1. Four types of end area
backbone area and standard area: 1, 2, 3, 4, 5, including 5 types of LSA, in order to reduce the LSA of some common areas (mainly types 4 and 5, sometimes even 3 Classes are also not needed), and the stub area is introduced. At the same time, in order to ensure that the data can go out, the general ABR will automatically import the default route. That is, "you don't need to tell me how wonderful it is outside, as long as I can go out".
Received LSA Default Route Configuration Commands
stub 1,2,3 Automatically delegated (Type 3 LSA) area x stub
totally stub 1,2 Automatically delegated (Type 3 LSA) area x stub no-su
NSSA 1,2,3,7 No decentralization area x nssa
totally NSSA 1,2,7 Automatic decentralization (type 3 LSA) area x nssa no-su
need to pay attention:
a) Indicates the special role of the router, such as ASBR, ABR, through V, B in type1 LSA B) All routers in the
stub area must be configured as stubs, otherwise the neighbors cannot be built, and there is a special area flag in hello
c) The no-summary commands of total stub and total nssa only need to be done on the ABR
d) Area 0 cannot be configured as a stub area, and no virtual link can pass through the stub area . e) The default route assigned is type 3 LSA (show ip ospf database), and the default cost=1. Change f
through area X defult-cost X
) When multiple ABRs decentralize the default route, the routers in the end area will automatically select the ABR closest to COST and put them in the LSDB
g) Assuming that there is an ASBR in area 1, there will be no Type 4 LSA of this ASBR in this area. Why? (Because the area has directly passed the location flag V/B/E of the type1 LSA sent by the ASBR, the ASBR is known). Area0 and area 2 (not the terminal area) will have 4 types of LSAs pointing to ASBRs in area 1, and these 4 types of LSAs are generated by ABRs.
h) NSSA does not allow type 5 LSAs to enter, because type 5 is filtered, but the NSSA area needs to retain the routes of the external AS where ASBR appears, so type 7 LSAs are defined, which are equivalent to type 5
i) NSSA or total NSSA in the area Type 7 LSAs will be converted to Type 5 LSAs when they pass through ABR.
j) All end areas do not have Type 5. Type 7 LSAs can only appear in this NSSA area. Type 7 of one NSSA will not be propagated to another NSSA because it is Converted to type 5 and rejected
k) NSSA does not delegate default routes by default, you can delegate default routes to area x default-information-orignate
l) Note that there is no type 4 LSA in the STUB area, because type 4 LSA is type 5 LSA Service, help locate the location of ASBR, since the 5th category can't come in, the 4th category does not need to come in.
2. Routing table and LSA
OSPF intra-domain: O
OSPF intra-domain type 3: O IA
OSPF external type 5: O E1, O E2
OSPF external type 7: O N1, O N2
3.
The main reason for the irregular area is cutover.
Irregular area type: away from area 0; divided area 0.
Methods to solve irregular areas:
a) Multi-process bidirectional redistribution: Take area0-area1-area2 as an example, start two ospf processes on the abr between area1 and area2, and then redistribute them in both directions.
b) tunnel: Establish a tunnel (GRE tunnel?) in area1, so that aera2 can connect to area0.
c) virtual link virtual link: similar to tunnel, only the configuration is different from tunnel. Just refer to the Router-IDs on both sides of each other, area x virtual-link xxxx.
4. LSA
4.1, LSA type
TYPE1 Router: generated by each device, LSA in the domain, describing its own link status and overhead.
TYPE2 Network: Generated by the DR, LSA in the domain, existing in the MA network, describing the information of all the networks connected to the DR. Help Drother redirect and avoid suboptimal paths, otherwise DROTHER and DROTHER traffic will go through DR.
TYPE3 Summary: ABR is generated, inter-area routing, and inter-area routing is advertised.
TYPE4 Summery ASB: ABR is generated and announces the location of ASBRs in other areas. Type4 has no area attribute, which is roughly the same as type5, only the purpose is changed to the host.
TYPE5 AS external: ASBR is generated, describes OSPF extra-area routes, and advertises it to all areas (except the end area).
TYPE7: Generated by ASBR, describing OSPF extra-area routes, only propagated in NSSA, and converted to type5 after ABR.
