, MS = 0)
LS Request
LS Update
LS Request
Full
The above figure indicates how the adjacency relation between two routers is set up through transmission of the 5 protocol packets, and how the neighbor state machine is moved.
An RT1's interface connected to a broadcast-type network activates the OSPF protocol, and transmits a HELLO packet (using the multicast address 224.0.0.5). Since at this time RTI has not found any neighbor in the stub network, so the Neighbor section in the HELLO packet is vacant.
After receiving the HELLO packets sent by RT1, RT2 creates a neighbor data structure for RT1, and set RT1's neighbor state machine as Init. RT2 transmits a HELLO packet in response to RT1, and fills out the Router ID of RT1 in the Neighbor section of the packet, thus indicating that it has received RT1's HELLO packet.
After receiving the responding HELLO packet from RT2, RT1 creates a neighbor data structure for RT2, and set the neighbor state machine as Exstart. To go further, each of the two parties starts to transmit its own link state database.
In order to increase the transmission efficiency, the two parties need to know about which LSAs in the opposite data base are what itself needs, for if a certain LSA is owned by itself, it will not be necessary to apply for it. The method is to first transmit the DD packets, which includes an abstract description of the LSA in the local data base (each abstract can uniquely identify one LSA, but the space it occupies will be much smaller). Since OSPF directly packs its own protocol packets using IP packets, so the packet transmission reliability during the transmission must be taken into account. For this purpose, the master/slave relation between the two parties need to be determined during the DD packet transmission. The side as the Master defines a sequence number "seq", and add 1 to the seq every time a new DD packet is transmitted. And the side as the Slave uses, every time it transmits the DD packets, the seq of the last DD packet received from the master. As a matter of fact, this sequence number mechanism is an implicit confirmation method, and with the time-out re-transmission for every packet, transmission reliability will be guaranteed.
RT1 first transmits a DD packet, declaring itself as the Master (MS=1), and sets the sequence number as x. The 1=1 indicates that this is the first DD packet, with no LSA abstract included in the packet, for the purpose of determining the Master/Slave relationship, while M=1 indicates that this is not the final packet.
After receiving the DD packet from RT1, RT2 changes RT1's neighbor state machine to Exstart, and replies the latter with a DD packet, which does not include any abstract information of LSA, either. As RT2's Router ID is relatively larger, so RT2 claims itself in the packet as the Master, and re-set the sequence number as "y".
Having received the packet, RT1 agrees that RT2 is the Master, and changes RT2's neighbor state machine to Exchange. Using the sequence number y of RT2, RT1 transmits a new DD packet, which officially starts to transmit LSA's abstract. In the packet, RT1 sets M=0, indicating itself as the Slave.
Having received the packet, RT2 changes RT1's neighbor state machine to Exchange, and transmits a new DD packet describing the LSA abstract of its own. Please note that at this time, RT2 has changed the packet sequence number to y+1.
The above procedures going on and on, RT1 confirms, by repeating RT2's sequence number, the packets it has received from RT2. And RT2 confirms, by adding +1 to the sequence number, the packets received from RT1. When RT2 transmits the final DD packet, it sets the M=0, indicating that this is the last DD packet.
After RT1 has received the final DD packet, it finds out that it does not have many of the LSAs in RT2's data base, so it changes the neighbor state machine to Loading. At the same time, RT2 also receives the last DD packet from RT1, finding that it already has all the LSAs owned by RT1 and no application is needed, so it directly changes RT1's neighbor state machine to Full.
RT1 applies to RT2 for the LSAs it needs by transmitting an LS Request packet. And RT2 responds to RT1's application by transmitting an LS Update packet. When RT1 receives the packet, it needs to confirm by transmitting an LS Ack packet. The above procedures will continue till the synchronization of the LSAs in RT1 with those in RT2. At this time, RT1 changes RT2's neighbor state machine to Full.
Note: The above process is a process from the time when two routers do not find each other's existence at first and to the time when they establish an adjacency relationship. The other way round, we can deem it a processing procedure where a router is newly introduced into a network. When the state machines between two routers both reach the Full state, and if any further route change occurs in the network, not all the above procedures need to be repeated. It will be enough that one party transmits an LS Update packet, informing of the items to be updated, and the other side responds by transmitting an LS Ack packet. During this process, no change will take place in terms of the state machine of both sides.
