Cisco router OSPF protocol classic learning notes

The OSPF protocol is very important for the use of Cisco routers. But how do I perform specific operations? The following article will give a detailed explanation.

OSPF protocol operation:

1. the ROUTER that declares OSPF sends a HELLO message from all interfaces that start the OSPF protocol. The two routers share a Public Data Link and can negotiate with each other the parameters specified in their HELLO packets. Then they become neighbors Neighbor)
2. Adjacency is determined by the router type used to exchange HELLO message information and the network type used to exchange HELLO message information.
3. Each ROUTER sends a Link State Advertisement (LSA) notification to all neighboring nodes that form an adjacent relationship. The LSA mainly refers to the notification describing all the ROUTER Link information OR interfaces) and link status information. Due to the diversity of chain status information. The OSPF protocol defines many LSA types.
4. Every ROUTER that receives the LSA announcement from the neighbor ROUTER records the LSA announcement in its link status database, and send an LSA copy to all other neighbors of the ROUTER.
5. spread to the entire region through LSA. All rourers form the same link status database.
6. When all the ROUTER databases are completely the same, Each ROUTER uses itself as the root and uses the SPF algorithm to calculate a non-loop topology. To describe the minimum path cost that it knows to reach every destination.) This topology is the SPF algorithm tree.
7. Each router will build its own route selection table from the SPF algorithm tree.

Note: When all the link status information spreads to all routers in the same region-that is, the chain status database is synchronized-and the route selection table is created successfully, OSPF becomes a "quiet" protocol. The HELLO packet exchanged by the neighbor is called KEEPALIVE keep. And the LSA is re-transmitted every 30 minutes.

Vroid ID is the IP address that uniquely identifies a vro in the ospf area. this IP address first selects the IP address with the highest value on all LOOPBACK interfaces. If the ROUTER does not configure the LOOPBACK interface with the IP address, the ROUTER selects the IP address with the highest number of physical interfaces. The ID interface used as the router does not have to run the OSPF protocol.

Using the LOOPBACK address as the router id has two advantages: one is that the LOOPBACK interface is more stable than any other physical interface, because as long as the ROUTER starts, this LOOPBACK interface processes the active status, it will only expire when the ROUTER fails. The second is: it has the ability to properly control the router id.
The ospf router uses the HELLO packet to advertise its router id to establish a relationship with its neighbors.

The HELLO Message Protocol serves the following purposes:

1. It is a method for finding a neighbor router;
2. Before the two routers become neighbors, the HELLO packet protocol must be used to notify the two routers of the parameters that must be recognized by each other;
3. HELLO messages act as KEEPALIVE between neighboring routers;
4. It ensures two-way communication between neighboring routers;
5. It is used to select the specified Router Designated Router and DR on a broadcast network OR non-broadcast Multiple Access nbma) and Backup the specified Router Backup Designated Router, BDR)

On a Cisco router, HELLO is sent once every 10 seconds by default, and can be changed through ip ospf hello-interval.
The router's invalid interval is four times the default HELLO interval, which can be changed through ip ospf dead-interval.

A hello message contains the following parts:

ID of the router of the originating router)
ID of the region where the router interface is originated. Area ID)
Address Mask of the interface of the originating Router
Authentication Type and authentication information of the originating router interface
HELLO interval of the originating router interface
Router invalid interval of the originating router interface
Router priority
Specify a router DR) and back up a vrobbdr)
5 Mark bits that identify the optional Performance
ID of the router of all valid neighbors of the originating Router

When a router receives a HELLO Message from its neighbor router, it checks whether the region ID, authentication information, network mask, HELLO interval, router invalid interval, and optional values carried by the HELLO Message correspond to the values configured on the receiving interface. yes, no, this HELLO is not required, and the adjacent relationship cannot be established,
The OSPF protocol defines five network types: Point-to-Point network (point-to-Point) Broadcast Network (Broadcast) non-Broadcast multiple access (NBMA) network Point to multi-point network Point-to-multipoint) virtual links ).