the most important part of OSPF understanding the getting to know LSA "link state advertisements" section
the understanding part should not only be for memorizing the LSA names
but you also need to know how to check each one of them in command line
and how is every LSA is generated and who generates the LSA
so basically as we learned that OSPF is uses hierarchical design and this is done by the backbone area 0
because all area must connect to it in order to reach the other area's
other than that, configuring areas will hide the routes from the other area and limit the LSA flooding that is happening in a single area
so Area 0 will not advertise its LSA flooding when change done internally to other areas like area 1 and area 2
now getting back to the LSA types we have the most important LSA:
LSA1 [ this is for all routers within the same area and each one say's i'm in this area ]
so LSA1 will be generated from all routers for every routers link to be sent to the ABR
LSA2 [ this is for IP address for DR interface within same area ]
all routers will have OSPF adjacency with the DR and BDR and they will generate LSA2 and this is only in OSPF network types where DR is elected, so its in [NMBA, Broadcast networks]
and then DR will send it to all routers so its like a map to be used internally between routers to reach each other.
LSA3 [ this is generated by the Area Border Router which has legs in two areas, and its summary for the networks advertised by the other area so its point of contact between area ]
LSA4 [ is when the ASBR gets routes from EIGRP or RIP into OSPF domain, the ASBR will send to the ABR of that area LSA1 messages and the ABR will generate LSA4 to send to next area.
LSA4 is like identified for the ASBR
LSA5 [ external routes that are coming from another router runs rip or eigrp or others ]
so when Rx redistributes EIGRP into OSPF domain it will send LSA1 to the ABR of that area, then the ABR will generate LSA5
LSA7 [ used in NSSA because routers will not allow LSA5 so it will set LSA5 to LSA7 to be allowed into OSPF domain ]
all of these LSA's can be seen under OSPF database using
#show ip ospf database and adding its arguments to find each LSA self originated or advertised by other routers
Ok, now how to filter LSA's?
STUB = NO LSA4 NO LSA5
TOTALLY STUB = NO LSA3,4,5
NSSA= NO LSA5 but LSA7 is ok and will be translated back at the borders.
TOTALLY NSSA= will generate LSA3 with default Route
the understanding part should not only be for memorizing the LSA names
but you also need to know how to check each one of them in command line
and how is every LSA is generated and who generates the LSA
so basically as we learned that OSPF is uses hierarchical design and this is done by the backbone area 0
because all area must connect to it in order to reach the other area's
other than that, configuring areas will hide the routes from the other area and limit the LSA flooding that is happening in a single area
so Area 0 will not advertise its LSA flooding when change done internally to other areas like area 1 and area 2
now getting back to the LSA types we have the most important LSA:
LSA1 [ this is for all routers within the same area and each one say's i'm in this area ]
so LSA1 will be generated from all routers for every routers link to be sent to the ABR
LSA2 [ this is for IP address for DR interface within same area ]
all routers will have OSPF adjacency with the DR and BDR and they will generate LSA2 and this is only in OSPF network types where DR is elected, so its in [NMBA, Broadcast networks]
and then DR will send it to all routers so its like a map to be used internally between routers to reach each other.
LSA3 [ this is generated by the Area Border Router which has legs in two areas, and its summary for the networks advertised by the other area so its point of contact between area ]
LSA4 [ is when the ASBR gets routes from EIGRP or RIP into OSPF domain, the ASBR will send to the ABR of that area LSA1 messages and the ABR will generate LSA4 to send to next area.
LSA4 is like identified for the ASBR
LSA5 [ external routes that are coming from another router runs rip or eigrp or others ]
so when Rx redistributes EIGRP into OSPF domain it will send LSA1 to the ABR of that area, then the ABR will generate LSA5
all of these LSA's can be seen under OSPF database using
#show ip ospf database and adding its arguments to find each LSA self originated or advertised by other routers
Ok, now how to filter LSA's?
STUB = NO LSA4 NO LSA5
TOTALLY STUB = NO LSA3,4,5
NSSA= NO LSA5 but LSA7 is ok and will be translated back at the borders.
TOTALLY NSSA= will generate LSA3 with default Route
