If I understood correctly, you are causing IA routes not to be added to the routing tables of the HUBs. These routes to the spokes are in the OSPF database but not in the routing tables.
Areas 1, 2, and 3 are configured as totally stub areas, so no type 3 LSAs will be seen in those areas. However, if you have also caused the HUBs to block any IA routes, so they don’t have any such routes, then there is no way, via OSPF that these HUBs will be able to know where to send traffic for networks found on the spokes.
If the routes to the spokes are not in the routing table via OSPF, they need to get into the routing table via another method. I’m not sure how that can be done.
If I have misunderstood your question, please let me know so I can clarify.
As you know, a Type 5 LSA will contain information about routes that are redistributed from other routing processes. As you can see in the diagram in the lesson, Type 5 LSAs will be generated by the ASBR and will have the information of the external prefixes, in this case 5.5.5.0/24.
This LSA will go to all areas, including the area in which the ASBR exists, which is Area 2. Now when R4 receives those Type 5 LSAs, it will be informed that the advertising router of 5.5.5.0/24 is R1. Since R1 is not in Area 51, we don’t know what the router ID is of R1 or how to get to it. How do we get this information? Using Type 4 LSAs.
As seen above, a Type 4 LSA is not required in Area 2 because routers in the same area as the ASBR know how to get to it. The routers in other areas, such as R4 in Area 51 must know the router ID of the ASBR associated with this particular external prefix learned via a Type 5 LSA. This of course is done with a Type 4 LSA.
Now one can say, why not combine both of these LSAs into a single one? Well, OSPF engineers found that it was more efficient to use two separate LSAs and to keep these functions as separate operations. You may find that inefficient, but think about it this way.
R1 does not generate a Type 5 and a Type 4 LSA and a Type 1 LSA. If it did that would indeed be redundant. What it does do however is, like all OSPF routers, it generate a Type 1 LSA for its routes (with a flipped bit) and the ABR R2 will generate the Type 4 LSA. R1 as an ASBR will also generate a Type 5 LSA for the external routes.
Keep in mind that OSPF routers will routinely combine multiple LSAs into a single update packet, so by having multiple LSA types, it doesn’t really cause inefficiency in the number of OSPF messages exchanged. It simply makes the operations discrete from one another.
I have few doubts for type4 LSA as per below statement
" This makes R1 an ASBR (Autonomous System Border Router). What happens is that R1 will flip a bit in the router LSA to identify itself as an ASBR. When R2 who is an ABR receives this router LSA it will create a type 4 summary ASBR LSA and flood it into area 0."
On Router LSA of ASBR , The E bit will be flipped , the ABR will receive it and generate type 4 . How this thing will work in NSSA . Does the ASBR in NSSA will do the same and NSSA ABR will generate type4 and also convert type7 to type5 ?
The functionality of the flipped E-bit occurs only in the Type 1 LSA. This means that in an NSSA situation, the ASBR will generate a type 1 LSA which all OSPF routers generate. In there the E-bit will be flipped, and thus the ABR will receive this and send out a Type 4 LSA into Area 0.
But the ASBR will also generate a Type 7 LSA to inform the rest of the network of its externally connected networks. This is then translated by the ABR into a Type 5 LSA into Area 0.
I have doubt in ASBR Router, Request you to clear that please,
in the below which i have attached a screenshot, can you let me know exactly
which is the ASBR Router? &
which router generate the Summary ASBR LSA.?
in first point you have told AS R1 is redistributing from rip to Ospf. this makes R1 as an ASBR Router.
in Second Point you have told R2 will generate the ASBR LSA. Hence it is the ASBR Router.
If R2 generate the ASBR LSA. R2 should be an ASBR Router? isnt it? & R2 is already an ABR Router. Again how it will become an ASBR Router.
is it possible one router act as both ABR & ASBR Router in the same area/ network.
Request you to understand my concern and help me as earliest.
As the name Autonomous System Border Router (ASBR) suggests, it is defined as the router at the border between OSPF and another autonomous system. An ASBR is an OSPF router that has at least one interface connected to an external autonomous system that is running another routing protocol, or static routing. It is the router that redistributes external routes into an OSPF area.
An Area Border Router (ABR) is a router that has one interface in area 0, and at least one interface in another OSPF area. It forms the border between OSPF areas.
Now only ABRs will generate Type 4 LSAs. Even though these LSAs are called Summary ASBR LSAs, it doesn’t mean that ASBRs generate them. It does mean however that they contain information on where to find the ASBR for those particular routes.
An OSPF router would be an ABR and an ASBR only if it has one interface in OSPF area 0, another interface in another OSPF area, and a third interfaces connected to an external non-OSPF area, but even then, it would not generate Type 4 LSAs.
What is the difference between Type 1 and Type 2 LSAs? In a multi-access segment what is the use of sending Type 1 LSA’s when all routers are going to send the Hello Packets to the DR/BDR. I am sure there is something more to the Type 1 LSA but I just dont get it.