LSDB
C
A
B
D
1
2
3
C
A
B
D
1
2
3
C
A
B
D
1
2
3
C
A
B
D
1
2
3
(4)Each router uses SPF algorithm to calculate a loop-free graph describing the shortest (lowest cost) path to every destination ,with itself as the root
RTA
RTB
RTC
RTD
3
2
1
5
(1)Network Topology
The above figure gives out a process of route calculating by OSPF protocol.
1. In the network composed of four routers, the number beside the connecting line indicates the cost of packet transmission from one router to another. To simplify the matter, we suppose that the cost from one router to the other and vice versa is the same.
2. Each router creates an LSA (Link State Advertisement) in accordance with the topological structure of its peripheral networks and, through the mutual transmission of protocol packets, sends this LSA to all the other routers of the networks. In this way, each router receives the LSA of other routers. And all the LSAs put together are called LSDB (Link Status DataBase). Evidently, the LSDB of each of the four routers is the same with each other.
3. An LSA is a description of the topological structure of a router's peripheral networks, then the LSDB is a description of the topological structure of the entire networks. It is very easy for a router to convert the LSDB into a weighted directional figure, which is a real reflection of the entire network's topological structure. Evidently, again, the four routers will receive a completely same figure.
4. Next, each router, taking itself as the root point in the figure, works out a shortest path tree by using the SPF algorithm. With this tree, a table of routes leading to each root point of the network will be obtained. Evidently, the routing tables obtained from the four routers will be different from each other.
In this way, each router works out its routes to other routers.
From the above analysis, it is known that the route calculating by OSPF involves the following three procedures:
Describing the topological structure of the router's peripheral networks, and creating the LSA.
Disseminating the LSAs created by itself throughout the autonomous system, and collecting the LSAs created by all the other routers.
Calculating the routes by using the LSAs collected.
Example 1 Of OSPF Calculation
OSPF using the LSA to describe the router’s link state information, so let’s see what is it inside the LSA .
AREA 0
Loopback0
2.2.2.2/32
Here is a quite simple networking, RTA and RTB is two OSPF router using the point to point protocol to connect each other. All the interface list in this figure are OSPF enabled . All the interface running OSFP are in the same area : area 0
Router LSA ( Type 1 LSA )
Router LSAs are produced by every router. This most fundamental LSA lists all of a router's links, or interfaces, along with the state and outgoing cost of each link. These LSAs are flooded only within the area in which they are originated.
Type : Router
Data : 255.255.255.252
Type : StubNet
Metric : 1562
Once RTA and RTB become the neighbor and form the adjacency relationship ,they will exchange the router LSA ,we can check that what it is stored in the LSA :
Router LSAs are produced by every router. This most fundamental LSA lists all of a router's links, or interfaces, along with the state and outgoing cost of each link. These LSAs are flooded only within the area in which they are originated.
All of the router’s link in an area must be describe in a single LSA .
As shown above ,LSA will divide into two part : LSA herder and LSA Data
LSA header indicate some information as follow
Type: router : indicate the LSA is a router LSA.
Adv rtr : 2.2.2.2 : indicate who generate the router LSA ,it is the router id for the OSPF router.
LS id : the same value as the Adv rtr for the router LSA
Link ID:1.1.1.1 Data : 255.255.255.255 indicate a link (subnet) that attach to the router
Type : StubNet : indicate the link is directly connect to the OSPF router 2.2.2.2
Metric : the cost for the OSPF router to this network,
In the figure show in the slide, all the data is only to describe the OSPF router before it form the neighbor with the RTB, after the adjacency is form between RTA and RTB ,the router LSA is change as follow:
<RTA>display ospf lsdb router self-originate
OSPF Process 1 with Router ID 1.1.1.1
Link State Database
Data : 255.255.255.252
Type : StubNet
Metric : 1562
again, All of the router’s link in an area must be describe in a single LSAs , so if there is something change in the router ,new router LSA will be generate and with the new sequence number that indicate a more recent Router LSA.
