06 – OSPF Concepts 10,577 views

Posted by Mo7sin in CCNP, CCNP Routing (642 - 902) On 18/03/2012 at 11:16 PM

OSPF Topology Database: Consists of lists of subnet numbers (links), list of routers (and links they are connected to).

-> Uniquely identifier each router in this database using OSPF Router ID (RID)
To select the RID
  • The router first checks for any loopback interfaces that are up, and chooses the highest numeric IP address of those.
  • If no loopback exists, router chooses highest IP address from interfaces that are up and up.

*Note: loopback interface is a virtual interface, configured with

interface loopback [interface #]

Each Router chooses RID when OSPY is initialized (during initial loading of IOS). If other interfaces come up after this, not used unless clear ip ospf process is issued.
Meeting OSPF Neighbors: Once router has assigned itself a RID, and some of its interfaces are up, the router is ready to meet its neighbors (connected routers).

  • Can become neighbors if connected to same subnet
  • Router multicasts OSPF Hello packets out each interface
  • Hello message follows IP packet header (port = 89)
  • Hello packets sent to (all OSPF speaking routers)
  • Routers learn several things from Hello Packets:
  • RID, Area ID, Hello Interval, Dead Interval, router priority, designated router, backup designated router, and a list of neighbors sending router already knew about.
  • To confirm that a Hello Packet was received, next Hello Message will include the sender’s RID within the list of neighbors.

-> Once router sees its RID included, two-way state achieved, and more detailed information can be exchanged.

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