Facts to remember about OSPF - Part2

OSPF summarization
Internal Must be done in the ABR which contains the subnets to be summarize, Apply this command at router ospf level configuration: "area # range ip address mask", The area must be different than 0 (area 0)

External Must be done in the ASBR which contains the subnets to be summarize, Apply this command at router ospf level configuration: "summary-address ip address mask"  



OSPF Stub area
Apply this command at router ospf configuration level in all the routers members of the stub area  "area # stub", the ABR will filter all type 5 LSA that come from area 0 into the stub area but it will allow type 3 LSAs. ABR will inject a type 3 default route

OSPF Totally Stubby area
Apply this command at router ospf configuration level in the ABR route totally stubby area "area # stub no-summary" and this command in the other routers of the totally stubby area "area # stub", the ABR will filter all type 5 and type 3 LSA that come from area 0 into the stub area but they still will reach external routes cause ABR will inject a type 3 default route (Notice that this is the only summary LSA allowed in a totally stubby area)

OSPF Not so Stubby area
Apply this command at router ospf configuration level in all the routers members of the not so stubby area: "area # nssa", the ABR will filter all type 5 LSA that come from area 0 into the stub area but it will allow type 3 LSAs, the ABR will not inject a type 3 default route, if you want to reach routes outside the ospf doman in a nssa you got to advertise a default route in the ABR with the command "area # nssa default-information-originate" , this will advertise a default route type 7 into nssa area (O*N2). Only type 7 LSA's are accepted inside NSSA, when the ABR from a NSSA receives a type 7 LSA it will turn into type 5 LSA and redistribute it into area 0. Type 7 LSA are created when extenal routes are redistributed by routers inside NSSA

OSPF Totally Not so Stubby area
Apply this command at router ospf configuration level the ABR route totally stubby area "area # nssa no-summary" and this command in the other routers of the totally stubby area "area # nssa", the ABR will filter all type 5 LSA that come from area 0 into the stub area, the ABR will inject a type 3 default route. Only type 7 LSA's are accepted inside NSSA, when the ABR from a NSSA receives a type 7 LSA it will turn into type 5 LSA and redistribute it into area 0. Type 7 LSA are created when extenal routes are redistributed by routers inside NSSA

Type-7 to Type-5 Translator Election
Only the ABR with the highest router-id will make the translation from a LSA type 7(NSSA area) to LSA type 5 (area 0)


When there is more than one exit point (ABR) into a NSSA, a sort of Traffic engineering can be made suppressing incoming intra area routes in one of he ABR's with the command area # nssa no-summary (turning it into a totally NSSA ABR)



OSPF NSSA Redistribution Filtering
There are situations where there is no need to inject type 7 external routes into the Totally NSSA. This situation usually occurs when an ASBR is also an Totally NSSA ABR and all the routers in the NSSA use it as default gateway. When redistribution takes place in this scenario, the router generates type 5 (into area 0) as well as type 7 LSAs (into nssa). You can prevent the router from creating type 7 LSAs for NSSA with this command:

router ospf 1
 redistribute connected subnets route-map CONNECTED->OSPF
 area 3 nssa no-redistribution no-summary

OSPF LSA Type-3 Filtering
This command aplies on the ABR at router level configuration. here some examples

area 3 filter-list prefix R1_LOOPBACK in (filter LSA leaving area 0 and entering area 3)
area 3 filter-list prefix AREA_3_ROUTES out (filter LSA leaving area 3 and entering area 0)
area 0 filter-list prefix R1_LOOPBACK out (filters LSA leaving area 0 and going to any area, that means that none areas behind the ABR will receive the route even new areas)


OSPF forwarding address supression
 When OSPF database lookups on external routes, the "Forward Address" field determines who the next recursive lookup should be performed toward. This "forward address" is preserved when the ABR makes a translation from LSA type 7 to LSA type5. But there will be a problem if that "forward address" was filtered in the ABR and nobody knows about it in the ospf domain. To overcome this use this command at router configuration level in the ABR which makes the translation area # nssa translate type7 suppress-fa where # is the area which originates the LSA type 7

router ospf 1
 area 3 nssa translate type7 suppress-fa

The whole idea behind forward address: If only R1 redistributes external routes from R100 to the OSPF domain, then R2 will always have go through R1 to reach those extrenal routes. Forward address let R2 to ask R100 directly. So when R1 makes the translation from type 7 to type 5 it will preserve the R100 address in the forward address parameter