13. The Flooding Procedure
Link State Update packets provide the mechanism for flooding link
state advertisements. A Link State Update packet may contain
several distinct advertisements, and floods each advertisement one
hop further from its point of origination. To make the flooding
procedure reliable, each advertisement must be acknowledged
separately. Acknowledgments are transmitted in Link State
Acknowledgment packets. Many separate acknowledgments can also be
Figure 16: Forwarding address example
grouped together into a single packet.
The flooding procedure starts when a Link State Update packet has
been received. Many consistency checks have been made on the
received packet before being handed to the flooding procedure (see
Section 8.2). In particular, the Link State Update packet has been
associated with a particular neighbor, and a particular area. If
the neighbor is in a lesser state than Exchange, the packet should
be dropped without further processing.
All types of link state advertisements, other than AS external link
advertisements, are associated with a specific area. However, link
state advertisements do not contain an area field. A link state
advertisement's area must be deduced from the Link State Update
For each link state advertisement contained in the packet, the
following steps are taken:
(1) Validate the advertisement's LS checksum. If the checksum turns
out to be invalid, discard the advertisement and get the next
one from the Link State Update packet.
(2) Examine the link state advertisement's LS type. If the LS type
is unknown, discard the advertisement and get the next one from
the Link State Update Packet. This specification defines LS
types 1-5 (see Section 4.3).
(3) Else if this is a AS external link advertisement (LS type = 5),
and the area has been configured as a stub area, discard the
advertisement and get the next one from the Link State Update
Packet. AS external link advertisements are not flooded
into/throughout stub areas (see Section 3.6).
(4) Else if the advertisement's LS age is equal to MaxAge, and there
is currently no instance of the advertisement in the router's
link state database, then take the following actions:
(a) Acknowledge the receipt of the advertisement by sending a
Link State Acknowledgment packet back to the sending
neighbor (see Section 13.5).
(b) Purge all outstanding requests for equal or previous
instances of the advertisement from the sending neighbor's
Link State Request list (see Section 10).
(c) If the sending neighbor is in state Exchange or in state
Loading, then install the MaxAge advertisement in the link
state database. Otherwise, simply discard the
advertisement. In either case, examine the next
advertisement (if any) listed in the Link State Update
(5) Otherwise, find the instance of this advertisement that is
currently contained in the router's link state database. If
there is no database copy, or the received advertisement is more
recent than the database copy (see Section 13.1 below for the
determination of which advertisement is more recent) the
following steps must be performed:
(a) If there is already a database copy, and if the database
copy was installed less than MinLSInterval seconds ago,
discard the new advertisement (without acknowledging it) and
examine the next advertisement (if any) listed in the Link
State Update packet.
(b) Otherwise immediately flood the new advertisement out some
subset of the router's interfaces (see Section 13.3). In
some cases (e.g., the state of the receiving interface is DR
and the advertisement was received from a router other than
the Backup DR) the advertisement will be flooded back out
the receiving interface. This occurrence should be noted
for later use by the acknowledgment process (Section 13.5).
(c) Remove the current database copy from all neighbors' Link
state retransmission lists.
(d) Install the new advertisement in the link state database
(replacing the current database copy). This may cause the
routing table calculation to be scheduled. In addition,
timestamp the new advertisement with the current time (i.e.,
the time it was received). The flooding procedure cannot
overwrite the newly installed advertisement until
MinLSInterval seconds have elapsed. The advertisement
installation process is discussed further in Section 13.2.
(e) Possibly acknowledge the receipt of the advertisement by
sending a Link State Acknowledgment packet back out the
receiving interface. This is explained below in Section
(f) If this new link state advertisement indicates that it was
originated by the receiving router itself (i.e., is
considered a self-originated advertisement), the router must
take special action, either updating the advertisement or in
some cases flushing it from the routing domain. For a
description of how self-originated advertisements are
detected and subsequently handled, see Section 13.4.
