Link State Routing Protocol OSPF Overview









Ospf  is link sate routing protocol. OSPF used SPF algorithm to find shortest path
 first. Ospf is multi vendor routing protocol that means we can use for any kind of routing device. Ospf send periodic update and send trigger update when change topology information. It maintains three type of table.
  1. Neighbor Table: These table tracks on all neighbors that are used ospf routing protocol are directly connected.
  2. Topology Table: Topology table maintain all of possible path of ospf area. Topology table is road map of possible path.
  3. Routing Table: Routing table also know as forwarding database.


Commonly used OSPF :


Link State Database: Link state database storing of topology data.
SPF: The name of the algorithm ospf uses to analyze the Link State Database.
Link State Update: LSU is ospf packet that holds the detailed topology information.
Link state Advertisement (LSA): LSA hold on topology base information.
AREA:  A group of router that are connected a domain.
ABR (Area Border Router): Area border router are connected as a point that have multiple area.
Designated Router (DR): Designated router is very important think in OSPF design and implementation plan. Designated area use multicast ip address as 224.0.0.6.
Backup Designated Router (BDR): When DR fail then BDR perform DR operation 


FIG: OSPF AREA Concept 

Building the LSDB:
  • The hallow protocol is used to define neighbors
  • Adjacent is established
  • Adjacent routers exchange LSAa
  • Each router builds an LSDB using LSA

Planning For OSPF
  • Ip Addressing Plan
  •  Network Topology
    1. Primary Vs Backup Link
    2. WAN Bandwidth utilization
  • Define hierarchical network design and areas
  • Evaluate OSPF scaling option
1.      Stigmatization where necessary
2.      Define stub area

OSPF Network Type:
·        Point to point: A network that joins a single pair of routers.
·        Broadcast : A multi-access broadcast network such as Ethrtnet
·        Nonbroadcast multi-access (NBMA): A network that is interconnecting more than two router but they has not broadcast capability. such as ATM,X.25,Frame Relay.
Electing The DR and BDR:
  • Hello packet are exchange via IP multicast
  • DR router with the highest OSPF priority
  • BDR the router with the second height priority value
  • The OSPF router ID is used as the tiebreaker
  • The DR election is no preemptive.

OSPF LSA Type: There are 11 type of OSPF link state advertisement exist hear.
·        LSA 1(Router):Every router have own type of LSA 1 each represent itself for each area to which it connect.
·        LSA 2 (Network): Created by the designated router on the subnet and represents the subnet and the router interface connected to the subnet.
·        LSA 3 (Net Summary): It created by Area Border Router.
·        LSA 4 (ASBR Summary): It created by Autonomous system Border Router. Except it advertises a host rout used to reach an ASBR.
·        LSA 5 (External AS): Generated by ASBR for external rout injected into ospf.