Quiz :
How many Router LSA will be generate by the a router in a point to point network within the one area at a time ?
A: 1
B: 2
C: 3
LSA data for RTB
Destination Cost Type NextHop AdvRouter Area
1.1.1.1/32 1 Stub 1.1.1.1 1.1.1.1 0.0.0.0
10.1.1.0/30 1562 Stub 10.1.1.1 1.1.1.1 0.0.0.0
2.2.2.2/32 1563 Stub 10.1.1.2 2.2.2.2 0.0.0.0
How OSPF router calculate the route for this simple network ?
In the router RTA ,it will first check the Router LSA originated itself ,find the link with the type :StubNet and install the network into the OSPF routing table directly.
RTA will find the neighbor router LSA ,in the LSA Data ,you can see that router-id 2.2.2.2 is the neighbor, router RTA will install the network with the type :StubNet of the RTB into the OSPF routing table ,with the sum cost indicate by the RTB router LSA and the cost to reach to the RTB .
Networking
Note: the cost for each interface running OSPF using the default formula to calculate
10.1.1.0/30
.1
.2
20.1.1.0/29
.1
.2
.3
.4
RTA
RTB
RTD
RTE
RTC
Loopback0
1.1.1.1/32
Loopback0
2.2.2.2/32
Loopback0
3.3.3.3/32
Loopback0
4.4.4.4/32
Loopback0
5.5.5.5/32
From the example 1,we know that how ospf calculate the OSPF route in the point to point network, now we will discuss the netwrok mixed the point to point link network and broadcast network .
We can see that RTB connect to the RTA with the point to point link like example 1,but RTB now connect to RTC / RTE / RTE using the Ethernet protocol. As we know that Ethernet is broadcast network in the OSPF, so what is the different about the SPF calculation between the point to point network and the broadcast network?
Link State Database
Stub 2.2.2.2 2.2.2.2 152 24 0 0 SpfTree
Stub 4.4.4.4 4.4.4.4 153 24 0 0 SpfTree
Stub 1.1.1.1 1.1.1.1 1663 24 0 0 SpfTree
Stub 10.1.1.0 1.1.1.1 152 24 0 0 SpfTree
Stub 3.3.3.3 3.3.3.3 758 24 0 0 SpfTree
Stub 5.5.5.5 5.5.5.5 150 24 0 0 SpfTree
Rtr 2.2.2.2 2.2.2.2 148 72 80000014 0 Clist
Rtr 4.4.4.4 4.4.4.4 146 48 8000000e 0 Clist
Rtr 1.1.1.1 1.1.1.1 1663 60 80000006 0 SpfTree
Rtr 3.3.3.3 3.3.3.3 760 48 80000010 0 Clist
Rtr 5.5.5.5 5.5.5.5 152 48 8000000d 0 Clist
Net 20.1.1.1 2.2.2.2 153 40 80000005 0 SpfTree
Stub network directly connect to the advertise router
Router LSA
Network LSA
As we know that before the SPF calculation, the OSPF router will form the neighbor and adjacent with other router and then flooding the LSAs .Once all the router in the OSPF routing domain synchronize the link state database, all the router within the same area will have the same LSDB ,so when we check the LSDB in the RTA ,we can see that in the network ,it will contain two type of link state advertisement : Router LSA and the Network LSA ?
As we know that router LSA will lists all of a router's links, or interfaces, along with the state and outgoing cost of each link .but what about the network LSA ?
….
