Hello,
Today, I would like to share with you the basic concepts of OSPF.
Introduction :
OSPF (Open Shortest Path First ) is a typical Link-state routing protocol, generally used in the same routing domain. Here, the routing domain refers to an Autonomous System, namely AS , which refers to a group of networks that exchange routing information with each other through a unified routing policy or routing protocol. In this AS , all OSPF routers maintain an identical database describing the structure of the AS . The database stores the state information of the corresponding links in the routing domain. It is through this database that the OSPF routers calculate their OSPF routing table.
As a link-state routing protocol, OSPF transmits link-state broadcast packets LSA ( Link State Advertisement ) to all routers in an area, which is different from distance vector routing protocols. A router running a distance vector routing protocol passes some or all of its routing table to its neighbors.
Here are some basic concepts of OSPF that need to be introduced :
Hello protocol:
1- Used to discover neighbors;
2- Before becoming a neighbor, some parameters in the Hello packet must be successfully negotiated;
3- Play the role of keepalive among neighbors ;
4- Allow two-way communication between neighbors;
5- Election of DR and BDR on NBMA (NonBroadcast Multi-Access) network .
Neighbor and Adjacency :
1- Two routers with ports connected to the same network segment are neighbors, and the neighbor relationship is maintained by the OSPF Hello protocol;
2- The relationship formed from the neighbor relationship to exchange routing information is called adjacency. Not all neighbor relationships can become adjacency relationships. Different network types have different rules for establishing adjacency relationships. For example, in a broadcast network, routers are adjacency only with DR and BDR ;
Router ID :
The OSPF protocol uses a 32 -bit unsigned integer called the Router ID to uniquely identify a router. For this purpose, every router running OSPF needs a Router ID . This Router ID generally needs to be configured manually, and is generally configured as the IP address of an interface of the router. Since the IP address is unique, it is easy to guarantee the uniqueness of the Router ID . Without manual configuration of the Router ID , some manufacturers' routers (including Quidway series) support the automatic election of the largest IP address as the Router ID from the IP addresses of all current interfaces .
DR and BDR :
Every broadcast network and NBMA network that contains at least two routers has a Designated Router ( DR ) and a Backup Designated Router ( BDR ).
1- Reduce the number of adjacencies. A router that is neither DR nor BDR only forms adjacencies with DR and BDR and exchanges link state information and routing information, which greatly reduces adjacencies in large broadcast networks and NBMA networks number of relationships.
2- In the LSDB describing the topology , an NBMA network segment or a broadcast network segment is described by a single LSA , which is generated by the DR on the network segment .
Area :
OSPF supports grouping together a group of network segments, such a combination is called an area, that is, an area is a collection of a group of network segments.
Area 0 is the backbone area. The backbone area is responsible for advertising routing information (not detailed link state information) summarized by area border routers between non-backbone areas. Advertise inter-area routing information to each other. Therefore, all area border routers have at least one interface belonging to Area 0 , i.e. every area must be connected to the backbone area.
Router Type :
Internal Router ( IR ) : A router where all connected network segments are in one area
Area Border Router : A router connected to multiple areas, ABR maintains an LSDB for each connected area .
Backbone Router : A router with at least one port (or virtual connection) connected to the backbone area, including all ABRs and routers with all ports in the backbone area.
AS Boundary Router : A router that exchanges routing information with routers in other ASs. This router advertises AS external routing information to the entire AS . The AS border router can be an internal router IR , or an ABR , and it can belong to the backbone area or not.
You are welcome to leave a message and exchange in the comment area. Thank you!