In a multi-access segment, it is true that an DR/BDR election will take place, and all DROTHER (non DR or BDR) routers will establish a neighbor adjacency with the DR. However, keep in mind that the type of LSA also has to do with the role of the router. All DROTHER routers will generate Type 1 LSAs. All DR routers will generate Type 2 LSAs.
Both of these must exist, because the DROTHER routers need to advertise the list of links they ahve with other routers or networks in the same area, along with their metrics. This is done using Type 1 LSAs. The DR on the other hand must send an LSA that lists all of the routers on the segment, that contains all of the destinations that have been collected using the Type 1 LSAs.
In a Totally stub area , how the router in this area will get to know the information of other routers ?As we read no type 3 and type 5 in this area type , I am wondering how this router will get to know the information of other routers in the topology ?
In a point to point network ospf type, lets assume 4 routers are connected and in same area. In this case if we login to router 1 what are the LSA types we are going to see and what are the LSA contents in it ?
I apologize for responding so late to this question. For Type2 LSAs, the link state ID is indeed the interface IP of the DR. Specifically it is the IP on the interface attached to the network segment for which that router is DR. This is also specified in the OSPF RFC 2328. The link state ID varies for each LSA type. For example, Type 1 LSAs use the router ID of the sending router as the link-state ID.
I was unable to find this statement in any of the lessons.
In a totally stub area, routers in such an area don’t need to know about information of other routers. Because total stub areas have only one exit point from the area via the ABR, they only need to know how to get out of their area, using a default route, without having additional information about specific routes in Area 0 and beyond. Take a look at the following lesson for more detailed information.
A strictly point to point network type will have only two routers, as defined by the term point to point. However, I assume you are referring to a point to point frame relay topology such as that described in the following lesson, but with one hub and three spokes:
In such a case, all routers are in the same OSPF area, and as far as the LSA types are concerned, because no DR/BDR elections take place, you will only see Type 1 LSAs.
It is important to remember that OSPF network types (non-broadcast, point to point, point to multipoint, broadcast) are different entities than the OSPF stub areas (stub, totally stub, NSSA, totally NSSA).
I wish if you could make videos for such topics (LSA Types-OSPF Borad Casting,etc) , Once I start OSPF here I got lost so I look for other resources over internet so I could follow you.
Thanks
A quick question on Type 5 LSAs. In a situation where as ASBR router is turned-up and network is redistributed, does ASBR generate router LSA and external LSA simultaneously? In general, what happens when the ABR router or other router downstream receive external LSA before router LSA or summary ASBR LSA? Will the router discard the LSA or install it into LSDB?
It is important to remember that an LSA is not an individual OSPF message. It is a component, or a subsection, of the LSDB that is sent to other OSPF routers within a Link State Update (LSU). Multiple LSAs can be consolidated into a single LSU. So when an ASBR sends out its LSU, it can contain both Type 1 and Type 5 LSA information. This information is received by the routers in the same area, and processed appropriately.
Specifically, an ABR in the same area as the ASBR will recieve the LSU, keep the Type 1 LSA information for itself, and forward an LSU containing Type 5 information to its other OSPF areas.
Hello Rene, why do we need type 3 LSA apart, because if i dnt use summarization it act as type LSA 1 only , i mean if i have 10 prefixes in non zero area, ABR will create 10 LSA type 3 ( similar to LSA 1) then what is the advantage of Type 3 in this case, i mean we type 3 was made.
The advantage that a Type 3 LSA provides is not summarization, even though its name “Summary LSA” seems to indicate this. The purpose of a Type 3 LSA is that it comes from a single router, that is, the ABR. Type 1 LSAs are sent by all OSPF routers to all OSPF routers within an area, so you may have multiple Type 1 LSAs in a routers database, but each of these LSAs point to a different router. Type 3 LSAs from an ABR point to the same router, the ABR itself. It is the entry point into another area.
So when a router receives a Type 3 LSA, it knows 1) that those LSAs originate from a different area, and 2) that the specific ABR is the route to those prefixes in that other area.
Hi Rene,
Since OSPF works as distance vector between areas, how do we ensure the loop avoidance.
Consider a topology - Area 0 has multiple ABR - B & C connected to area 1, which ABR will generate Summary LSA for Area 1. What if for some prefix B is shorted path while for some prefixes C is shorted path.
It is actually considered by some to be best practice to have two ABRs that connect a non-backbone area to the backbone. This is perfectly fine, and you will have no issues with loops.
Let’s use the topology you described as an example. Any Type 3 LSA that router B injects into area 1 will be received by router C as well, but the routes it contains are already in the routing table as intra-area routes (for area 0). This means that the router will prefer the intra-area routes it already knows to the inter-area routes this LSA is providing.
Two ABRs will actually have an equal cost for the routes they advertise into area 0 from area 1, therefore they will even load balance traffic between them providing a more efficient network.
