Routing with OSPF – Part 6 - :
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OSPF Network Types

There are some OSPF network types used to design OSPF network. Some of these types are general standard and some of them are Cisco proprietary. These networks types are mentioned below:

• Broadcast Multiaccess (Cisco Proprietary)

• Point-to-Point (Cisco Proprietary)

• Point-to-Multipoint Broadcast (Standard)

• Point-to-Multipoint Nonbroadcast (Cisco Proprietary)

• Non Broadcast Multi Access(NBMA) (Standard)

OSPF Virtual Links

Virtual links are used for connecting areas to the backbone area, that is not directly connected to the backbone area. For this, there is one transit area that helps to connect the area to the backbone area. Both end of the transit area is configured with virtual-links. And this area can not be a stub area.

OSPF (Open Shortest Path First) Virtual Link

In the above picture, Area 1 is the transit area. And the virtual-link will be configured between R2 and R3 routers to connect Area 2 to the backbone area, Area 0.

This is a temporary solution to connect one area to the backbone area. So it is not recommended to use this solution as a permanent soultion.

OSPF Summarization

In OSPF, summarization can be done in two different location. These locations are ABRs and ASBRs.
In an ABR, the Type 3 LSA is summary LSAs and this LSA produced by the below command:

Router(config-router)#area area-id range address mask [advertise-not advertise] cost cost

Here, the not-advertised command is used to mention the routes that you do not want to advertised other areas.

The other summarization is done in ASBR. The external routes is advertised by this summary routes. To configure an ASBR for summarization you an use the below command:

Router(config-router)#summary-address address mask [not- advertise] [tag tag]

Here, the not-advertised command has the same function like above, but this time the networks do not exist in Type 5 LSAs. Lastly, Tag value is used for redistribution with route maps.

With this article series, we talked about the common OSPF topic. In the following article series you can find articles about the detailed OSPF features. So, keep on, ;)

Routing with OSPF – Part 1
Routing with OSPF – Part 2 (OSPF Adjacency)
Routing with OSPF – Part 3 (OSPF Packet Types)
Routing with OSPF – Part 4 (OSPF LSA Types)
Routing with OSPF – Part 5 (OSPF Area Types)
Routing with OSPF – Part 6 (OSPF Network Types, Virtual Links, Summarization)

OSPF Single Area Configuration on Cisco

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About the Author
Gokhan Kosem is a telecommunation and network engineer. His ambition to IP networks and end-to-end system installation made him to prepare this web-site. By sharing his experiences about various networking protocols beside different system installation experiences and Cisco, Juniper, Alcatel-Lucent devices configurations, he is aimed to be helpful for his collegues in all over the world. He is currently lives in Istanbul, Turkey.

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