(6) Else, if there is an instance of the advertisement on the
sending neighbor's Link state request list, an error has
occurred in the Database Exchange process. In this case,
restart the Database Exchange process by generating the neighbor
event BadLSReq for the sending neighbor and stop processing the
Link State Update packet.
(7) Else, if the received advertisement is the same instance as the
database copy (i.e., neither one is more recent) the following
two steps should be performed:
(a) If the advertisement is listed in the Link state
retransmission list for the receiving adjacency, the router
itself is expecting an acknowledgment for this
advertisement. The router should treat the received
advertisement as an acknowledgment, by removing the
advertisement from the Link state retransmission list. This
is termed an "implied acknowledgment". Its occurrence
should be noted for later use by the acknowledgment process
(b) Possibly acknowledge the receipt of the advertisement by
sending a Link State Acknowledgment packet back out the
receiving interface. This is explained below in Section
(8) Else, the database copy is more recent. Note an unusual event
to network management, discard the advertisement and process the
next link state advertisement contained in the Link State Update
13.1. Determining which link state is newer
When a router encounters two instances of a link state
advertisement, it must determine which is more recent. This
occurred above when comparing a received advertisement to its
database copy. This comparison must also be done during the
Database Exchange procedure which occurs during adjacency
A link state advertisement is identified by its LS type, Link
State ID and Advertising Router. For two instances of the same
advertisement, the LS sequence number, LS age, and LS checksum
fields are used to determine which instance is more recent:
o The advertisement having the newer LS sequence number is
more recent. See Section 12.1.6 for an explanation of the
LS sequence number space. If both instances have the same
LS sequence number, then:
o If the two instances have different LS checksums, then the
instance having the larger LS checksum (when considered as a
16-bit unsigned integer) is considered more recent.
o Else, if only one of the instances has its LS age field set
to MaxAge, the instance of age MaxAge is considered to be
o Else, if the LS age fields of the two instances differ by
more than MaxAgeDiff, the instance having the smaller
(younger) LS age is considered to be more recent.
o Else, the two instances are considered to be identical.
13.2. Installing link state advertisements in the database
Installing a new link state advertisement in the database,
either as the result of flooding or a newly self-originated
advertisement, may cause the OSPF routing table structure to be
recalculated. The contents of the new advertisement should be
compared to the old instance, if present. If there is no
difference, there is no need to recalculate the routing table.
(Note that even if the contents are the same, the LS checksum
will probably be different, since the checksum covers the LS
If the contents are different, the following pieces of the
routing table must be recalculated, depending on the new
advertisement's LS type field:
Router links and network links advertisements
The entire routing table must be recalculated, starting with
the shortest path calculations for each area (not just the
area whose topological database has changed). The reason
that the shortest path calculation cannot be restricted to
the single changed area has to do with the fact that AS
boundary routers may belong to multiple areas. A change in
the area currently providing the best route may force the
router to use an intra-area route provided by a different
Summary link advertisements
The best route to the destination described by the summary
link advertisement must be recalculated (see Section 16.5).
If this destination is an AS boundary router, it may also be
necessary to re-examine all the AS external link
AS external link advertisements
The best route to the destination described by the AS
external link advertisement must be recalculated (see
Also, any old instance of the advertisement must be removed from
the database when the new advertisement is installed. This old
instance must also be removed from all neighbors' Link state
retransmission lists (see Section 10).
13.3. Next step in the flooding procedure
When a new (and more recent) advertisement has been received, it
must be flooded out some set of the router's interfaces. This
section describes the second part of flooding procedure (the
first part being the processing that occurred in Section 13),
namely, selecting the outgoing interfaces and adding the
advertisement to the appropriate neighbors' Link state
retransmission lists. Also included in this part of the
flooding procedure is the maintenance of the neighbors' Link
state request lists.
This section is equally applicable to the flooding of an
advertisement that the router itself has just originated (see
Section 12.4). For these advertisements, this section provides
the entirety of the flooding procedure (i.e., the processing of
Section 13 is not performed, since, for example, the
advertisement has not been received from a neighbor and
therefore does not need to be acknowledged).
