Analysis of link status Routing Protocol

Establish the connection relationship between link status Routing Protocol

NetworkVroEstablish contact with your adjacent vro.

This connection method is mainly implemented by passing the Hello message. The main functions of Hello messages:

Establish an adjacent relationship

Monitoring the handshake signal of the adjacent relationship

Link Status routing protocol-flood LSA

The router sends a group of LSA (link status notification) to its neighbors, also known as LSP (link status message ).

After each neighbor router receives the updated LSA, it sends the LSA to its neighbor (except the neighbor router that sends the LSA to the router.

Flooding:

Sort

Because it only relies on the TTL value, it is difficult to stop flooding when the LSA spreads completely. Therefore, it is necessary to identify LSA. This identifier is the serial number.

Comparison between the LSA received by the router and the LSA saved by the router:

If the information is the same and the serial number is the same, the LSA is discarded;

When the information is the same and the serial number is updated, the LSA is retained and the related LSA in the database is updated;

When the information is the same and the serial number is old, the LSA is discarded;

The serial number is divided:

Linear serial number Space

Circular serial number Space

Lollipop serial number Space

Aging

An age field similar to TTL. Several important constants in OSPF:

LSRefeshTime:

LSA refresh time-this timer can be used if an important LSA does not want it to be deleted. The value of LSRefeshTime in OSFP is 30 minutes.

MaxAge

Maximum aging time. The router discards the LSA that exceeds this time. The value of MaxAge in OSPF is 1 hour.

MaxAgeDiff

MaxAgeDiff (the largest age gap) is a means to ensure network reliability. The MaxAgeDiff value in OSFP is 15 minutes.

The router receives multiple LSAs, but these LSAs have the same serial number and different ages. The router can process these LSAs in the following two cases:

(1) The age difference is less than MaxAgeDiff

The Network considers these LSAs belong to the same LSA. The old LSA is retained, and the new LSA does not spread any more.

(2) The age difference is greater than MaxAgeDiff

If a fault occurs in the network, the new LSA is retained and the old LSA is discarded.

Link Status routing selects protocol updates and backs up LSA to generate a complete topology database (link status database)

If the protocol works normally and the network is stable, the topology data of all routers should be completely consistent.

There are two common LSA information types:

Router link information

Peripheral Network Information

Finding the shortest path for Link Status Routing

Use the Dijkstra algorithm to calculate the shortest path to the destination. Because the network expands to a certain extent, it takes too much time to use the shortest path algorithm to calculate the shortest path. Therefore, you need to divide a large network into several small networks. These small networks are called regions.

This is the introduction of the link status routing protocol.