by danang.wijanarko@gmail.com
In particular purpose you will need OSPF for your specific reason. There are many other AS routing method, but in this article i just interested with OSPF.
LSA
The get the feeling of OSPF it is better to understand about its LSA. LSA (Link State Advertisement) is the basic communication means of the OSPF routing protocol. The important type used in this article are:
An Area Border Router (ABR) takes information it has learned on one of its attached areas and it can summarize it (but not by default) before sending it out on other areas it is connected to.
Generated by ABR or ASBR.
- Type 5: General External LSA
This LSA contain information imported into OSPF network from other routing processes (RIP for example). It is flooded to all areas (except stub areas).
Generated by ASBR
- Type 4: ASBR Summary External LSA
It is similiar with Type 5 function, but it is not flooded to all areas, but instead it makes the summary. It is needed because Type 5 flooded to all areas.
Generated by ASBR
- Type 7: External LSA that are mumbling in NSSA only
Routers in NSSA do not receive LSA Type 5 and 4 from other ABR, but are allowed to send Type 5 and 4 for redistribution.
Generated by ASBR.
They use this LSAs to tell the ABRs about these external routes, which the ABR then translates to type 5 external LSAs and floods as normal to the rest of the OSPF network.
Area
Areas are logical groupings of hosts and networks, including their routers. An area maintains a link state database whose information may be summarized towards the rest of the network by the interconnecting router. Thus, the topology of an area is unknown outside of the area. This reduces the amount of routing traffic between parts of an autonomous system.
Type of area:
Backbone Area / Area 0 is the top level of the OSPF heirarchy. All other areas should border area 0 (what I'm saying is, if you build an OSPF network where areas do not border area 0, you're on your own)
Looking on the picture above, the A, B, C, D, and E are backbone routers that are part of two areas, area 0, and another other area. This makes them acting as border routers in OSPF parlance.
This is an area which
1. Receive inter-area routes
2. Does not receive external routes but with 1 exception, it is still receive the default route
Will not accept external routes (type 5 LSAs); type 5 LSAs are replaced by a default route. It is usually the area where normal PC or nodes connected.
- Totally Stubby Area: Not Standard - Cisco
It is similar to a stub area, with 1 more rule added, that it is not allowed to send its own summary routes.
Will not accept LSAs of type 3, 4, or 5; routes are replaced by the ABR with a default route;
- Not So Stubby Area (NSSA): Standard
NSSA ASBR TYPE 7, which can exist only in an NSSA area. An NSSA ASBR generates this LSA, and an NSSA ABR router translates it into type 5 LSA which gets propagated into the OSPF domain.
We could say that NSSA is a stub areas which contain one or more ASBRs; ASBRs in a NSSA generate type 7 LSAs which are then converted to type 5 by the receiving ABR.
Router Type
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Router Type
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Description
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Internal Router
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A router with all interfaces connected to the same area. Internal routers each have a single LSDB.
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Area Border Router (ABR)
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A router with interfaces connected to different areas. ABRs have multiple LSDBs, one for each attached area.
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Backbone Router
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A router with an interface on the backbone area. This includes all ABRs and internal routers of the backbone area.
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AS Boundary Router (ASBR)
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A router that exchanges routes with sources outside of the OSPF AS. ASBRs advertise external routes throughout the OSPF AS.
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Some picture to give it more detail
ospf_area_2
- d