Net mask : 255.255.255.248
Attached Router 2.2.2.2
A Network LSA is generated for every transit broadcast or NBMA network . ( a transit network is a network having two or more attached routers) . The network LSA describe all the routers that are attached to the network
Indicate this is a network LSA
Indicate the transit network
indicate the routers attached to the network
A Network LSA is generated for every transit broadcast or NBMA network . ( a trnasit network is a network having two or more attached routers) . The network LSA describe all the routers that are attached to the network
Network LSAs are produced by the DR on every multi-access network ,As discussed earlier, the DR represents the multi-access network and all attached routers as a "pseudo node," or a single virtual router. In this sense, a Network LSA represents a pseudo node just as a Router LSA represents a single physical router. The Network LSA lists all attached routers, including the DR itself. Like Router LSAs, network LSAs are flooded only within the originating area . We can check the OSPF network LSA with the following command:
<RTA>dis ospf lsdb network
OSPF Process 1 with Router ID 1.1.1.1
Link State Database
Network LSA for RTB
RTA treat itself as the root , check the router LSA generated itself , and install the link with the type: StubNet into the OSPF routing table, so the result would be as follow :
<RTA>display ospf routing
Routing Tables
1.1.1.1/32 1 Stub 1.1.1.1 1.1.1.1 0.0.0.0
10.1.1.0/30 1562 Stub 10.1.1.1 1.1.1.1 0.0.0.0
From the Router LSA with the type: point to point, RTA find its neighbor RTB, so RTA will check the LSDB to install the RTB’s router LSA link step 1, and get the result as follow:
Routing Tables
1.1.1.1/32 1 Stub 1.1.1.1 1.1.1.1 0.0.0.0
10.1.1.0/30 1562 Stub 10.1.1.1 1.1.1.1 0.0.0.0
2.2.2.2/32 1563 Stub 10.1.1.2 2.2.2.2 0.0.0.0
20.1.1.0/29 1563 Net 10.1.1.2 2.2.2.2 0.0.0.0
From RTB’s LSA ,RTA know that RTB have a neighbor by the point to point link ,which is RTA itself, so this neighbor (RTA) will be ignored.
RTA check RTB’s Router LSA and find a link with the type :transNet ,which indicate that RTB is connectted to a multi-Access network, so RTA will check the LSDB for RTB’s network LSA.
From the RTB’s network LSA ,RTA know that RTB have the neighbor with RTC (router id 3.3.3.3) /RTD (router id 4.4.4.4) / RTE (router id 5.5.5.5)
Data : 20.1.1.3
Type : TransNet
Metric : 1
From the RTB’s Network LSA, RTA will check RTC /RTD /RTE router LSA and install the link into the OSPF routing table, following this procedure ,RTA will finish the SPF calculation and install all the link into OSPF routing table .
Routing Tables
Total Nets: 7
Intra Area: 7 Inter Area: 0 ASE: 0 NSSA: 0
RTA
RTB
1562
RTC
1
RTD
1
RTE
1
For this example ,RTA treat itself as the root , install itself as the root of the SPF tree .then RTA add it’s neighbor RTB in the SPF tree, recording the cost to the RTB in the SPF tree,. Once all RTA’s neighbor have installed the SPF tree, RTA will find the RTB’s neighbors and install them into the SPF tree of RTA ,of course ,RTA will record the cost also, through this method , RTA build the SPF tree ,by calculation the cost associate with the link, RTA Build the route into the OSPF routing table.
Why Is OSPF Loop-Free?
Each LSA has marked out the creator. The rest routers only take charge of transmission. In such a way, there will be no change or misunderstanding to the messages in the course of transmission.
The route algorithm is SPF. The calculation result is a tree and the routers are the leaf nodes. From the root node to the leaf nodes is a unidirectional and un-returnable path.
The route self-loop refers to the self-loop formed in the network by the route to a certain target address. Here is a simplest example. Router A has a route 10.0.0.0/8 with the next hop being Router B, and in the routing table of router B the next hop of the route is directed to A. If A receives a packet to 10.0.0.1, it will forward it to B which in turn forwards the packet to A according to the routing table. Thus the packet will keep oscillating between A and B and will not drop it until TTL=0. In the worst case, it may oscillate 255 times. The route self-loop is most hazardous to network. It will not only cause the route unreachable, but also waste tremendous network bandwidth. The route self-loop is a problem that has to be solved by all of the route protocols. It has been an important sign of the quality of a route protocol.