Depending upon the advertisement's LS type, the advertisement
can be flooded out only certain interfaces. These interfaces,
defined by the following, are called the eligible interfaces:
AS external link advertisements (LS Type = 5)
AS external link advertisements are flooded throughout the
entire AS, with the exception of stub areas (see Section
3.6). The eligible interfaces are all the router's
interfaces, excluding virtual links and those interfaces
attaching to stub areas.
All other LS types
All other types are specific to a single area (Area A). The
eligible interfaces are all those interfaces attaching to
the Area A. If Area A is the backbone, this includes all
the virtual links.
Link state databases must remain synchronized over all
adjacencies associated with the above eligible interfaces. This
is accomplished by executing the following steps on each
eligible interface. It should be noted that this procedure may
decide not to flood a link state advertisement out a particular
interface, if there is a high probability that the attached
neighbors have already received the advertisement. However, in
these cases the flooding procedure must be absolutely sure that
the neighbors eventually do receive the advertisement, so the
advertisement is still added to each adjacency's Link state
retransmission list. For each eligible interface:
(1) Each of the neighbors attached to this interface are
examined, to determine whether they must receive the new
advertisement. The following steps are executed for each
(a) If the neighbor is in a lesser state than Exchange, it
does not participate in flooding, and the next neighbor
should be examined.
(b) Else, if the adjacency is not yet full (neighbor state
is Exchange or Loading), examine the Link state request
list associated with this adjacency. If there is an
instance of the new advertisement on the list, it
indicates that the neighboring router has an instance of
the advertisement already. Compare the new
advertisement to the neighbor's copy:
o If the new advertisement is less recent, then
examine the next neighbor.
o If the two copies are the same instance, then delete
the advertisement from the Link state request list,
and examine the next neighbor.
o Else, the new advertisement is more recent. Delete
the advertisement from the Link state request list.
(c) If the new advertisement was received from this
neighbor, examine the next neighbor.
(d) At this point we are not positive that the neighbor has
an up-to-date instance of this new advertisement. Add
the new advertisement to the Link state retransmission
list for the adjacency. This ensures that the flooding
procedure is reliable; the advertisement will be
retransmitted at intervals until an acknowledgment is
seen from the neighbor.
(2) The router must now decide whether to flood the new link
state advertisement out this interface. If in the previous
step, the link state advertisement was NOT added to any of
the Link state retransmission lists, there is no need to
flood the advertisement out the interface and the next
interface should be examined.
(3) If the new advertisement was received on this interface, and
it was received from either the Designated Router or the
Backup Designated Router, chances are that all the neighbors
have received the advertisement already. Therefore, examine
the next interface.
(4) If the new advertisement was received on this interface, and
the interface state is Backup (i.e., the router itself is
the Backup Designated Router), examine the next interface.
The Designated Router will do the flooding on this
interface. If the Designated Router fails, this router will
end up retransmitting the updates.
(5) If this step is reached, the advertisement must be flooded
out the interface. Send a Link State Update packet (with
the new advertisement as contents) out the interface. The
advertisement's LS age must be incremented by InfTransDelay
(which must be > 0) when copied into the outgoing Link State
Update packet (until the LS age field reaches its maximum
value of MaxAge).
On broadcast networks, the Link State Update packets are
multicast. The destination IP address specified for the
Link State Update Packet depends on the state of the
interface. If the interface state is DR or Backup, the
address AllSPFRouters should be used. Otherwise, the
address AllDRouters should be used.
On non-broadcast, multi-access networks, separate Link State
Update packets must be sent, as unicasts, to each adjacent
neighbor (i.e., those in state Exchange or greater). The
destination IP addresses for these packets are the
neighbors' IP addresses.
13.4. Receiving self-originated link state
It is a common occurrence for a router to receive self-
originated link state advertisements via the flooding procedure.