Summarize:
Propagation range Advertiser Link ID Router ID Content
TYPE1 Each router in this area RID RID topology/route (IP prefix of adjacent network segment)
TYPE2 This area DR DR's IP DR's RID topology/mask (topology in this MA network)
TYPE3 The entire OSPF domain ABR inter-domain routing ABR's RID inter-domain routing
TYPE4 The entire OSPF domain (except the area where ASBR is located) ABR ASBR's RID ABR's RID ASBR Router ID
TYPE5 The entire OSPF domain ASBR extra-domain routing ASBR's RID extra-domain routing
4.2, LSA description
a) Type 2 LSA redirects the MA segment, and other routers in the area receive this MA segment as a result of the combination of Type 1 and Type 2 LSAs.
b) TYPE6 multicast is basically not used now.
c) It can be seen in the show ip ospf database that the type 5 lsa does not belong to any area, and the type 7 lsa belongs to the NSSA area.
d) Different LSAs have different link ID meanings. For TYPE3, Link ID is a Link ID for each route. For TYPE1, it means that a router is adjacent to all interfaces, so if a router advertises multiple adjacent interfaces in this area, there is only one LSA, and if it advertises multiple adjacent interfaces in other areas, there are multiple LSAs. (For example, the loopback interface on the router is advertised in the local area and outside the area, and the type and number of LSAs generated are different)
e) Since the advertiser of the five types of LSAs is always the Router ID of the ASBR, but many other areas The router does not know where the ASBR is, so it introduces four types of LSAs.
f) The area where the ASBR is located does not contain the four types of LSAs of the ASBR, because it can be done through one type of LSA. After an ASBR starts redistribution (that is, it officially becomes an ASBR), the corresponding class of LSAs spread in the area will mark the router as an ASBR.
g) Type 7 LSA: ①TYPE7 is equivalent to TYPE5, why does TYPE7 appear? Because (totally) NSSA needs to retain the 5 categories from ASBR in the area, while filtering out the 5 categories from other places, the 7 categories are redefined. ②Type 7 LSAs in the NSSA area will be converted to Type 5 LSAs through abr when they spread to other areas (area0) of the AS. The abr can also be called asbr. (asbr definition: as long as a router that can generate 5 types of lsa is asbr)
4.3, check the LSDB (LSA table)
show ip ospf database to see:
linkid, advrouter, age, seq#, checksum
where advrouter is the advertised router, for example, summary LSAs are advertised by ABR, and the display is the router- id
LSA correspondence in LSDB:
router link states (areax) : Type 1 LSA
net link states (areax) : Type 2 LSA
summary net link states (aerax) : Type 3 LSA
summary asb link states (areax) : Type 4 LSA
type -5 as external link states : Category 5 LSA
4.4, OSPF LSDB overload protection
exceeds the set amount of LSAs, and can alarm/reset the neighbor
limit is not generated by the LSA
command: max-lsa number (percentage), the default percentage is 75%
5. NSSA Supplementary Detailed Explanation
5.1, Route summary on
NSSA ABR NSSA ABR will convert type7 to type5. You can use summary-address to advertise summary type 5 LSAs (only for externally imported routes). If you want to summarize internal routes in the NSSA area Need to use the area range command.
5.2, NSSA ABR, to some extent, it can be understood as ASBR
because NSSA ABR undertakes 7 to 5, it is responsible for producing 5, other areas do not need to know the real ASBR (the router that produces 7), so NSSA ABR is not necessary. Generate 4 points to the real ASBR in regular areas such as area0. Other areas only need to treat NSSA ABRs as ASBRs.
5.3, NSSA double ABR problem
In the above figure, both ABRs receive 7, but only the ABR with the larger router-id performs 7 to 5.
Both ABRs will flood the NSSA area and learn of each other's existence.
5.4, the N/P bit in the
Hello packet is used to indicate the NSSA. In the LSA message, it is used to indicate whether the ABR in the NSSA area can convert this 7 into a 5.
5.5, Delivery of default routes in
NSSA 1) In an NSSA area, ABR advertises a default route. There are two methods:
configure the area type as totally NSSA, and ABR will automatically deliver the default route (type3 LSA); at the same time, it will block LSA3 to NSSA Regional delivery.
Use area x nssa default-information-originate under the process to automatically delegate the default route (type7 LSA); ABR does not block type3 LSA from the backbone area.
2) In the NSSA area, the ASBR advertises the default route
using area x nssa default-information-originate under the process, and the default route (type7 LSA) will not be automatically delegated. There must be a default route in the routing table in advance.
Note: In the NSSA area, whether it is ASBR or ABR, using default-information-originate (with the default of this router), or default-information-originate always, cannot inject default routes. The reason is that using this method produces type5, and NSSA does not allow type5 to exist.
Summary: In 5.5, it was found that only the total NSSA will automatically delegate the default route of the type3 LSA, and the NSSA will not.