D-V Algorithm and Route Self-Loop
The D-V algorithm is also known as "Distance — Vector" algorithm. Its core idea is that each router in the network sends the routing table known by it to the neighboring router, and each router determines the next hop with the best path according to all the routes received. The limitation of this algorithm (mainly the D-V algorithm realized by RIP and IGPR) is:
The fidelity of the routes received by each router completely depends on the neighboring router. But a router can only be responsible for the correctness of its own local routes (the interface routes of the local router), but can not be also responsible for the routes sent by other routers.
As the creator's information is not marked out in the route packet, the packet may be transferred back to the original creator after many times of transfer in the network. But the creator can not know whether the packet was issued by himself, which gives birth to the risk of a self-loop.
When the topology structure has some change, an already invalid route may be still in the process of transfer in the network before it is cleaned out completely. When it is transferred to its creator, a mistake may happen in the calculation for the next hop, which produces a route self-loop.
OSPF and Route Self-Loop
The OSPF is a protocol based on the link status algorithm. Its core idea is: each router describes the link status (including the direct routes of interface, connected routers and other packet) around it, and sends the status to all the routers in the network. After receiving the link status packet sent from all the rest routers, each router runs the SPF algorithm and calculates the route.
The route calculated by the OSPF has no self-loop because:
What is described by each router is the packet that it can guarantee its correctness --the neighboring network topology structure. It also marks out the creator of the packet in the LSA created --and writes it into its own Router ID. The rest routers are only responsible for the transmission of the packet in the network and will not cause any change. This ensures that the router can receive the topology structure figure of the whole network, no matter how the network topology structure is and what position the router is in the network.
The route algorithm is SPF. The calculation result is a tree and the routes are the leaf nodes. From the root node to the leaf nodes is a unidirectional and unreturnable path.
When there is a change in the network topology structure (It is the time that the route self-loop is most likely to happen), one (or more) router(s) will sense the change, describe the topology structure anew and notify it to the rest routers. After receiving the updating packet, each router will restart to run the SPF algorithm and get a new route.
Does OSPF Really Have No Self-Loop?
What is described above is the case without the division of areas or rather, it is the case that the OSPF runs in the same area. When the OSPF is divided into different areas, ABR sends out the internal area routes that have been already calculated and packaged into LSA of Type 3. But please pay special attention to the fact that, in this case, the ABR describes only the routers instead of the status of the links. Or rather, in calculating the internal area routes, the OSPF uses the D-V algorithm. Here may come the route self-loop which is solved by the OSPF through the backbone area. (For details, see the Backbone Area and the Virtual Link).
But when the OSPF redistributes the external routes from outside the autonomous system, what the ASBR packages in the LSA of Type 5 is also the route packet instead of link status. Here may also come a route self-loop. But this time the OSPF does not take any measures against the creation of self-loop, because the external route redistributed from outside the autonomous system is itself unreliable. It may come from the unreliable routes such as the RIP and IGRP or the static routes that are incorrectly configured. They may have a self-loop in themselves. As the routes calculated inside the area at the time of area division are correct and mistake proof, every effort should make to avoid the creation of new self-loops as a result of area division. The external routes redistributed from the outside have unreliable sources, so there is no need to conduct further operations.
The exact meaning of the sentence "The OSPF Protocol will not produce a self-loop" should be: the internal area routes of the autonomous system created by the OSPF Protocol are free from self-loops whereas the external routes redistributed from outside the autonomous system are not guaranteed.
Maintain a big link-state database in the router
SPF calculation become more complex
Route flapping impact the network stability
The route entry size is too big for a router
Management and troubleshooting become more difficulty
As the network grow up , more and more router join into the network, so more and more LSA will generate in the area, there are some problem with OSPF in a single area:
Maintain a big link state database in the router ,as we know that all the OSPF router in the single area maintain the same LSDB, so all the routes in the same areas will require the same memory and CPU power.
SPF calculation become more complex: more and more router join the network ,more and more LSA generate ,so SPF calculation will become more complex.
OSPF is link state routing protocol ,so as the network grown up ,the chance for the link fail grown up also ,sometimes there is a link up and down frequency, this is call route flapping ,and the route flapping will impact the network stability .
As the network grown up, The route entry size is too big for a router.