A self-originated advertisement is detected when either 1) the
advertisement's Advertising Router is equal to the router's own
Router ID or 2) the advertisement is a network links
advertisement and its Link State ID is equal to one of the
router's own IP interface addresses.
However, if the received self-originated advertisement is newer
than the last instance that the router actually originated, the
router must take special action. The reception of such an
advertisement indicates that there are link state advertisements
in the routing domain that were originated before the last time
the router was restarted. In most cases, the router must then
advance the advertisement's LS sequence number one past the
received LS sequence number, and originate a new instance of the
It may be the case the router no longer wishes to originate the
received advertisement. Possible examples include: 1) the
advertisement is a summary link or AS external link and the
router no longer has an (advertisable) route to the destination,
2) the advertisement is a network links advertisement but the
router is no longer Designated Router for the network or 3) the
advertisement is a network links advertisement whose Link State
ID is one of the router's own IP interface addresses but whose
Advertising Router is not equal to the router's own Router ID
(this latter case should be rare, and it indicates that the
router's Router ID has changed since originating the
advertisement). In all these cases, instead of updating the
advertisement, the advertisement should be flushed from the
routing domain by incrementing the received advertisement's LS
age to MaxAge and reflooding (see Section 14.1).
13.5. Sending Link State Acknowledgment packets
Each newly received link state advertisement must be
acknowledged. This is usually done by sending Link State
Acknowledgment packets. However, acknowledgments can also be
accomplished implicitly by sending Link State Update packets
(see step 7a of Section 13).
Many acknowledgments may be grouped together into a single Link
State Acknowledgment packet. Such a packet is sent back out the
interface that has received the advertisements. The packet can
be sent in one of two ways: delayed and sent on an interval
timer, or sent directly (as a unicast) to a particular neighbor.
The particular acknowledgment strategy used depends on the
circumstances surrounding the receipt of the advertisement.
Sending delayed acknowledgments accomplishes several things: it
facilitates the packaging of multiple acknowledgments in a
single Link State Acknowledgment packet; it enables a single
Link State Acknowledgment packet to indicate acknowledgments to
several neighbors at once (through multicasting); and it
randomizes the Link State Acknowledgment packets sent by the
various routers attached to a multi-access network. The fixed
interval between a router's delayed transmissions must be short
(less than RxmtInterval) or needless retransmissions will ensue.
Direct acknowledgments are sent to a particular neighbor in
response to the receipt of duplicate link state advertisements.
These acknowledgments are sent as unicasts, and are sent
immediately when the duplicate is received.
The precise procedure for sending Link State Acknowledgment
packets is described in Table 19. The circumstances surrounding
the receipt of the advertisement are listed in the left column.
The acknowledgment action then taken is listed in one of the two
right columns. This action depends on the state of the
concerned interface; interfaces in state Backup behave
differently from interfaces in all other states. Delayed
acknowledgments must be delivered to all adjacent routers
associated with the interface. On broadcast networks, this is
accomplished by sending the delayed Link State Acknowledgment
packets as multicasts. The Destination IP address used depends
on the state of the interface. If the state is DR or Backup,
the destination AllSPFRouters is used. In other states, the
destination AllDRouters is used. On non-broadcast networks,
delayed Link State Acknowledgment packets must be unicast
separately over each adjacency (i.e., neighbor whose state is >=
The reasoning behind sending the above packets as multicasts is
best explained by an example. Consider the network
configuration depicted in Figure 15. Suppose RT4 has been
elected as Designated Router, and RT3 as Backup Designated
Router for the network N3. When Router RT4 floods a new
advertisement to Network N3, it is received by routers RT1, RT2,
and RT3. These routers will not flood the advertisement back
onto net N3, but they still must ensure that their topological
databases remain synchronized with their adjacent neighbors. So
RT1, RT2, and RT4 are waiting to see an acknowledgment from RT3.
Likewise, RT4 and RT3 are both waiting to see acknowledgments
from RT1 and RT2. This is best achieved by sending the
acknowledgments as multicasts.
