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• Neighbor - connected (adjacent) router that is running OSPF with the adjacent interface assigned to the same area. Neighbors are found by Hello packets (unless manually configured).
• Adjacency - logical connection between a router and its corresponding DR and BDR. No routing information is exchanged unless adjacencies are formed.
• Link - link refers to a network or router interface assigned to any given network.
• Interface - physical interface on the router. The interface is considered as a link when it is added to OSPF. Used to build link database.
• LSA - Link State Advertisement, data packet contains link-state and routing information, that is shared among OSPF Neighbors.
• DR - Designated Router, chosen router to minimize the number of adjacencies formed. The option is used in broadcast networks.
• BDR -Backup Designated Router, hot standby for the DR. BDR receives all routing updates from adjacent routers, but it does not flood LSA updates.
• Area - areas are used to establish a hierarchical network.
• ABR - Area Border Router, router connected to multiple areas. ABRs are responsible for summarization and update suppression between connected areas.
• ASBR - Autonomous System Boundary Router, router connected to an external network (in a different AS). If you import other protocol routes into OSPF from the router it is now considered ASBR.
• NBMA - Non-broadcast multi-access, networks allow multi-access but have no broadcast capability. Additional OSPF neighbor configuration is required for those networks.
• Broadcast - Network that allows broadcasting, for example, Ethernet.
• Point-to-point - Network type eliminates the need for DRs and BDRs
• Router-ID - IP address used to identify OSPF router. If the OSPF Router-ID is not configured manually, a router uses one of the IP addresses assigned to the router as its Router-ID.
• Link State - The term link-state refers to the status of a link between two routers. It defines the relationship between a router's interface and its neighboring routers.
• Cost - Link-state protocols assign a value to each link called cost. the cost value depends on to the speed of the media. A cost is associated with the outside of each router interface. This is referred to as interface output cost.
• Autonomous System - An autonomous system is a group of routers that use a common routing protocol to exchange routing information.

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A distinctive feature of OSPF is the possibility to divide AS into multiple routing Areas which contains contain their own set of neighbors.
Imagine a large network with 300+ routers and multiple links between them. Whenever link flaps or some other topology change happens in the network, this change will be flooded to all OSPF devices in the network resulting in a quite heavy load on the network and even downtime since network convergence may take some time for such a large network. 

Introduction The introduction of areas allows for better resource management since topology change inside one area is not flooded to other areas in the network. The concept of areas enables simplicity in network administration as well as routing summarization between areas significantly reducing the database size that needs to be stored on each OSPF neighbor.

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Link-state routing protocols are distributing, replicating database that describes the routing topology. The link-state protocol's flooding algorithm ensures that each router has an identical link-state database and the routing table is calculated based on this database.

After all the steps above are completed link-state database on each neighbor contains full routing domain topology (how many other routers are in the network, how many interfaces routers have, what networks link between router connects, cost of each link, and so on).

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• Hello packet - used to discover OSPF neighbours neighbors and build adjacencies.
• Database Description (DD) - check for Database synchronization between routers. Exchanged after adjacencies are built.
• Link-State Request (LSR) - used to request up-to-date pieces of the neighbourneighbor's database. Out of date parts of routes routing database are determined after DD exchange.
• Link-State Update (LSU) - carries a collection of specifically requested link-state records.
• Link-State Acknowledgment (LSack) - is used to acknowledge other packet types that way introducing reliable communication.

Neighbors Discovery

OSPF discovers potential neighbours neighbors by periodically sending Hello packets out of configured interfaces. By default Hello packets are sent out with 10 second interval which can be changed by setting hello interval in OSPF interface settings. Router The router learns the existence of a neighbouring neighboring router when it receives the neighbourneighbor's Hello in return with matching parameters.

The transmission and reception of Hello packets also allows allow a router to detect the failure of the neighbourneighbor. If Hello packets are not received within Dead interval (which by default is 40s) router starts to route packets around the failure. Hello protocol ensures that the neighbouring neighboring routers agree on the Hello interval and Dead interval parameters, preventing situations when not in time received Hello packets mistakenly bring the link down.

On each type of network segment Hello protocol works a little differentdifferently. It is clear that on point-to-point segments only one neighbour neighbor is possible and no additional actions are required. However, if more than one neighbour neighbor can be on the segment additional actions are taken to make OSPF functionality even more efficient.

Two routers do not become neighbours neighbors unless the following conditions are met.

• Two-way communication between routers is possible. Determined by flooding Hello packets.
• Interface The interface should belong to the same area;
• Interface The interface should belong to the same subnet and have the same network mask , unless it has network-type configured as point-to-point;
• Routers should have the same authentication options, and have to exchange the same password (if any);
• Hello and Dead intervals should be the same in Hello packets;
• External routing and NSSA flags should be the same in Hello packets.