As the network grown up , Management and troubleshooting become more difficulty
SPF calculation become easy to implementation
Reduce the impact of flapping route with the route summary
Reduce the route entry in routing table with route summary
Easy to manage the OSPF router in a area
How to solve the problem with OSPF running in a single area ?
The answer is divide the OSPF routing domain into several logical router groups. This logical group is called OSPF area. OSPF uses areas to reduce these adverse effects
Routers within an area will have no detailed knowledge of the topology outside of their area. Because of this condition:
A router must share an identical link state database only with the other routers in its area, not with the entire internetwork. The reduced size of the database reduces the impact on a router's memory.
The smaller link state databases mean fewer LSAs to process and therefore less impact on the CPU.
Because the link state database must be maintained only within an area, most flooding is also limited to the area.
So there are some benefit with the OSPF area division :
Reduce the requirement of router memory and CPU power
SPF calculation become easy to implementation
Reduce the impact of flapping route with the route summary
Reduce the route entry in routing table with route summary
Easy to manage the OSPF router in a area
Area 0
(backbone )
Internal Routers are routers whose interfaces all belong to the same area.
Area Border Routers (ABRs) connect one or more areas to the backbone and act as a gateway for inter-area traffic.
Backbone Routers are routers with at least one interface attached to the backbone
Area 2
Area 18
Area 1
As shown in the figure , a OSPF routing domain is divide into for OSPF areas.
Area 0, also known as the backbone area ,if there are more than one area in the OSPF routing domain, backbone area must be exist in such conditions.
OSPF using the area ID to identify the different areas, the value for the area id is from 0 to 232 .
After the area division on the OSPF routing domain, there is some router role for different OSPF routers.
Internal Routers are routers whose interfaces all belong to the same area.
Area Border Routers (ABRs) connect one or more areas to the backbone and act as a gateway for inter-area traffic.
Backbone Routers are routers with at least one interface attached to the backbone
Router LSA and Network LSA only flooding within area
SPF Calculation only execute within area
Router is the boundary between areas
All inter-area traffic must pass through the backbone area
Non-backbone areas cannot exchange packets directly
All non-backbone areas must connect to backbone area physically or logically
RTA
RTB
Metric : 2
Before discuss the new type LSA ,one router role in the OSPF network we should understand is the ABR : area border router .an ABR is a router that has multiple area assignments .an interface may belong to only one area, if a router has multiple interfaces and if any of these interfaces belong to different areas ,this router is considered an ABR.
After the division of the autonomous system into different area by OSPF, many changes have also taken place in the route-calculating algorithm:
LSDB synchronization is ensured only between routers belonging to the same area, and the change in network topological structure will be updated first within the area.
Network Summary LSAs are originated by ABRs. They are sent into a single area to advertise destinations outside that area . In effect, these LSAs are the means by which an ABR tells the Internal Routers of an attached area what destinations the ABR can reach. An ABR also advertises the destinations within its attached areas into the backbone with Network Summary LSAs. Default routes external to the area but internal to the OSPF autonomous system are also advertised by this LSA type
Simple explanation for the network summary LSA
SumNet : indicate a network summary LSA
LS id : indicate the destination network ,it is always work with the net mask
Adv rtt : indicate the ABR who generate the LSA
Net mask : net mask for the destination network.
Metric: indicate the cost from the ABR to the destination network.
All rights reserved
Inter-area Route Calculation
OSPF using the router LSA & network LSA to calculate the best route within a area
Each network summary LSA is treat as a StubNet directly attached to the ABR
Router installed the destination network advertised by ABR, and the total cost is the sum of the cost of the router to the ABR and the cost advertise by the network summary LSA. This behavior is distance vector behavior
When an ABR originates a Network Summary LSA, it includes the cost from itself to the destination the LSA is advertising. The ABR will originate only a single Network Summary LSA for each destination even if it knows of multiple routes to the destination. Therefore, if an ABR knows of multiple routes to a destination within its own attached area, it originates a single Network Summary LSA into the backbone with the lowest cost of the multiple routes. Likewise, if an ABR receives multiple Network Summary LSAs from other ABRs across the backbone, the original ABR will choose the lowest cost advertised in the LSAs and advertise that one cost into its attached non-backbone areas.