The reason that the acknowledgment logic for Backup DRs is
slightly different is because they perform differently during
the flooding of link state advertisements (see Section 13.3,
13.6. Retransmitting link state advertisements
Advertisements flooded out an adjacency are placed on the
adjacency's Link state retransmission list. In order to ensure
that flooding is reliable, these advertisements are
retransmitted until they are acknowledged. The length of time
between retransmissions is a configurable per-interface value,
RxmtInterval. If this is set too low for an interface, needless
retransmissions will ensue. If the value is set too high, the
speed of the flooding, in the face of lost packets, may be
Action taken in state
Circumstances Backup All other states
Advertisement has No acknowledgment No acknowledgment
been flooded back sent. sent.
out receiving in-
terface (see Sec-
tion 13, step 5b).
Advertisement is Delayed acknowledg- Delayed ack-
more recent than ment sent if adver- nowledgment sent.
database copy, but tisement received
was not flooded from Designated
back out receiving Router, otherwise
interface do nothing
Advertisement is a Delayed acknowledg- No acknowledgment
duplicate, and was ment sent if adver- sent.
treated as an im- tisement received
plied acknowledg- from Designated
ment (see Section Router, otherwise
13, step 7a). do nothing
Advertisement is a Direct acknowledg- Direct acknowledg-
duplicate, and was ment sent. ment sent.
not treated as an
Advertisement's LS Direct acknowledg- Direct acknowledg-
age is equal to ment sent. ment sent.
MaxAge, and there is
no current instance
of the advertisement
in the link state
Section 13, step 4).
Table 19: Sending link state acknowledgements.
Several retransmitted advertisements may fit into a single Link
State Update packet. When advertisements are to be
retransmitted, only the number fitting in a single Link State
Update packet should be transmitted. Another packet of
retransmissions can be sent when some of the advertisements are
acknowledged, or on the next firing of the retransmission timer.
Link State Update Packets carrying retransmissions are always
sent as unicasts (directly to the physical address of the
neighbor). They are never sent as multicasts. Each
advertisement's LS age must be incremented by InfTransDelay
(which must be > 0) when copied into the outgoing Link State
Update packet (until the LS age field reaches its maximum value
If the adjacent router goes down, retransmissions may occur
until the adjacency is destroyed by OSPF's Hello Protocol. When
the adjacency is destroyed, the Link state retransmission list
13.7. Receiving link state acknowledgments
Many consistency checks have been made on a received Link State
Acknowledgment packet before it is handed to the flooding
procedure. In particular, it has been associated with a
particular neighbor. If this neighbor is in a lesser state than
Exchange, the Link State Acknowledgment packet is discarded.
Otherwise, for each acknowledgment in the Link State
Acknowledgment packet, the following steps are performed:
o Does the advertisement acknowledged have an instance on the
Link state retransmission list for the neighbor? If not,
examine the next acknowledgment. Otherwise:
o If the acknowledgment is for the same instance that is
contained on the list, remove the item from the list and
examine the next acknowledgment. Otherwise:
o Log the questionable acknowledgment, and examine the next
14. Aging The Link State Database
Each link state advertisement has an LS age field. The LS age is
expressed in seconds. An advertisement's LS age field is
incremented while it is contained in a router's database. Also,
when copied into a Link State Update Packet for flooding out a
particular interface, the advertisement's LS age is incremented by
An advertisement's LS age is never incremented past the value
MaxAge. Advertisements having age MaxAge are not used in the
routing table calculation. As a router ages its link state
database, an advertisement's LS age may reach MaxAge. At this
time, the router must attempt to flush the advertisement from the
routing domain. This is done simply by reflooding the MaxAge
advertisement just as if it was a newly originated advertisement
(see Section 13.3).
When creating a Database summary list for a newly forming adjacency,
any MaxAge advertisements present in the link state database are
added to the neighbor's Link state retransmission list instead of
the neighbor's Database summary list. See Section 10.3 for more
A MaxAge advertisement must be removed immediately from the router's
link state database as soon as both a) it is no longer contained on
any neighbor Link state retransmission lists and b) none of the
router's neighbors are in states Exchange or Loading.
