The Type-1 LSA OSPF is the most important LSA among all the available LSAs. Because the informations carried in the Body.
These informations are the collection states of Router's Links or interfaces.
The Type-1 LSA is structured with two fields or two pieces:
1-Link ID
2-Link Data
Both Link ID and Link Data are called Link Type. The Link Type is the core component of the Body's Type-1 LSA that provides a complete view of the topology using the informations carried by the Link ID and Link Data.
There are four Link Types. Each Link Type has two fields as mentioned previously, Link ID and Link Data.
Link Type 1: The Link ID describes a point-to-point connection in the link which means a Point to Point or Point to Multipoint network, in this case the Link ID helps to identify the neighbor connected to this link with its Router ID. The Link Data helps to include the IP Address in this link.
Link Type 2: The Link ID describes a Transit connection in the Link, which means a Broadcast Network, the Link ID helps to identity the IP Address of the DR while the Link Data identifies the IP address of the local router in this link.
Link Type 3: The Link ID describes a connection to a Stub Network and identifies the subnet connected to this link while the Link Data provides the information of the Mask. This Link Type-3 is the once that allows OSPF routers to discover remotes subnets.
Link Type 4: The Link ID describes a connection to Virtual Link and identifies the remote Router ID that is connected to. The Link Data identifies the IP address of the Link that establishes a neighboring relationship through this virtual link.
If we focus on the Link ID of Link Type 1 and Link Type 2. Both describe the Network Type (P-2-P and P-2-M or Broadcast).
When two neighbors exchange Type-1 LSA, Analyzing the Link ID in the Link Type is amazing and important to detect potential problem of Network Type Mismatch, especially when you have access only to one router. R1 for example advertises a Type-1 LSA with Link Type 1 and the Link ID field informs R2: the connection is a Point-to-Point while the Router R2 advertises a Type-1 LSA with Link Type 2 and the Link ID field informs R1: the connection is a Transit Network and DR election occurs.
To summarize, R1 sees its neighbor through a Link Type 1 “connected to R2 as a point-to-point”, while R2 sees its neighbor through a Link Type 2 “link connected to Designated Router in a Transit Network”. This creates a discrepancy in the link-state database, which means no routes are installed in the routing table even if the corresponding LSAs are present in the LSDB.