When another router receives a Network Summary LSA from an ABR, it does not run the SPF algorithm. Rather, it simply adds the cost of the route to the ABR and the cost included in the LSA. A route to the advertised destination, via the ABR, is entered into the route table along with the calculated cost. This behavior—depending on an intermediate router instead of determining the full route to the destination—is distance vector behavior. So, while OSPF is a link state protocol within an area, it uses a distance vector algorithm to find inter-area routes
ASBR
…
200.1.1.0/24
RIP
metric: 1562
ASBR : an autonomous system boundary routter ,is a router with an interface connected to an external network or to a different AS .an external network or autonomous system refers to an interface belonging to a different routing protocol such as RIP, an ASBR is reponsible for injecting route information learned by other routing protocols into OSPF .
ASBR Summary LSAs are also originated by ABRs. ASBR Summary LSAs are identical to Network Summary LSAs except that the destination they advertise is an ASBR , not a network. The command show ip ospf database asbr-summary is used to display ASBR Summary LSAs , Note in the illustration that the destination is a host address, and the mask is zero; the destination advertised by an ASBR Summary LSA will always be a host address because it is a route to a router.
Autonomous System External LSAs, or External LSAs, are originated by ASBRs and advertise either a destination external to the OSPF autonomous system , AS External LSAs are the only LSA types in the database that are not associated with a particular area; external LSAs are flooded throughout the autonomous system .
Simple explanation for the AS external LSA
ASE : indicate the as external LSA
LS id : the AS external network that ASBR can reached
Adv rtr : ASBR router ID
Net mask the network mask for the destination network
Metric : the cost for the ASBR to the AS external destination network.
E type :1 indicate that this is a AS external path type 1
Simple explanation for the ASBR summary LSA
sumASB : indicate that this is a ASBR summary LSA
LS id : ABR router id
Adv rtr : the ASBR router id
Type 1 external paths are to destinations outside the OSPF autonomous system. When an external route is redistributed into any autonomous system, it must be assigned a metric that is meaningful to the routing protocol of the autonomous system. Within OSPF, the ASBR is responsible for assigning a cost to the external routes they advertise. Type 1 external paths have a cost that is the sum of this external cost plus the cost of the path to the ASBR
<RTC>display ospf routing
…
ASBR
metric: 1562
Type 2 external paths (E2) are also to destinations outside the OSPF autonomous system, but do not take into account the cost of the path to the ASBR. E2 routes provide the network administrator with the option of telling OSPF to consider only the external cost of an external route, disregarding the internal cost of reaching the ASBR. OSPF external routes are, by default, E2 paths.
Till now ,we should know that in the OSPF network, there are four type of route available for us: intra-area route ,inter-area route ,type 1 external paths (routes), type 2 external paths (routes) .
How may type of OSPF route type available in a OSPF network ?
A: 1
B: 2
C: 3
D: 4
What are the OPSF network route type available in the OSPF network ?
A: intra-area route ,inter-area route ,type 1 external paths (routes), type 2 external paths (routes)
B: point to point network ,point to multi-point network ,NBMA network and the broadcast network
Which of the following statement is true ,choose all apply ( )
A: DRother will form the adjacent with the DR and BDR
B: DRother will form the adjacent with the DR only.
C: DRother will form the adjacent with the DRother
D: DRother will never form the adjacent with the DRother
How many Router LSA will be generate by the a router in a point to point network within the one area at a time ?
A: 1
B: 2
C: 3
D: depending on the network topology
How may type of OSPF route type available in a OSPF network ?
A: 1
B: 2
C: 3
D: 4
What are the OPSF network route type available in the OSPF network ?
A: intra-area route ,inter-area route ,type 1 external paths (routes), type 2 external paths (routes)
B: point to point network ,point to multi-point network ,NBMA network and the broadcast network
area 0
ABR can perform the address summarization from one area to another area, and this can reduce the network summary LSA flooding and improve the network performances
20.1.0.0/20
OSPF can perform two types of address summarization: inter-area summarization and external route summarization. Inter-area summarization is, as the name implies, the summarization of addresses between areas; this type of summarization is always configured on ABRs..
