· Huawei is called external import routing, Cisco is called routing information redistribution, and the meaning is the same.
Premise summary: In the study of this chapter, we must first detail a few concepts
①: External routing: External routing is that the route does not belong to the OSPF area, but routing information enters OSPF through some means, then this is called external routing
②: This chapter It contains explanations of AS-External-LSA and ASBR-Summary-LSA, external route types, forwarding address attributes of external routes, configuration of external route import and aggregation, configuration of OSPF multi-process, etc.
③: Difference in AS concept: A concept called AS autonomous system is mentioned in OSPF. This concept means that the area using the same routing protocol is called autonomous system AS; the concept of AS in BGP (Border Gateway Protocol) is different.
In the above picture, it can be known that the OSPF intra-domain routing transmits routing information and topology information through the first-class LSA (Router-LSA) and second-class LSA (Network-LSA). The inter-domain routing information is composed of three types of LSA (Network-LSA) Summary-LSA). Area 0, 1, 2 are in the same AS. For the device of AR2, it is ABR and ASBR (Autonomous System Border Routing) device, so there are both OSPF routing information and RIP routing information on AR2, but RIP and OSPF routing protocols are different Yes, when no operation is performed, routing information cannot be imported into each other.
Therefore, five types of LSA (AS-External-LSA) are required.
Five types of LSA (AS-External-LSA)
1. Role: transfer external routes
2. Generate: ASBR equipment
3. Scope: entire OSPF area
· Scenario: When OSPF and RIP are connected through ASBR, the routing information of RIP will be converted from ASBR to five types of LSA (AS-External-LSA) for transmission. The transmission range of five types of LSA is the entire OSPF area .
Importing external routes
In the above figure, you can see that RTC is both in OSPF and RIP. In this case, rip can be introduced in OSPF. In fact, the routing information of RIP will become five types of LSA (AS-External- LSA) is transmitted in the OSPF area. If it is not RIP that is imported on RTC, but static routing, then there is no problem,For OSPF, it is the same for external routing protocolsAs long as it is introduced, it will be converted into five types of LSA for delivery.
· When the five types of LSAs were introduced, a problem could be found in the OSPF area, that is, the five types of LSAs on RTA and RTB are exactly the same as the five types of LSAs issued by RTC, but for RTA, it does not know how to go to RTC. For this kind of problem, you need to use four types of LSA (ASBR-Summary-LSA)
Commands to query the five types of LSAs:
display ospf lsdb ase self-originate // Query the information of AS-external-LSA generated by the device
-the five types of LS id are the same as the three types of LS id, all for the destination network Segment address
-advertise the router on RTC as RTC itself
-the mask is the network mask of the target network segment, and the information is the same as the three types of LSA-
TOS 0 Metric is the cost value-
E type is the type, there are 1 and 2. Five types of LSA E typeThe default is 2。
The difference between 1 and 2:
Type 2: For OSPF, in general, the type overhead of 2 is considered to be not the same level as the OSPF overhead, that is, the type 2 type is more expensive than any link type of OSPF. After introduction in this case, the cost value of the external route is 20. At the same time, because OSPF believes that the type cost value of Type 2 is much larger than the internal cost of OSPF, the Cost value will never change during the transmission process because Type The order of magnitude of type 2 is not in the same plane as the order of magnitude of OSPF, so there is no way to increment, so 20 is never changed.
Type 1: If Type 1 is used, the cost of transferring the link within OSPF will be added during the introduction. OSPF believes that the routing information of Type 1 is the same level as the OSPF internal routing cost, so it will be accumulated.
· Forwading Addresss: Forwarding addresses, which may be used when encountering sub-optimal paths.
· Tag (routing tag): By default, the routing tag is 1, and it can be modified when the routing strategy is done later.
After importing external routes, you can enter the display ospf abr-asbr command in the router in the OSPF area to query the ASBR and ABR for the specific router.
Four types of LSA (ASBR-Summary-LSA)
1. Function: Describe how to reach the relevant ASBR
2. Generate: ABR
3. Scope: Except for the area where the ASBR is located
Answer: For the pictures mentioned above, RTC exists as ASBR Yes, it will send five types of LSA (AS-External-LSA). For the router in Area 0, the router ID of RTC is known, but for the router in Area 1, it is not known, so it is on the RTB ASBR-Summary-LSA will also be generated for delivery, telling the router in Area 1 how to go to RTC. [Area 0 does not need to have four types of LSA (AS-External-LSA), because the router ID of RTC has been known through the first and second types of LSA]
Use display ospf lsdb asbr self-originate in RTB to perform four types of LSA queries as shown below:
The information of the parameters contained in this LSA indicates that:
If you want to go to 3.3.3.3, you can go through the router 2.2.2.2, and in this In this case, 2.2.2.2 must be the router in Area 1, and 2.2.2.2 can allow other routers in Area 1 to learn through Class 1 LSA and Class 2 LSA.
The possibility of generating sub-optimal external routes
When the link type is a broadcast link, if the external route is introduced through RTB, then in this case, the five types of LSAs are announced by RTB, so in this kind of In this case, if RTA initiates communication with an external router, it will cause the link to be RTA-> RTB-> RTC instead of directly RTA-> RTC, which is not the optimal path.
Solution: Through Forwarding Address, it can be achieved.
Set Forwarding Address ---- Physical Topology Description
①: In the above figure, RTB is configured with static routing information to reach 20.1.1.0/24, and the two network segments of RTA and RTB are in the OSPF area, then Forwading Address is The next hop address to import routing information, which is 10.1.1.3/24 in the figure
②: After RTA receives five types of LSAs, and there is Forwading Address in this LSA, in this case, RTA does not need to directly submit Send to ASBR (RTB), but can be directly delivered to the router with FA address
When viewing the five types of LSAs, the FA address is the next hop address of the static route
[Prerequisite: The next hop address must be in the OSPF routing domain. If OSPF is not running, there will be no FA]
ASBR can perform route summarization for five types of LSAs. Purpose: ① Reduce the number of detailed routing entries ②: In order to prevent the network from flapping due to excessive details (OSPF triggers update, when there are 100 details, it is assumed that one of the routing entries occurs Problem, then the entire network will be affected)
Configuration: For the five types of LSA (ASBR-summary-LSA)
premise: First of all, the five types of LSA do not belong to any area, so in this case, simply perform asbr-summary in the OSPF process to summarize
Configure OSPF multi-process-physical topology
· Application scenario: use in discontinuous areas
· For example, if two companies merge, if there are two backbone areas, then in this case, direct communication is not possible, and multiple OSPFs are required. Introduce each other in the process, you can also carry out the desired filtering during the introduction, which is more flexible. But for RTA, the imported routes are all external routes, and the same is true for RTC.
