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Ospf lsa types

jcarrillo26

Hello everyone,

first off i would like to thank everyone that reads this post. I am having a difficult time understanding the types of lsa in ospf. I understand that lsa are used for neighboring routers to learn about the link state database to build the topology. The dr sends this information flooding the network and the bdr is used if the dr goes down, it is a back up. The administrative distance of ospf is 110 and the routing protocol is based on cost (bandwidth). There are certain elements that ospf uses to build a relationship like they have to have the same area, the hello and dead intervals have to be the same, on the same contiguous addressing space, unique router ids. The router id is the highest loopback address if configured and if not it goes with the highest ip address configured. I guess i am having a hard time with the fact of all the different types used in different areas. And are there only 3 types of lsas in a single area versus a multiple area config.

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Comments

EdTheLad

If you have a single area topology, there are only 2 LSAs used, type 1 and type 2, if within that single area you don't have any multiaccess networks, then you only have type 1 LSAs. Type 1 LSA will describe all ospf enabled interfaces on the local router, type 2 is sent by the DR describing all other routers connected to the multiaccess network.

If you have multiple areas, the ABR which is a router connected to Area 0 and at least one other Area, it will convert all the type 1 LSA's known in one area into type 3 summary LSAs and send them to the other area. All type 1 LSAs sent by the ABR will have the B bit set so that every other router in the area will know that this area router is an ABR, hence how the command "show ip ospf border-routers" works.

If you are performing redistribution on a router, that router is know as an ASBR, it will generate type 1 LSAs with the e bit set, letting other routers within the area know its an ASBR, when the ABR receives a type 1 LSA with the e-bit set, it will generate a type 4 LSA into all other attached areas. The routes redistributed will be type 5 LSA's, they get flooded unmodified throughout all other areas. To reduce type 5 flooding you can configure an area as a stub area.

Within a stub area if you want to redistribute external routes, you need to make the stub area a NSSA, this will block the type 5 LSAs from the backbone and allow you to flood type 7 LSAs throughout the NSSA area. When the ABR receives a type 7 LSA it will convert it to type 5 before flooding it to the backbone.

That's a quick overview on how the lsa's work!

jcarrillo26

thanks i really appreciate the replay the lsa type got really confusing for me. i know the general stuff, so in general basically all the other different types just send different information to the ABR and the ASBR regarding the information that is taking place in each router that is in the area right and all the areas in one way or another tie back to the backbone?

EdTheLad

All the routers within an area flood type 1 & 2 ( not gonna mention type 2 anymore, as its only if DR is present in the Area ) , so every router within an area has exactly the same LSAs as every other router. If you wanna connect 2 area's together, one of the areas needs to be the backbone. A router that has an active interface in Area 0 is an ABR, it will have 2 databases, these databases are identical to the database on every other router in the respective area. So you dont really look at routers within an area sending lsa's to the ABR, the ABR just receives the flooded LSAs the same as any other router in the area. It just so happens to perform an additional function, converting all received type 1 LSAs into type 3 LSAs, flooding them into the backbone and any other directly attached area.
If you have two areas 1 and 2, and these are interconnected by a router, this router is not an ABR as it doesn't have backbone connectivity. They will not convert the type 1 to type 3, so routes will not be exchanged between areas. This is why the backbone area 0 is so important. This means all inter-area routes are advertised by the backbone, which means all inter-area traffic must flow through the backbone.

jcarrillo26

so the area boarder router should be in the backbone (area 0) and the ABR has as many databases as there are areas? so type 1 and type 3 are exchanged with the ABR to update the information that is advertised in topology per area?

tomtom1

jcarrillo26 wrote: »

so the area boarder router should be in the backbone (area 0) and the ABR has as many databases as there are areas? so type 1 and type 3 are exchanged with the ABR to update the information that is advertised in topology per area?

The definition of an ABR is a router with 1 (OSPF enabled) interface in area 0 and one in another area. To my knowledge, the ABR keeps a separate database for each area it is connected to.

jcarrillo26

i see its like a spoke and hub kind of topology that keeps record of all the areas connected to it.

EdTheLad

Yes, this design is prevent routing loops, the type 3 lsa's are essentially distant vector updates, the routers internal to an area are trusting routing info received from the ABR.

jcarrillo26

hey thank you so much i just watched a video on multiarea ospf and it really cleared a lot up for me. i do plan on going to the ccnp level stuff so i do like to have a clear understanding of what it is going to lead on to.