When, in the process of aging the link state database, an
advertisement's LS age hits a multiple of CheckAge, its LS checksum
should be verified. If the LS checksum is incorrect, a program or
memory error has been detected, and at the very least the router
itself should be restarted.
14.1. Premature aging of advertisements
A link state advertisement can be flushed from the routing
domain by setting its LS age to MaxAge and reflooding the
advertisement. This procedure follows the same course as
flushing an advertisement whose LS age has naturally reached the
value MaxAge (see Section 14). In particular, the MaxAge
advertisement is removed from the router's link state database
as soon as a) it is no longer contained on any neighbor Link
state retransmission lists and b) none of the router's neighbors
are in states Exchange or Loading. We call the setting of an
advertisement's LS age to MaxAge premature aging.
Premature aging is used when it is time for a self-originated
advertisement's sequence number field to wrap. At this point,
the current advertisement instance (having LS sequence number of
0x7fffffff) must be prematurely aged and flushed from the
routing domain before a new instance with sequence number
0x80000001 can be originated. See Section 12.1.6 for more
Premature aging can also be used when, for example, one of the
router's previously advertised external routes is no longer
reachable. In this circumstance, the router can flush its
external advertisement from the routing domain via premature
aging. This procedure is preferable to the alternative, which is
to originate a new advertisement for the destination specifying
a metric of LSInfinity. Premature aging is also be used when
unexpectedly receiving self-originated advertisements during the
flooding procedure (see Section 13.4).
A router may only prematurely age its own self-originated link
state advertisements. The router may not prematurely age
advertisements that have been originated by other routers. An
advertisement is considered self-originated when either 1) the
advertisement's Advertising Router is equal to the router's own
Router ID or 2) the advertisement is a network links
advertisement and its Link State ID is equal to one of the
router's own IP interface addresses.
15. Virtual Links
The single backbone area (Area ID = 0.0.0.0) cannot be disconnected,
or some areas of the Autonomous System will become unreachable. To
establish/maintain connectivity of the backbone, virtual links can
be configured through non-backbone areas. Virtual links serve to
connect physically separate components of the backbone. The two
endpoints of a virtual link are area border routers. The virtual
link must be configured in both routers. The configuration
information in each router consists of the other virtual endpoint
(the other area border router), and the non-backbone area the two
routers have in common (called the transit area). Virtual links
cannot be configured through stub areas (see Section 3.6).
The virtual link is treated as if it were an unnumbered point-to-
point network (belonging to the backbone) joining the two area
border routers. An attempt is made to establish an adjacency over
the virtual link. When this adjacency is established, the virtual
link will be included in backbone router links advertisements, and
OSPF packets pertaining to the backbone area will flow over the
adjacency. Such an adjacency has been referred to in this document
as a "virtual adjacency".
In each endpoint router, the cost and viability of the virtual link
is discovered by examining the routing table entry for the other
endpoint router. (The entry's associated area must be the
configured transit area). Actually, there may be a separate routing
table entry for each Type of Service. These are called the virtual
link's corresponding routing table entries. The InterfaceUp event
occurs for a virtual link when its corresponding TOS 0 routing table
entry becomes reachable. Conversely, the InterfaceDown event occurs
when its TOS 0 routing table entry becomes unreachable. In other
words, the virtual link's viability is determined by the existence
of an intra-area path, through the transit area, between the two
endpoints. Note that a virtual link whose underlying path has cost
greater than hexadecimal 0xffff (the maximum size of an interface
cost in a router links advertisement) should be considered
inoperational (i.e., treated the same as if the path did not exist).
The other details concerning virtual links are as follows:
o AS external links are NEVER flooded over virtual adjacencies.
