12.4. Originating link state advertisements

Into any given OSPF area, 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.

Area border routers originate a single summary link advertisement for each known inter-area destination. AS boundary routers originate a single AS external link advertisement for each known AS external destination. Destinations are advertised one at a time so that the change in any single route can be flooded without reflooding the entire collection of routes. During the flooding procedure, many link state advertisements can be carried by a single Link State Update packet.

As an example, consider Router RT4 in Figure 6. It is an area border router, having a connection to Area 1 and the backbone. Router RT4 originates 5 distinct link state advertisements into the backbone (one router links, and one summary link for each of the networks N1-N4). Router RT4 will also originate 8 distinct link state advertisements into Area 1 (one router links and seven summary link advertisements as pictured in Figure 7). If RT4 has been selected as Designated Router for Network N3, it will also originate a network links advertisement for N3 into Area 1.

In this same figure, Router RT5 will be originating 3 distinct AS external link advertisements (one for each of the networks N12-N14). These will be flooded throughout the entire AS, assuming that none of the areas have been configured as stubs. However, if area 3 has been configured as a stub area, the external advertisements for networks N12-N14 will not be flooded into area 3 (see Section 3.6). Instead, Router RT11 would originate a default summary link advertisement that would be flooded throughout area 3 (see Section 12.4.3). This instructs all of area 3's internal routers to send their AS external traffic to RT11.

Whenever a new instance of a link state advertisement is originated, its LS sequence number is incremented, its LS age is set to 0, its LS checksum is calculated, and the advertisement is added to the link state database and flooded out the appropriate interfaces. See Section 13.2 for details concerning the installation of the advertisement into the link state database. See Section 13.3 for details concerning the flooding of newly originated advertisements.

The ten events that can cause a new instance of a link state advertisement to be originated are:

1. The LS age field of one of the router's self-originated advertisements reaches the value LSRefreshTime. In this case, a new instance of the link state advertisement is originated, even though the contents of the advertisement (apart from the link state advertisement header) will be the same. This guarantees periodic originations of all link state advertisements. This periodic updating of link state advertisements adds robustness to the link state algorithm. Link state advertisements that solely describe unreachable destinations should not be refreshed, but should instead be flushed from the routing domain (see Section 14.1).

   When whatever is being described by a link state advertisement changes, a new advertisement is originated. However, two instances of the same link state advertisement may not be originated within the time period MinLSInterval. This may require that the generation of the next instance be delayed by up to MinLSInterval. The following events may cause the contents of a link state advertisement to change. These events should cause new originations if and only if the contents of the new advertisement would be different:

2. An interface's state changes (see Section 9.1). This may mean that it is necessary to produce a new instance of the router links advertisement.

3. An attached network's Designated Router changes. A new router links advertisement should be originated. Also, if the router itself is now the Designated Router, a new network links advertisement should be produced. If the router itself is no longer the Designated Router, any network links advertisement that it might have originated for the network should be flushed from the routing domain (see Section 14.1).

4. One of the neighboring routers changes to/from the FULL state. This may mean that it is necessary to produce a new instance of the router links advertisement. Also, if the router is itself the Designated Router for the attached network, a new network links advertisement should be produced.

   The next four events concern area border routers only:

5. An intra-area route has been added/deleted/modified in the routing table. This may cause a new instance of a summary links advertisement (for this route) to be originated in each attached area (possibly including the backbone).

6. An inter-area route has been added/deleted/modified in the routing table. This may cause a new instance of a summary links advertisement (for this route) to be originated in each attached area (but NEVER for the backbone).

7. The router becomes newly attached to an area. The router must then originate summary link advertisements into the newly attached area for all pertinent intra-area and inter- area routes in the router's routing table. See Section 12.4.3 for more details.

8. When the state of one of the router's configured virtual links changes, it may be necessary to originate a new router links advertisement into the virtual link's transit area (see the discussion of the router links advertisement's bit V in Section 12.4.1), as well as originating a new router links advertisement into the backbone.

   The last two events concern AS boundary routers (and former AS boundary routers) only:

9. An external route gained through direct experience with an external routing protocol (like EGP) changes. This will cause an AS boundary router to originate a new instance of an AS external link advertisement.

10. A router ceases to be an AS boundary router, perhaps after restarting. In this situation the router should flush all AS external link advertisements that it had previously originated. These advertisements can be flushed via the premature aging procedure specified in Section 14.1.

The construction of each type of link state advertisement is explained in detail below. In general, these sections describe the contents of the advertisement body (i.e., the part coming after the 20-byte advertisement header). For information concerning the building of the link state advertisement header, see Section 12.1.

• 12.4.1. Router links
• 12.4.2. Network links
• 12.4.3. Summary links
• 12.4.4. Originating summary links into stub areas
• 12.4.5. AS external links