As we all are aware about OSPF Routing protocols and its basic features and characteristics. But OSPF is very Vast and useful protocol for every environment. Here we will discuss about their advance features like Area type, Stater and DR/DBR selection.
OSPF Neighbor States
Neighbor adjacencies will progress through several states, including:
Down – indicates that no Hellos have been heard from the neighbouring router.
Init – indicates a Hello packet has been heard from the neighbor, but two-way communication has not yet been initialized.
2-Way – indicates that bidirectional communication has been established. Recall that Hello packets contain a neighbor field. Thus, communication is considered 2-Way once a router sees its own Router ID in its neighbour’s Hello Packet. Designated and Backup Designated Routers are elected at this stage.
ExStart – indicates that the routers are preparing to share link state information. Master/slave relationships are formed between routers to determine who will begin the exchange.
Exchange – indicates that the routers are exchanging Database Descriptors (DBDs). DBDs contain a description of the router’s Topology Database. A router will examine a neighbour’s DBD to determine if it has information to share.
Loading – indicates the routers are finally exchanging Link State Advertisements, containing information about all links connected to each router. Essentially, routers are sharing their topology tables with each other.
Full – indicates that the routers are fully synchronized. The topology table of all routers in the area should now be identical. Depending on the “role” of the neighbor, the state may appear as:
- Full/DR – indicating that the neighbor is a Designated Router (DR)
- Full/BDR – indicating that the neighbor is a Backup Designated Router (BDR)
- Full/DR Other – indicating that the neighbor is neither the DR nor BDR On a multi-access network, OSPF routers will only form Full adjacencies with DRs and BDRs. Non-DRs and non-BDRs will still form adjacencies, but will remain in a 2-Way State. This is normal OSPF behaviour.
What is DR/BDR
In Multi-access network such as Ethernet, all router is connected in same LAN and there are possible to many neighbours’ relationship of same physical segment. To avoid this OSPF elect DR-Designated Router and BDR-Back-up Designated Router over multicast address 224.0.0.6.
DR/BDR Selection Criteria
- DR/BDR works on NBMA network. Means all nodes are in same broadcast Lan network .
- DR/DBR selection will be happened during 2-Way state.
- In NBMA or Same LAN Network, al Lan router will send packet to all router multiple time with each to make Identical database
- To avoid this, a concept called DR/BDR is advertise. There are DR, BDR and DRother.
- During neighbor selection process, Router elected as server called DR and Backup server—BDR and DR other
- DR responsibility to send all updated info to other router.
- BDR work as backup DR if DR router is failed.
- DRother are the router who are not DR/BDR
- It works on 224.0.0.6 multicast address
- If not want to start DR/BDR process on any router or don’t want participate any router in this selection then we can set priority 0.
- In 2-Way state there are wait timer of 60 sec for DR/BDR selection.
- When a new topology built and first router boots up, it will wait till 60sec to received hello from DR
- If not recd till 60 sec then it will declare himself as DR and send DR hello to all other RTRs and same as for BDR.
- The router with the highest priority becomes the DR; second highest becomes the BDR. If there is a tie in priority, whichever router has the highest Router ID will become the DR.
- Default priority on Cisco routers is 1. A priority of 0 will prevent the router from being elected DR or BDR.
Note: The DR election process is not pre-emptive
Neighbor Adjacency
Once neighbours have been identified, adjacencies must be established so that routing (LSA) information can be exchanged. There are two steps required to change a neighbouring OSPF router into an adjacent OSPF router:
- Establish two-way communication (achieved via the Hello protocol).
- Establish database synchronization—this consists of five packet types being exchanged between routers:
- Hello packets—Establish communication with directly attached neighbours.
- Database Descriptor (DBD)—Sends a list of router IDs from whom the router has an LSA and the current sequence number. This information is used to compare information about the network.
- Link State Requests (LSR)—Follow DBDs to ask for any missing LSAs.
- Link State update (LSU)—Replies to a link-state request with the requested data.
Link-state acknowledgements (LSAck)—Confirm receipt of link-state information
OSPF Network Types
OSPF’s functionality is different across several different network topology types.
Broadcast Multi-Access – indicates a topology where broadcast occurs.
Examples include Ethernet, Token Ring, and ATM.
OSPF will elect DRs and BDRs.
Traffic to DRs and BDRs is multicast to 224.0.0.6. Traffic from DRs and BDRs to other routers is multicast to 224.0.0.5.
Neighbours do not need to be manually specified.
Point-to-Point – indicates a topology where two routers are directly connected.
An example would be a point-to-point T1.
OSPF will not elect DRs and BDRs.
All OSPF traffic is multicast to 224.0.0.5.
Neighbours do not need to be manually specified.
Point-to-Multipoint – indicates a topology where one interface can connect to multiple destinations. Each connection between a source and destination is treated as a point-to-point link.
An example would be Point-to-Multipoint Frame Relay.
OSPF will not elect DRs and BDRs.
All OSPF traffic is multicast to 224.0.0.5.
Neighbours do not need to be manually specified.
Non-broadcast Multi-access Network (NBMA) – indicates a topology where one interface can connect to multiple destinations; however, broadcasts cannot be sent across a NBMA network.
An example would be Frame Relay.
OSPF will elect DRs and BDRs.
OSPF neighbours must be manually defined, thus All OSPF traffic is unicast instead of multicast.
Read more- https://techblog.kbrosistechnologies.com/ospf-routing-protocol-basic/
Remember: on non-broadcast networks, neighbours must be manually specified, as multicast Hello’s are not allowed.
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