In this figure, area 1 contains sixteen subnets: 20.1.0.0/24 through 20.1.15.0/24, and all these addresses can be represented with the single summary address 20.1.0.0/20 .
ASBR
OSPF
ASBR can perform the address summarization from Non-OSPF routing domain to OSPF routing and this can reduce the AS external LSA flooding and improve the network performances
20.1.0.0/24
20.1.1.0/24
20.1.2.0/24
20.1.15.0/24
RIP
20.1.0.0/20
External route summarization allows a set of external addresses to be redistributed into an OSPF domain as a summary address and is configured on ASBRs . ASBR can perform the address summarization from Non-OSPF routing domain to OSPF routing and this can reduce the AS external LSA flooding and improve the network performances
√
√
ABR router will stop the type 5 LSA to enter the stub area, all the traffic forward outside the OSPF routing domain will be using the default route generated by the ABR .
Type 3 LSA
Type 4 LSA
Type 5 LSA
×
×
In some Autonomous Systems, the majority of the link-state database may consist of AS-external-LSAs. An OSPF AS-external-LSA is usually flooded throughout the entire AS. However, OSPF allows certain areas to be configured as "stub areas". AS-external-LSAs are not flooded into/throughout stub areas; routing to AS external destinations in these areas is based on a (per-area) default only. This reduces the link-state database size, and therefore the memory requirements, for a stub area's internal routers.
In order to take advantage of the OSPF stub area support, default routing must be used in the stub area. This is accomplished as follows. One or more of the stub area's area border routers must advertise a default route into the stub area via summary-LSAs. These summary defaults are flooded throughout the stub area, but no further. (For this reason these defaults pertain only to the particular stub area). These summary default routes will be used for any destination that is not explicitly reachable by an intra-area or inter-area path (i.e., AS external destinations).
√
ABR will stop the type 3/4/5 LSA to enter total stub area except the one type 3 LSA: the default for route the traffic outside the total stub area
Type 3 LSA (default route)
×
×
×
For all total stub area ,ABR will stop the type 3/4/5 LSA to enter the total stub area only one exception : the ABR attached to the total stub area will generate a default route type 3 LSA and flooding in this total stub area ,the total stub area can reduce the LSA heavily. So in the total stub area ,there are type 1 / 2 LSAs and one type 3 default route LSA available in the LSDB .
area 0
NSSA area
NSSA area can import the Non-OSPF route into the OSPF routing domain using the type 7 LSA, type 7 LSA only flooding in the NSSA area and will be translate into type 5 LSA by the ABR
NSSA External LSAs are originated by ASBRs within not-so-stubby areas (NSSAs). NSSAs are described in the following section. An NSSA External LSA (type 7 LSA )is almost identical to an AS External LSA, as the section on OSPF packet formats shows. Unlike AS External LSAs, which are flooded throughout an OSPF autonomous system, NSSA external LSAs are flooded only within the not-so-stubby area in which it was originated. At NSSA area border routers will translate selected type 7 LSA from the NSA into type 5 LSA,these type 5 lsa wil be flooded to all type 5 capable areas .
Virtual Link
area 0
area 1
area 2
OSPF require all non backbone area must connect to the backbone directly or logically ,for a area not connect to the OSPF backbone directly, a virtual link must be configure between two ABR router in the non backbone area.
Virtual Links
The OSPF backbone is the special OSPF Area 0 ,The OSPF backbone always contains all area border routers. The backbone is responsible for distributing routing information between non-backbone areas. The backbone must be contiguous. However, it need not be physically contiguous; backbone connectivity can be established/maintained through the configuration of virtual links.
Virtual links can be configured between any two backbone routers that have an interface to a common non-backbone area. Virtual links belong to the backbone. The protocol treats two routers joined by a virtual link as if they were connected by an unnumbered point-to-point backbone network. On the graph of the backbone, two such routers are joined by arcs whose costs are the intra-area distances between the two routers. The routing protocol traffic that flows along the virtual link uses intra-area routing only.

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07.2 OSPF Routing Protocol