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:
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:
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.
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).
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:
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).
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).
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.
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:
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.
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.
