What is OSPF (Open Shortest Path First) Protocol

Open Shortest Path First Protocol

OSPF is used inside the networks as RIP, the operation is very simple. Each router knows the nearby routers and the addresses each router has. In addition, each router (measured on routers) knows how far each router is. So when you need to send a packet, you send it the same way you have to do fewer skips.

For example, a router with a local network with one workstation, and a router with three network connections (A) with a fast 48Mbps square relay network and a 64Kbps ISDN (B) line. Three packets from the local network to A and B to two packets go to W. The packet passes through B regardless of the saturation of the line or the bandwidth of the line.

OSPF Types

Undoubtedly, OSPF is a complex protocol and requires a lot of work to understand how it works, and makes many practices to master. One of the most important concepts in OSPF is the design and operation of different areas, which is quite confusing when this protocol is known.

To explain how each works, it is necessary to know the types of LSA (Link State Ads) that the OSPF uses to communicate between neighbors and transmit routing information between them.

Type 1 (LSA Router)

Each router in area X sends a type 1 LSA to its neighbors. This LSA never leaves the domain to which it belongs and contains the Router ID of the sender and all connections that connect it.

Type 2 (Network LSA)

Sent by the DR (Private Router) within the network. Tells others the networks and masks it is connected to. This LSA never leaves the area it corresponds to. So, an ABR does not transmit to another region.

Type 3 (Summary LSA)

They are sent by an ABR to transfer information from one region to another. OSPF calls them a “summary”.

Type 4 (ASBR-Summary LSA)

Represents an ASBR (Autonomous System Border Router)

Type 5 (External LSA)

Represents an external route redistributed from another protocol within the OSPF (Ex: EIGRP). The ASBR takes the route from the external protocol and transmits them as type 5 to all internal areas except Stub type.

Type 7

OSPF rules say that redistribution is required only in a Spine zone (Area 0). In an NSSA domain, a router with a connection to another external routing protocol (eg RIP) can be connected and the ASBR sends these networks in type 7 format so that the ABR receives them and redistributes them as type 5.

Type 1 and 2 LSAs are found in all areas and are not shipped anywhere they belong. Other LSAs are sent between fields, depending on the function they perform.

Field Types

  • Standard
  • Backbone (Area 0)
  • Stub Area
  • Totally Stubby Area
  • Not-so-stubby Area (NSSA)
  • Totally Stubby NSSA

Standard

This is the default field and allows links to be updated, summary paths, and external paths.

Backbone (Area 0)

OSPF is the main area of ​​topology. It must be present and all others must depend on it. The field is labeled 0 and has the same properties as a standard field.

Stub Area

Such a field does not accept information about external routes to the autonomous system (redistribution), such as paths from non-OSPF sources. If routers need to route to networks outside the autonomous OSPF system, they use a default route (0.0.0.0/0) sent by ABR to other internal routers in the Stub area. ASBR is not allowed in this area (unless ABR is also an ASBR)

Totally Stubby Area

This area belongs to Cisco and does not accept routes from external autonomous systems (redistribution) or abstracts from other internal regions of the autonomous system. As with the Stub fields, ABRs send a default route for all external and SUMMARY routes (this is the difference with Stub). ASBR is not allowed in this area (unless ABR is also an ASBR)

Not-so-stubby Area (NSSA)

Almost the worst name in the world chose this name. There are type 7 LSAs in this field, which are similar to the Stub field because they do not accept information from external routes to the autonomous system (OSPF world) and replace them with a default route originating from ABR. However, the difference is that the NSSA accepts an ASBR directly connected to another routing protocol (eg, RIP, EIGRP, etc.). The NSSA ASBR transmits pathways within the site as LSA 7, and the corresponding ABR converts them to type 5 for normal treatment.

Totally Stubby NSSA

If the old is almost the worst name, make sure it is the worst. Fully Stubby Not Left-Handed Area or Fully Stubby NSSA is a proprietary Cisco area that functions in the same way as the Fully Stubby Area, does not allow external or summary routes, but permits an ASBR such as NSSA.

Related Posts

What is EIGRP?
How to Install Packet Tracer
Download Packet Tracer
What is RIP?
What is RIPv2?

Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_ospf/configuration/xe-16/iro-xe-16-book/iro-cfg.html