perform the following parameter checks based on the receive Hellos:
■ Must pass the authentication process
■ Must be in the same primary subnet, including same subnet mask
■ Must be in the same OSPF area
■ Must be of the same area type (stub, NSSA, and so on)
■ Must not have duplicate RIDs
■ OSPF Hello and Dead timers must be equal
■ MTU must be equal for the DD packets to be successfully sent between neighbors, but this parameter check is technically not part of the Hello process.
2-Mixing & Matching Different OSPF Network Types:
Here is a quick list of which combinations will work:Broadcast to Broadcast
Non-Broadcast to Non-Broadcast
Point-to-Point to Point-to-Point
Point-to-Multipoint to Point-to-Multipoint
Broadcast to Non-Broadcast (adjust hello/dead timers)
Point-to-Point to Point-to-Multipoint (adjust hello/dead timers)
The reason why some network types are not compatible with some other is based on a fact and that is - Network Types that elects/requires DR election in first place will be only compatible with other types that also elects/requires DR election. On the other hand Network Types that doesn't elect/requires DR are only compatible with network type that also doesn't elect/requires DR election. So that's why these two categories don't work with each other.Now if you try to break this criteria and try to match Network Type that Elects/Requires DR with Network Type that doesn't Elects/Requires DR ,you will see that although the neighborship comes up between OSPF routers but routes are not getting exchanged.
3- SPF on a router finds all possible routes to each subnet,
adds the cost for each outgoing interface in that route, and then picks the path with the least cost.
4- ABRs do not forward type 1 and 2 LSAs from one area into another, but instead create type 3 LSAs
for each subnet defined in the type 1 and 2 LSAs. Type 3 LSAs do not contain detailed information
about the topology of the originating area; instead, each type 3 LSA represents a subnet, and a cost
from the ABR to that subnet.
5- ABRs ignore LSAs created by other ABRs, when learned through a nonbackbone area,
when calculating least-cost paths. This prevents an ABR from choosing a path that goes
into one nonbackbone area and then back into area 0 through some other ABR.
6-OSPF Filtering
■ Filtering routes, not LSAs—Using the distribute-list in command, a router can filter the
routes its SPF process is attempting to add to its routing table, without affecting the LSDB.
- Distribute lists can be used only for inbound filtering, because filtering any outbound OSPF
information would mean filtering LSAs, not routes.
- The inbound logic does not filter inbound LSAs; it instead filters the routes that SPF chooses
to add to that one router’s routing table.
- If the distribute list includes the incoming interface parameter, the incoming interface is
checked as if it were the outgoing interface of the route.
■ ABR type 3 LSA filtering—A process of preventing an ABR from creating particular type 3
summary LSAs.
The OSPF ABR type 3 LSA filtering feature allows an ABR to filter type 3 LSAs at the point where the
LSAs would normally be created. By filtering at the ABR, before the type 3 LSA is injected into another
area, the requirement for identical LSDBs inside the area can be met, while still filtering LSAs.
To configure type 3 LSA filtering, you use the area number filter-list prefix name in | out command
under router ospf. The referenced prefix list is used to match the subnets and masks to be filtered. The
area number and the in | out option of the area filter-list command work together, as follows:
- When in is configured, IOS filters prefixes going into the configured area.
- When out is configured, IOS filters prefixes coming out of the configured area.
■ Using the area range no-advertise option—Another process to prevent an ABR from
creating specific type 3 summary LSAs.
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