This would be duplication of effort, since the same AS external
links are already flooded throughout the virtual link's transit
area. For this same reason, AS external link advertisements are
not summarized over virtual adjacencies during the Database
o The cost of a virtual link is NOT configured. It is defined to
be the cost of the intra-area path between the two defining area
border routers. This cost appears in the virtual link's
corresponding routing table entry. When the cost of a virtual
link changes, a new router links advertisement should be
originated for the backbone area.
o Just as the virtual link's cost and viability are determined by
the routing table build process (through construction of the
routing table entry for the other endpoint), so are the IP
interface address for the virtual interface and the virtual
neighbor's IP address. These are used when sending OSPF
protocol packets over the virtual link. Note that when one (or
both) of the virtual link endpoints connect to the transit area
via an unnumbered point-to-point link, it may be impossible to
calculate either the virtual interface's IP address and/or the
virtual neighbor's IP address, thereby causing the virtual link
o In each endpoint's router links advertisement for the backbone,
the virtual link is represented as a Type 4 link whose Link ID
is set to the virtual neighbor's OSPF Router ID and whose Link
Data is set to the virtual interface's IP address. See Section
12.4.1 for more information. Note that it may be the case that
there is a TOS 0 path, but no non-zero TOS paths, between the
two endpoint routers. In this case, both routers must revert to
being non-TOS-capable, clearing the T-bit in the Options field
of their backbone router links advertisements.
o When virtual links are configured for the backbone, information
concerning backbone networks should not be condensed before
being summarized for the transit areas. In other words, each
backbone network should be advertised into the transit areas in
a separate summary link advertisement, regardless of the
backbone's configured area address ranges. See Section 12.4.3
for more information.
o The time between link state retransmissions, RxmtInterval, is
configured for a virtual link. This should be well over the
expected round-trip delay between the two routers. This may be
hard to estimate for a virtual link; it is better to err on the
side of making it too large.
16. Calculation Of The Routing Table
This section details the OSPF routing table calculation. Using its
attached areas' link state databases as input, a router runs the
following algorithm, building its routing table step by step. At
each step, the router must access individual pieces of the link
state databases (e.g., a router links advertisement originated by a
certain router). This access is performed by the lookup function
discussed in Section 12.2. The lookup process may return a link
state advertisement whose LS age is equal to MaxAge. Such an
advertisement should not be used in the routing table calculation,
and is treated just as if the lookup process had failed.
The OSPF routing table's organization is explained in Section 11.
Two examples of the routing table build process are presented in
Sections 11.2 and 11.3. This process can be broken into the
(1) The present routing table is invalidated. The routing table is
built again from scratch. The old routing table is saved so
that changes in routing table entries can be identified.
(2) The intra-area routes are calculated by building the shortest-
path tree for each attached area. In particular, all routing
table entries whose Destination Type is "area border router" are
calculated in this step. This step is described in two parts.
At first the tree is constructed by only considering those links
between routers and transit networks. Then the stub networks
are incorporated into the tree. During the area's shortest-path
tree calculation, the area's TransitCapability is also
calculated for later use in Step 4.
(3) The inter-area routes are calculated, through examination of
summary link advertisements. If the router is attached to
multiple areas (i.e., it is an area border router), only
backbone summary link advertisements are examined.
(4) In area border routers connecting to one or more transit areas
(i.e, non-backbone areas whose TransitCapability is found to be
TRUE), the transit areas' summary link advertisements are
examined to see whether better paths exist using the transit
areas than were found in Steps 2-3 above.
(5) Routes to external destinations are calculated, through
examination of AS external link advertisements. The locations
of the AS boundary routers (which originate the AS external link
advertisements) have been determined in steps 2-4.
Steps 2-5 are explained in further detail below. The explanations
describe the calculations for TOS 0 only. It may also be necessary
to perform each step (separately) for each of the non-zero TOS
values. For more information concerning the building of non-zero
TOS routes see Section 16.9.
Changes made to routing table entries as a result of these
calculations can cause the OSPF protocol to take further actions.
For example, a change to an intra-area route will cause an area
border router to originate new summary link advertisements (see
Section 12.4). See Section 16.7 for a complete list of the OSPF
protocol actions resulting from routing table changes.
