OSPF Header, OSPF packet types and OSPF LSA types

Georgecooldude

OK confused here.

OSPF packets share a common 24 byte header (or maybe 20 - will come to that later).

<IP header etc><Version,type,packet length, router id, area id, checksum, auth type, auth data><OSPF data></IP link trailer>
Inside that OSPF header we have "type" which can be from 1-5 (hello, dbd, lsr, lsu, ls ack).

Once the OSPF header is done we have the OSPF data field. If a router decides to send an LSA type 1-7 where does this go? Does it go in the data field? If so what would the type be flagged as? Maybe someone can clear that up for me as I haven't come across any documentation that is really clear about that.

Second question. Cisco says the common OSPF header is 24 bytes, but I've seen other sources saying its 20 bytes (including juniper documentation). Which is it? Or they are referring to 2 common OSPF packet headers?

Appreciate any assistance.

Plazma

They are defined in the "Type" field.. it can be an LSA, LSU, LSACK, Hello... or so forth.

tech-airman

Georgecooldude,

Georgecooldude wrote:

OK confused here.

OSPF packets share a common 24 byte header (or maybe 20 - will come to that later).

<IP header etc><Version,type,packet length, router id, area id, checksum, auth type, auth data><OSPF data></IP link trailer>
Inside that OSPF header we have "type" which can be from 1-5 (hello, dbd, lsr, lsu, ls ack).

Once the OSPF header is done we have the OSPF data field. If a router decides to send an LSA type 1-7 where does this go? Does it go in the data field? If so what would the type be flagged as? Maybe someone can clear that up for me as I haven't come across any documentation that is really clear about that.

The OSPF packet types of: 1) Hello 2) Database Description 3) Link-State Request 4) Link-State Update and 5) Link-State Acknowledgement are used in the overall OSPF Neighbor State management process of:

1. Down
2. Attempt
3. Init
4. 2-Way
5. Exstart
6. Exchange
7. Loading
8. Full

After the OSPF routers have reached the OSPF neighbor state of "Full" are LSAs exchanged between the OSPF routers. LSAs are sent to the OSPF multicast address of 224.0.0.5. Therefore, the terms "OSPF Packet" and "Link State Advertisements" are separate types of packets. That means, an LSA isn't of any of the OSPF packet types of 1 through 7.

Source:

1. OSPF Neighbor States [IP Routing] - Cisco Systems - http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f0e.shtml#loading

Georgecooldude wrote:

Second question. Cisco says the common OSPF header is 24 bytes, but I've seen other sources saying its 20 bytes (including juniper documentation). Which is it? Or they are referring to 2 common OSPF packet headers?

According to the Packet Format section of the Open Shortest Path First webpage of the Internetworking Technologies Handbook, it states "All OSPF packets begin with a 24-byte header, as illustrated in Figure 46-2." It is the LSA header that is 20 bytes long.

Source:

1. Internetworking Technology Handbook - Open Shortest Path First (OSPF) - Packet Format - http://www.cisco.com/en/US/docs/internetworking/technology/handbook/OSPF.html#wp1020593
2. Open Shorted Path First v3 - Cisco Systems - http://www.cisco.com/application/pdf/en/us/guest/tech/tk480/c1550/ccmigration_09186a0080187c6d.pdf (PDF page 27 of 93)

Georgecooldude wrote:

Appreciate any assistance.

kpjungle

LSA's are sent in an LSU (multiple LSA's can exist within an LSU). The LSU is sent in response to a LSR. (in the initial neighbor adjecency state).

If in a DR/BDR (multiaccess) environment, the LSU, and thereby LSA, are sent to the multicast of 224.0.0.6 (DR and BDR), and the DR will notify all adjacent routers of the change.

I would say that the OSPF Packet, can be one of the 5 types, and if its the LSU type, you've got your 11 different LSA types to choose from