Discovery on Broadcast Subnets

Attached The attached node to the broadcast subnet can send a single packet and that packet is received by all other attached nodes. This is very useful for auto-configuration and information replication. Another useful capability in broadcast subnets is multicast. This capability allows to send sending a single packet which will be received by nodes configured to receive multicast packetpackets. OSPF is using this capability to find OSPF neighbours neighbors and detect bidirectional connectivity.

Consider the Ethernet network illustrated in the image below.

!!!!!!bilde!!!!!! OSPF Broadcast network

Each OSPF router joins the IP multicast group AllSPFRouters (224.0.0.5), then the router periodically multicasts its Hello packets to the IP address 224.0.0.5. All other routers that joined the same group will receive a multicasted Hello packet. In that way, OSPF routers maintain relationships with all other OSPF routers by sending a single packet instead of sending a separate packet to each neighbour neighbor on the segment.

This approach has several advantages:

Automatic neighbour neighbor discovery by multicasting or broadcasting Hello packets. Less bandwidth usage compared to other subnet types. On the broadcast segment, there are n*(n-1)/2 neighbor relations, but those relations are maintained by sending only n Hellos. If broadcast has the multicast capability, then OSPF operates without disturbing non-OSPF nodes on the broadcast segment. If the multicast capability is not supported all routers will receive broadcasted Hello packet even if the node is not an OSPF router.

Discovery on NBMA Subnets

Nonbroadcast multiaccess (NBMA) segments similar to broadcast supports more than two routers, the only difference is that NBMA do does not support a data-link broadcast capability. Due to this limitation, OSPF neighbours neighbors must be discovered initially through configuration. On RouterOS NBMA configuration is possible in/routing ospf nbma-neighbor menu. To reduce the amount of Hello traffic, most routers attached to the NBMA subnet should be assigned Router Priority of 0 (set by default in RouterOS). Routers that are eligible to become Designated Routers should have priority values other than 0. It ensures that during the election of DR and BDR Hellos are sent only to eligible routers.

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On PTMP subnets Hello protocol is used only to detect active OSPF neighbours neighbors and to detect bidirectional communication between neighboursneighbors. Routers on PTMP subnets send Hello packets to all other routers that are directly connected to them. Designated Routers and Backup Designated routers Routers are not elected on Point-to-multipoint subnets.

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Before database synchronization can begin, a hierarchy order of exchanging information must be established, which determines which router sends Database Descriptor (DD) packets first (Master). Master router is elected based on highest priority and if priority is not set then router ID will be used. Note that it is a router priority-based relation to arrange arranging the exchanging data between neighbours neighbors which does not affect DR/BDR election (meaning that DR does not always have to be Master).

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Link-state Database synchronization between OSPF routers are is very important. Unsynchronized databases may lead to incorrectly calculated routing table tables which could cause routing loops or black holeholes.

There are two types of database synchronizations:

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When the connection between two neighbours neighbors first come comes up, initial database synchronization will happen. OSPF is using explicit database download when neighbour neighbor connections first come up. This procedure is called Database exchange. Instead of sending the entire database, the OSPF router sends only its LSA headers in a sequence of OSPF Database Description (DD) packets. Router The router will send the next DD packet only when the previous packet is acknowledged. When an entire sequence of DD packets has been received, the router knows which LSAs it does not have and which LSAs are more recent. The router then sends Link-State Request (LSR) packets requesting desired LSAs, and the neighbour neighbor responds by flooding LSAs in Link-State Update (LSU) packets. After all the updates are received neighbours neighbors are said to be fully adjacent.

Reliable flooding is another database synchronization method. It is used when adjacencies are already established and the OSPF router wants to inform other routers about LSA changes. When the OSPF router receives such Link State Update, it installs new LSA in the link-state database, sends an acknowledgement acknowledgment packet back to the sender, repackages LSA in new LSU, and sends it out all interfaces except the one that received the LSA in the first place.

OSPF determines if LSAs are up to date by comparing sequence numbers. Sequence numbers start with 0×80000001, the larger the number, the more recent the LSA is. Sequence A sequence number is incremented each time the record is flooded and neighbour the neighbor receiving the update resets the Maximum age timer. LSAs are refreshed every 30 minutes, but without a refresh, LSA remains in the database for the maximum age of 60 minutes.

Databases are not always synchronized between all OSPF neighboursneighbors, OSPF decides whether databases needs need to be synchronized depending on the network segment, for example, on point-to-point links databases are always synchronized between routers, but on Ethernet networks databases are synchronized between certain neighbour neighbor pairs.

Synchronization on Broadcast Subnets

On the broadcast segment there are n*(n-1)/2 neighbor relations, it will be a huge amount of Link State Updates and Acknowledgements sent over the subnet if the OSPF router will try to synchronize with each OSPF router on the subnet.

