OSPF's message types

[trane.m]

Hi,

I have a simple question. I tried searching for the answer, but dit not find any.

OSPF has 5 different message types, Hello, DBD, LSR, LSU and LSAck. What about LSA? What is Link State Advertisements, if not a OSPF message type?

[MHM Cisco World]

ospf use the Five message you mention, 

in message the prefix are classify with lsa,

for example 

LSU message contain prefix 1.1.1.1 which is lsa 5.

https://www.firewall.cx/networking-topics/routing/ospf-routing-protocol/1147-ospf-lsu-lsa-packet-structure-lsa-types-overview.html

[pman]

Hi,

---

@trane.m wrote:

What is Link State Advertisements, if not a OSPF message type?

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Link state advertisement

Describes the local state of a router or network.
This includes the state of the router's interfaces and adjacencies.
Each link state advertisement is flooded throughout the routing domain.
The collected link state advertisements of all routers and networks forms the
protocol's topological database.

[page 8]

Originating link state advertisements
a router will originate several link state advertisements.
Each router originates a router links advertisement.
If the router is also the Designated Router for any of the area's networks, it will originate network links
advertisements for those networks.

[Page 109]

https://www.rfc-editor.org/rfc/rfc1583.html

here is another example:

Link-state advertisement (LSA)

The LSAs (Link-State Advertisements) are used by routers running OSPF to exchange topology information. An LSA contains routing and topology information that describe a part of an OSPF network. Routers exchange LSAs and learn the complete topology of the network until all routers have the exact same topology database.

When two neighbors decide to exchange routes, they send each other a list of all LSAa in their respective topology database. Each router then checks its topology database and sends a Link State Request (LSR) requesting all LSAs not found in its topology table. The other router responds with the Link State Update (LSU) that contains all LSAs requested by the neighbor.

To better understand how LSAs are used in OSPF, consider the following example:

In the example above we have a two routers and a single computer. After configuring OSPF on both routers, routers exchange LSAs to describe their respective topology database. Router R1 sends an LSA header for its directly connected subnet 10.0.0.0/24. Router R2 check its topology database and determines that it doesn’t have information about that network. Router R2 then sends Link State Request message requesting further information about that subnet. Router R1 responds with Link State Update which contains information about subnet 10.0.0.0/24 (such as the next hop address, cost…).

https://geek-university.com/link-state-advertisement-lsa/

[Peter Paluch]

Hello,

To add to the other friends' answers here: Indeed, OSPF has 5 message types as you mentioned. The Link State Advertisement, or LSA, is not one of them because it is not a standalone message.

OSPF builds the entire map of the network as a jigsaw puzzle. For example, every router gets its own piece in this jigsaw puzzle. Every piece of this puzzle is an LSA. For certain reasons (which I'm not going to dive into just yet), OSPF has multiple types of jigsaw puzzle pieces - LSAs - for different components of the network:

1. Puzzle piece type for every single router, its interfaces and directly connected networks (LSA-1, Router LSA)
2. Puzzle piece type for every multi-access network connecting 2 or more routers together (LSA-2, Network LSA)
3. Puzzle piece type for every IP subnet in a different area (LSA-3, Network Summary LSA)
4. Puzzle piece type for every boundary router performing redistribution in a different area (LSA-4, ASBR Summary LSA)
5. Puzzle piece type for every external IP subnet redistributed on a boundary router (LSA-5, External LSA)

These LSAs are stored on every router in its link-state database, and if they need to be sent to another router, they are carried inside Link State Update packets, one or more as necessary. So the LSAs are carried as payloads of LSU messages.

Two more message types are also related to LSAs - DBD and LSR. Both of them, however, carry only the LSA headers but not their bodies. LSA headers are enough to uniquely identify the LSAs. If I, as a router, need to demonstrate to you the list of all LSAs I have in my link-state database, I will send you one or more DBD packets with the LSA headers of all LSAs in my database, and you can pick which ones you want to download from me. You can ask for them by putting their headers in an LSR packet to me, and I will then respond with an LSU with the complete LSAs in its payload.

Please feel welcome to ask further!

Best regards,
Peter

[Justinas Ciuplys]

I know I am replying to an old message, but... This is just amazing explanation! Can't thank you enough, this post clearly shows difference between knowing and understanding.

[Giuseppe Larosa]

Hello @trane.m ,

the LSAs are not OSPF messages, they are data structures that are built to provide information. Many types of LSAs exist in a single area the router LSA is built by each node in the area and the NEtwork LSA is build the OSPF DR in a LAN segment.

Devices in the same area share the same set of  LSAs type 1 and type 2 and this allows to run the SPF on each node and to build loop free shortest path for each destination prefix from the point of view of each node.

OSPF nodes have the concept of OSPF RID that has to be unique ( it is an IP address) and it represents the node in the database.

To be noted only the LSU messages contain the full content of LSAs. The LSR contains a list of LSAs requested.

Hope to help

Giuseppe

[iLoveBGP]

Think of OSPF like a group of neighbors sharing information about their local streets to build a collective map of the neighborhood. The messages they use are like notes passed between them, and there are five main types of notes:

1. Hello Messages: These are like friendly introductions: "Hey, I'm here! Are you my neighbor?" They help routers discover and maintain relationships (adjacencies) with each other.

2. Database Description (DBD) Messages: These are like summaries of what each router knows: "Here’s a list of the streets I’ve mapped so far." It’s a way for routers to compare their knowledge.

3. Link State Request (LSR) Messages: When a router realizes it’s missing some details, it sends a request: "Can you tell me more about Street X?"

4. Link State Update (LSU) Messages: These are the detailed replies: "Sure, here’s everything I know about Street X!" LSU messages carry the actual Link State Advertisements (LSAs).

5. Link State Acknowledgment (LSAck) Messages: After getting the LSU, the router sends a polite "Got it!" to confirm receipt.

Now, about LSAs—they’re the contents of the LSU message. If an LSU is a package, LSAs are the items inside. LSAs describe the state of a router’s connections (links) to its neighbors. These are what routers use to piece together the complete map of the network. So, LSAs aren’t a separate message type—they’re the data being exchanged within LSU messages.