OSPF LSA types (RFC 2328)

LSA type 1: Router LSA (Point-to-point networks) (RFC 2328 Section 12.4.1) (Grey arrows in image):

Sent by every OSPF enabled device within an area
Advertises it's presence (flooded) on the LAN segment within an area
Contains directly connected links on the device and the link type
Type 1 LSA's include the link-state ID as the RID of the advertising router
Use the show ip ospf database router self-originate command to inspect router LSAs generated by a router.
This command reveals Type-1 router LSAs, which offer insights into an individual router and its OSPF-enabled interfaces.
Each interface is represented as a link in the OSPF database and is accompanied by an associated metric value.
These links are categorized into the following types:
1. Point-to-point networks: Links connected to a single OSPF-speaking device.
2. Transit networks: Networks capable of carrying non-locally generated or destined traffic, often involving multiple OSPF-speaking devices.
3. Stub networks: Networks without other OSPF-speaking devices; only local traffic flows to or from the network.
The OSPF metric value can be manually configured on a per-interface basis or determined by the IOS.
By default, IOS employs a reference bandwidth of 100,000Kbps, which corresponds to a 100Mbps FastEthernet connection.
Interface cost values are expressed as ratios of the reference bandwidth to the actual interface bandwidth, calculated using the formula: reference-bandwidth / interface-bandwidth.

LSA type 2: Network LSA (RFC 2328 Section 12.4.2) (Green arrows in image):

Generated by the Designated Router (DR) on a multi-access network, within an area
Contains a list of all the devices connected a particular LAN segment.
The link-state ID of the type 2 LSA is the IP address of the (DR)

LSA type 3: Summary LSAs (RFC 2328 Section 12.4.3)** (Orange arrows in image):

Generated by the Area Border Router (ABR) of each area and sent into the backbone area (Area 0) so the area knows how to reach other areas through the ABR
Summarizes detailed area information found in Type 1 and Type 2 LSA's
Summary LSA's use the network address as the link-state ID
Listed as O IA routes in the RIB

LSA type 4: ASBR Summary (RFC 2328 Section 12.4.3) (Grey arrows in image):

Generated by the ABR and is flooded within an area to all areas
It contains the information aboute the ASBR and sets the next-hop to the area ABR
The link-state ID is set to the RID of the ASBR

Type-4 ASBR summary LSAs provide reachability information for ASBRs within the OSPF domain.
ABRs distribute Type-4 ASBR summary LSAs when a Type-5 external LSA is flooded, and it wasn't originally advertised by a router within the Area where the Type-5 external LSA is being disseminated.
Areas that are not configured as NSSAs do not allow Type-7 NSSA external LSAs. Therefore, NSSA ABRs must convert these LSAs into Type-5 external LSAs if connectivity from other OSPF domains is desired.
NSSA ABRs take on the role of pseudo ASBRs when translating Type-7 NSSA external LSAs into Type-5 external LSAs.
Type-4 ASBR summary LSAs are unnecessary in the area where the ASBR is located.

LSA type 5: External LSAs (RFC 2328 Section 12.4.4) (Green arrows in image):

Generated by the ASBR
Flooded throughout the entire OSPF domain unaltered (with the exception of NSSA and Stub areas)
unaltered meaning The ASBR R5 sends the routes redistributed routes and every router in every area sees the route with the advertising router as the ASBR 5.5.5.5 for example
None of the other Areas may know how to reach the redistributed network. Because in the Type 1s and 2s we do not get the adverting router-id of the ASBR. We may have a summary to the ASBR but sometimes we may not
that's why we need type 4 LSAs which is a shown earlier an ASBR summary which is genrated by the ABR. This tells us that to get to the ASBR we need to go through the ABR
Contains a list of all redistributed routes
Shows up as E1/E2 routes in the RIB

**LSA type 6: Network LSA (RFC 1583-1585)

Certain networking vendors support the RFC 1584 Multicast Extensions to OSPF, allowing network devices to send and receive Type-6 LSAs.
However, some router manufacturers, such as Cisco, do not support Type 6 LSAs. When a router configured for OSPF receives Type-6 LSA packets, it generates syslog messages indicating that the LSA type is unsupported.
Depending on the network size and the number of active multicast groups, logging these syslog messages can consume CPU resources on the router.
In such scenarios, it's advisable for the router to silently discard Type-6 LSAs without generating log messages.
The solution is to configure the router to ignore Type 6 LSAs using the "ignore lsa mospf " command under the router process.

LSA type 7: NSSA (RFC 3101) (Orange arrows in image):