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This problem is solved by electing one Designated Router and one Backup Designated Router for each broadcast subnet. All other routers are synchronizing and forming adjacencies only with those two elected routers. This approach reduces amount the number of adjacencies from n*(n-1)/2 to only 2n-3.

Image The image on the right illustrates adjacency formations on broadcast subnets. Routers R1 and R2 are Designated Router and Backup Designated router routers respectively. For example, R3 wants to flood Link State Update (LSU) to both R1 and R2, a router sends LSU to IP multicast address AllDRouters (224.0.0.6) and only DR and BDR listens to this multicast address. Then Designated Router sends LSU addressed to AllSPFRouters, updating the rest of the routers.

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DR and BDR routers are elected from data received in the Hello packet. The first OSPF router on a subnet is always elected as Designated Router, when a second router is added it becomes Backup Designated Router. When existing DR or BDR fails new DR or BDR is elected taking to take into account configured router priority. Router The router with the highest priority becomes the new DR or BDR.

Being Designated Router or Backup Designated Router consumes additional resources. If Router Priority is set to 0, then the router is not participating in the election process. This is very useful if certain slower routers are not capable of being DR or BDR.

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Database synchronization on NBMA networks are is similar as to on broadcast networks. DR and BDR are elected, databases initially are exchanged only with DR and BDR routers and flooding always goes through the DR. The only difference is that Link State Updates must be replicated and sent to each adjacent router separately.

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• type 1 - (Router LSA) Sent by routers within the Area, including the list of directly attached links. Does not cross the ABR or ASBR.
• type 2 - (Network LSA) Generated for every "transit network" within an area. A transit network has at least two directly attached OSPF routers. Ethernet is an example of a Transit Network. A Type 2 LSA lists each of the attached routers that make up the transit network and is generated by the DR.
• type 3 - (Summary LSA) The ABR sends Type 3 Summary LSAs. A Type 3 LSA advertises any networks owned by an area to the rest of the areas in the OSPF AS. By default, OSPF advertises Type 3 LSAs for every subnet defined in the originating area, which can cause flooding problems, so it´s a good idea to use a manual summarization at the ABR.
• type 4 - (ASBR-Summary LSA) It announces the ASBR address, it shows “where” the ASBR is located, announcing it´s its address instead of it´s its routing table.
• type 5 - (External LSA) Announces the Routes learned through the ASBR, is flooded to all areas except Stub areas. This LSA divides in into two sub-types: external type 1 and external type 2.
• type 6 - (Group Membership LSA) This was defined for Multicast extensions to OSPF and is not used by RouterOS.
• type 7 - type 7 LSAs are used to tell the ABRs about these external routes imported in into the NSSA area. Area Border Router then translates these LSAs to type 5 external LSAs and floods as normal to the rest of the OSPF network
• type 8 - External Attributes LSA (OSPFv2) / link-local LSA (OSPFv3)
• type 9 - Link-Local Scope Opaque (OSPFv2) / Intra Area Prefix LSA (OSPFv3). LSA of this type is not flooded beyond the local (sub)network.
• type 10 - Area Local Scope Opaque. LSA of this type is not flooded beyond the scope of its associated area.
• type 11 - Opaque LSA which is flooded throughout the AS (scope is the same as type 5). It is not flooded in stub areas and NSSAs.

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When link-state databases are synchronized OSPF routers are able to calculate the routing table.
Link state database describes the routers and links that interconnect them and are appropriate for forwarding. It also contains the cost (metric) of each link. This metric is used to calculate the shortest path to the destination network.
Each router can advertise a different cost for the router's own link direction, making it possible to have asymmetric links (packets to destination travels travel over one path, but response travels a different path). Asymmetric paths are not very popular, because it makes it harder to find routing problems.
The Cost in RouterOS is set to 10 on all interfaces by default. Value can be changed in ospf OSPF interface configuration menu, for example, to add ether2 interface with a cost of 100:

/routing ospf interface add interface=ether2 cost=100

The cost of an interface on Cisco routers is inversely proportional to the bandwidth of that interface. Higher A higher bandwidth indicates a lower cost. If similar costs are necessary on RouterOS, then use the following formula:

Cost = 100000000/bw in bps.

OSPF router is using Dijkstra's Shortest Path First (SPF) algorithm to calculate the shortest path. The algorithm places router at the root of a tree and calculates the shortest path to each destination based on the cumulative cost required to reach the destination. Each router calculates its own tree even though all routers are using the same link-state database.

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Assume we have the following network. Network The network consists of 4(four) routers. OSPF costs for outgoing interfaces are shown near the line that represents the link. In order to build the shortest-path tree for router R1, we need to make R1 the root and calculate the smallest cost for each destination.

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