[cisco] notes - CCNP OSPF - Fund1: Link State Protocol Operation

Link State Updates(LSUs): contain Link State Advertisements (LSAs) that palced into a link state database

To ensure that OSPF routers have the most recent infomation pssible in their database, the LSAs are assigned sequence numbers.

The role of the DR and BDR

THe detecting router will send a muticast to 224.0.0.6, the All Designated Routers address, where both the DR and BDR will hear it.

The DR then sends a multicast to 224.0.0.5, the All OSPF Router address, where every OSPF-speaking router on that segment will hear it. The BDR updates its OSPF database in order to stay ready to step into the DR role if needed, but only the DR 

FOur Routers ENter, One ROuter Leaves

Let;s take a close look at the rules and regulations regarding the DR and BDR elections on an OSPF broadcast segment. One router will become the DR, anothe the BDT, and the other two DROthers

interface priority

RID: the loopback interface will be the RID, or the highest IP address interface in a router

changing the OSPF interface priority with ip ospf priority

setting the OSPF RID manually with router-id

setting the OSPF RID manually with router-ID

setting the OSPF RID to an appropriate value with a lookback interface's IP address

DR 

show ip ospf int fast 0/0

 Building an NBMA network

withe a hub-and-spoke topology, a spoke router cannot get a ulticast to the other spoke. All spoke-to-spoke traffic goes through the hub router, and routers do not forward broadcasts or multicasts

The OSPF Point-to-Point and Point-to-Multipoint Networks

no need to have a DR or BDR, because only one peer for each router

Virtual Link

The area through which the virtual link is buildt, the transit area, cannot be a stub area of any kind. We're going to spend more time with stub areas later in this part of the course, and for right no it's enought to know that our transit area(Area 34) is not a stub. 

Building a virtual link is actually pretty easy. You just have to knnow to ignore the message you'll get halfway trought!

R3
router ospf 1
area 34 virutal-link 4.4.4.4

OSPF adjacency states

Down: No hellos received from that neighbor. Nothing much going on yet

Attempt: Unicast hello packets are being sent to the neighbor. You'll only see this stage onthe hub router in an NBM network

Init: Hey, we're getting somewhere! The first hello packet has been received from the neighbor

2-Way: Whe you're here, you're almost gold. At this point, each router has received a Hello packet containing its own RID, indicting bidirectional communication. When a router received a hello packet containing its own RID, it's not just talking to itself; that's the remote router's way of saying "I received hat hello packer your sent me earlier."

Exstart: Following the DR/BDR election, the exchange of link state database ingo can begin, The router withe the highest RID will begin the exchange and increment the initial sequence number, which is determined during this stage.

Exchange: Database Descriptor(DBD) packets are xchanged. As you'd expect, these packets contain a description of the link state database.

Loading: Routers now send Link State Request (LSR) packets to the almost-neighbor

Full: Router databases are synced and the adjacency has formed.

OSPF Router Types

ABR: Area Border ROuter: a router with at least one interface in Area 0 and another in a non-backbone area. All ABRs are backbone routers, but not all backone routers are ABRs.

ASBR(Autonomous system boundry router): a ruter injecting routes into the OSPF domain via route redistribution. Toverify the ABR and ASBR status of the local router, run show ip ospf.

The LSA Types

Type 1s: (ROuter Link States) are generated by each router for every area the touter has a link in. These are flooded to a single area only. The name is the recipe, as LSA Type 1s contain the "router link states" for this particular router.

Type 2: snet oute only by DRs. The only Type 2 LSA in R3's OSPF database for Area 0 is from Advertising Router 1.1.1.1, the OSPF RID of R1.

Type 1 and 2 are confined to a single area, which helps multi-area OSPF reduce the load on router resources. If your had only ine large OSPF aream every router in the area would receive every single Type 1 and Type 2 LSA!

Type 3: Summary Net Link States, THese summary link advertisements are generated by ABRs and describe inter-area routers. They summarize the networks from one area to another, and are not flooded into a total stub area.

Type 4: Generated only by ABRs and descrive the path to the ASBR. Type 4 LSAs are not flooded into a total area.

Type 5: describe links external to the OSPF domain. This link describes the network injected into the OSPF domain via route redistribution on R1, verified by the address listed as the advertising router. Tupe 5 LSAs are generated only by ASBRs, and they're flooded to all areas except stub and total stub areas.

Type 6: a specialty LSA type generated only by routers using multicast extensions to OSPF(MOSPF)

Type 7: generated only by an ASBR and sent into a not-so-stub stub area

NSSAs act as stub areas, but some of the more-specific routes are advertised, rathe than just a default route. Type 7 LSA are flooded throughout the NSSA, but thet don't get to leave! Instead, they're converted to Type 5 LSAs and are then sent out of the NSSA.

OSPF route redistribution and Stub Area

router ospf 1 
redistribute connected subnets