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why OSPF / LSA

Hi experts

               i am reading some doc about OSPF and i am wondering why they have the idea to make different types of LSA? .   is it only to be able to filter the ip routing table from external routes ?

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Hall of Fame Super Silver

Re: why OSPF / LSA

Hello Ohassairi,

>> is it only to be able to filter the ip routing table from external routes ?

the reasons for using multiple types of LSA data structures come from the hierarchical design of OSPF in a multi area domain.

LSA type1 (Router LSA) and LSA type 2 (Network LSA) have detailed topological information that allow to draw the complete topology within an area, but they are confined within the area they belong to.

All areas have to connect to the 0.0.0.0 backbone area through the ABR nodes that are member of area 0 and at least another area at the same time (on different L3 interfaces).

The inter area routing is performed in a distance vector way allowing a router to choice the best ABR exit path based on the internal cost to reach the ABR added to the cost of the summary routes generated by the ABR node itself and injected in the non zero area X.

These LSA type 3 do not contain topological details about distant areas but just IPv4 prefixes and an associated metric.

This allows for scalabilty of OSPF, because a change in area Y is not propagated in area X ( unless the change is related to an external route).

At the end external routes are propagated to all non stub areas unchanged as LSA type 5. Each router uses ASBR summary route ()LSA type 4) as a way to check that the ASBR node is actually present and active in the OSPF domain. It is a check to decide if an LSA type 5 is usable or not.

ABR nodes perform LSA type 3 and LSA type 4 regeneration as needed at area boundaries.

ABR nodes can also perform route filtering of internal routes ( LSA type 3 level) in a selective way (via the area filter-list command in router ospf mode) at area boundaries.

LSA type 5 for external routes cannot be filtered selectively: or an area gets all of them (standard area) or an area gets none of them (form of stub area including NSSA for the direction from backbone to area).

The NSSA is a special type of area that:

-does not receive external routes from backbone

- it can host an ASBR that generates LSA type 7

- some NSSA external routes might be confined to the NSSA area itself.

The ABR nodes  for the NSSA area Z check the propagation bit P in the LSA type 7 and generally convert them in standard LSA type 5 that are sent in the backbone area. (Actually ony one of the ABR nodes do the conversion for efficiency purposes).

Hope to help

Giuseppe

2 REPLIES
Hall of Fame Super Silver

Re: why OSPF / LSA

Hello Ohassairi,

>> is it only to be able to filter the ip routing table from external routes ?

the reasons for using multiple types of LSA data structures come from the hierarchical design of OSPF in a multi area domain.

LSA type1 (Router LSA) and LSA type 2 (Network LSA) have detailed topological information that allow to draw the complete topology within an area, but they are confined within the area they belong to.

All areas have to connect to the 0.0.0.0 backbone area through the ABR nodes that are member of area 0 and at least another area at the same time (on different L3 interfaces).

The inter area routing is performed in a distance vector way allowing a router to choice the best ABR exit path based on the internal cost to reach the ABR added to the cost of the summary routes generated by the ABR node itself and injected in the non zero area X.

These LSA type 3 do not contain topological details about distant areas but just IPv4 prefixes and an associated metric.

This allows for scalabilty of OSPF, because a change in area Y is not propagated in area X ( unless the change is related to an external route).

At the end external routes are propagated to all non stub areas unchanged as LSA type 5. Each router uses ASBR summary route ()LSA type 4) as a way to check that the ASBR node is actually present and active in the OSPF domain. It is a check to decide if an LSA type 5 is usable or not.

ABR nodes perform LSA type 3 and LSA type 4 regeneration as needed at area boundaries.

ABR nodes can also perform route filtering of internal routes ( LSA type 3 level) in a selective way (via the area filter-list command in router ospf mode) at area boundaries.

LSA type 5 for external routes cannot be filtered selectively: or an area gets all of them (standard area) or an area gets none of them (form of stub area including NSSA for the direction from backbone to area).

The NSSA is a special type of area that:

-does not receive external routes from backbone

- it can host an ASBR that generates LSA type 7

- some NSSA external routes might be confined to the NSSA area itself.

The ABR nodes  for the NSSA area Z check the propagation bit P in the LSA type 7 and generally convert them in standard LSA type 5 that are sent in the backbone area. (Actually ony one of the ABR nodes do the conversion for efficiency purposes).

Hope to help

Giuseppe

Silver

Re: why OSPF / LSA

thanks Giuseppe. i liked the summary you did.

you said type 5 are propagated unchanged. that's why we need type 4. ok.

why they are not propagated as type 3 simply?

you may say to let the router know the asbr. but why it is important for a router to know the id of the asbr?

you may say also to let the route know if this is an external route. but in this case we can just add a flag in the type 3 to indicate if it is an external route or just coming from an other area.

thanks

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