The reason for the emergence of special areas:
Early routers relied on CPU calculation and forwarding. Due to hardware technology limitations, resources are not particularly sufficient. Therefore, to save resource usage, planning is very necessary. However, OSPF needs to maintain the same LSDB for devices in the same area. As the number of devices increases, the number of LSA entries will increase, which will result in excessive use of device resources. If the device performance is not strong enough, it will result in a series of problem. Therefore, we will introduce special areas to reduce the number of LSA entries, so as to minimize the conditional restrictions imposed by device performance on function implementation.
Of course, the current routers are basically multi-core CPUs, or NP processors, and there is no problem with performance, but in order to make the entire network more reasonable and clear, planning is also recommended.
The
above figure shows an example of the stub area . As an example, RTA is used as an ASBR to import external routes and convert it into five types of LSA (AS-external-LSA) for transmission. Therefore, after receiving the ABR in Area 2, it will send five to area 0. Type LSA and Type 4 LSA (ASBR-Summary-LSA), when RTB is received, it should have sent Type 5 and Type 4 LSA directly to Area 1, but because area 1 is now a stub area, in this case In fact, RTB as ABR will send three types of LSA (Network-Summary-LSA), then if the router in the area 1 area wants to communicate with devices in other areas, it will be handed over to RTB, RTB LSDB The above has external routing information, so it can be done in this case.
Transmission area and end area:
For each OSPF area, it can be divided into two types:
== Transmission area: In addition to carrying traffic initiated by this area and traffic visiting this area, it also carries source IP and destination IP. The traffic in the area, that is, "traversing traffic", such as Area 0. =
End area: only carry the traffic initiated by this area and the traffic that visits this area, such as Area 1.
For the end area, the following issues need to be considered:
the necessity of saving detailed routes to other areas: access to other areas through a single exit, and the "summary" route is more concise than the detailed route.
Equipment performance: Network construction and maintenance must consider cost factors. In the end area, routers with relatively low performance can be selected.
OSPF routers calculate intra-area, inter-area, and external routes all rely on collecting a large number of LSAs in the network. A large number of LSAs will occupy LSDB storage space, so the key to solving the problem is to reduce the number of LSAs without affecting the normal routing.
Stub area features:
· OSPF allows a specific area to be configured as a stub area
. There is no area 0 in a specific area. Area 0 cannot be configured as a stub area.
A virtual link is configured as a stub area
. LSAs are not allowed to be published in the stub area. The route to the outside of the AS can only be based on a default route generated by the ABR.
Since AS-External-LSA is not allowed to be advertised in the stub area, in this case, the import of OSPF external routes is useless for the stub area, so only A default route can be generated based on ABR
. Stub area technology can reduce the size of the LSDB on the internal router of the stub area and the memory requirements. It
reduces the five types of LSA, which is equivalent to reducing the size of the LSDB in the area, and the memory requirements will be corresponding. Reduce
· Virtual connection cannot cross Stub area
If the Network-Summary-LSA of the Stub area is configured as the Stub area, then in this case, if you query the LSDB on the RTA, you will find that there are no five types of LSA (AS-External-LSA), but there is one The default route. And there are other routing information in LSDB.
Suppose that the performance of RTA is very poor, so bad that it cannot accommodate the three types of LSA passed from Area 0. In this case, you need to use the full stub area to reject the three types of LSAs, and use a default instead.
Complete stub area
Comparing with the above figure, we can know that there are routing information of Area 0 in the stub area. When configured as a full stub area, all three types of LSA, four types of LSA and five types of LSA are not receiving, However, ordinary Stub areas just do not accept category 4 and category 5 LSAs, and routing information between OSPF areas will actually be received.
Difference: Complete stub area: Type 3, Type 4 and Type 5 LSAs have no
stub area: Type 4 and Type 5 LSAs do not exist.
In a complete stub area, the default route is advertised by ABR and is still Type 3.
Not So Stubby Area (NSSA)
application scenario: If you want to import external routing information in the Stub area, in this case, because the external routing information will use ASBR-Summary-LSA (Type 4 LSA), but the Stub area Type 4 and type 5 LSAs cannot be generated, so the goal cannot be achieved, but if you want to import external routes into the stub area, you must consider NSSA.
RTC is used as ASBR. If external routes are imported, seven types of LSAs will be generated on RTC, and the transmission range is only NSSA area. Then they will be transmitted in Area 1. On RTB, seven types of LSAs will be converted into five types of LSAs and passed to Area 0. Routing.
Special note: Now when Area 0 is adjacent to Area 2, RTA is used as ABR. In this case, if Area 2 introduces external routes, then the five types of LSA will not be converted to seven types of LSA on RTB. To the NSSA area
NSSA-LSA is
actually not much different from the five types of LSA, but the Type is changed from AS-External-LSA to NSSA. And for the seven types of LSA also use two types, Type 1 and Type 2, which is the same effect as the five types of LSA. The default value is also 2.
The characteristics of NSSA:
①: It does not receive
Type 4 and Type 5 LSAs ②: It can import external routes from this area (type 7 LSAs are used)
Detailed NSSA FA address
Convert NSSA-LSA to AS-Exteral-LSA
Question: What if there are multiple ABRs in the area, that is, multiple ABRs appear in the NSSA area and choose the one that converts the seven types of LSAs to five types of LSAs?
Answer: In fact, one ABR will be elected from multiple ABRs, and the election will be based on the Router ID. The larger the Router ID, the higher the priority, so how to get the router ID? In fact, it is known according to the first and second class LSA.
NSSA-LSA advertises the default route to the
complete Nssa area
-in fact, based on the characteristics of the original Nssa area, it will no longer receive the type 3 LSA, and then one more default route will be sent out to each device in the special area through the ABR.
to sum up
| Types of | Link ID | advertise ID | Transmission range | Send content |
|---|---|---|---|---|
| [Class 1] Router-LSA | RID | RID | The area | Topology information |
| [Class 2] Network-LSA | DR interface ID | DR RID | The area | Topology information |
| [Three categories] Network–Summary-LSA | destination network | ABR RID | The AS | Routing information |
| [Four categories] ASBR-LSA | ASBR RID | ABR RID | The AS | Routing information |
| [Category 5] AS-External-LSA | destination network | ASBR RID | The AS | Routing information |
| [Seven categories] AS-External-LSA | destination network | ASBR RID | The AS | Routing information |
Feeling:
For the Stub area and Nssa area, what they actually want to accomplish is to reduce the number of LSAs on the device. The origin of the Stub area is earlier than that of the Nssa area. The Stub area is found to be inoperable later. (It cannot import external routing information), So the new protocol is adopted to form the Nssa area, so the Nssa area is an upgraded version of the Stub area, which has the function of importing routing information from the outside, and can also be adjusted according to the performance of the device. If the device cannot bear If there are too many LSAs, you can also use the complete Nssa area to process the three types of LSAs and replace them by default.