16.1. Calculating the shortest-path tree for an area
This calculation yields the set of intra-area routes associated
with an area (called hereafter Area A). A router calculates the
shortest-path tree using itself as the root. The formation
of the shortest path tree is done here in two stages. In the
first stage, only links between routers and transit networks are
considered. Using the Dijkstra algorithm, a tree is formed from
this subset of the link state database. In the second stage,
leaves are added to the tree by considering the links to stub
The procedure will be explained using the graph terminology that
was introduced in Section 2. The area's link state database is
represented as a directed graph. The graph's vertices are
routers, transit networks and stub networks. The first stage of
the procedure concerns only the transit vertices (routers and
transit networks) and their connecting links. Throughout the
shortest path calculation, the following data is also associated
with each transit vertex:
Vertex (node) ID
A 32-bit number uniquely identifying the vertex. For router
vertices this is the router's OSPF Router ID. For network
vertices, this is the IP address of the network's Designated
A link state advertisement
Each transit vertex has an associated link state
advertisement. For router vertices, this is a router links
advertisement. For transit networks, this is a network
links advertisement (which is actually originated by the
network's Designated Router). In any case, the
advertisement's Link State ID is always equal to the above
List of next hops
The list of next hops for the current set of shortest paths
from the root to this vertex. There can be multiple
shortest paths due to the equal-cost multipath capability.
Each next hop indicates the outgoing router interface to use
when forwarding traffic to the destination. On multi-access
networks, the next hop also includes the IP address of the
next router (if any) in the path towards the destination.
Distance from root
The link state cost of the current set of shortest paths
from the root to the vertex. The link state cost of a path
is calculated as the sum of the costs of the path's
constituent links (as advertised in router links and network
links advertisements). One path is said to be "shorter"
than another if it has a smaller link state cost.
The first stage of the procedure (i.e., the Dijkstra algorithm)
can now be summarized as follows. At each iteration of the
algorithm, there is a list of candidate vertices. Paths from
the root to these vertices have been found, but not necessarily
the shortest ones. However, the paths to the candidate vertex
that is closest to the root are guaranteed to be shortest; this
vertex is added to the shortest-path tree, removed from the
candidate list, and its adjacent vertices are examined for
possible addition to/modification of the candidate list. The
algorithm then iterates again. It terminates when the candidate
list becomes empty.
The following steps describe the algorithm in detail. Remember
that we are computing the shortest path tree for Area A. All
references to link state database lookup below are from Area A's
(1) Initialize the algorithm's data structures. Clear the list
of candidate vertices. Initialize the shortest-path tree to
only the root (which is the router doing the calculation).
Set Area A's TransitCapability to FALSE.
(2) Call the vertex just added to the tree vertex V. Examine
the link state advertisement associated with vertex V. This
is a lookup in the Area A's link state database based on the
Vertex ID. If this is a router links advertisement, and bit
V of the router links advertisement (see Section A.4.2) is
set, set Area A's TransitCapability to TRUE. In any case,
each link described by the advertisement gives the cost to
an adjacent vertex. For each described link, (say it joins
vertex V to vertex W):
(a) If this is a link to a stub network, examine the next
link in V's advertisement. Links to stub networks will
be considered in the second stage of the shortest path
(b) Otherwise, W is a transit vertex (router or transit
network). Look up the vertex W's link state
advertisement (router links or network links) in Area
A's link state database. If the advertisement does not
exist, or its LS age is equal to MaxAge, or it does not
have a link back to vertex V, examine the next link in
(c) If vertex W is already on the shortest-path tree,
examine the next link in the advertisement.
(d) Calculate the link state cost D of the resulting path
from the root to vertex W. D is equal to the sum of the
link state cost of the (already calculated) shortest
path to vertex V and the advertised cost of the link
between vertices V and W. If D is:
o Greater than the value that already appears for
vertex W on the candidate list, then examine the
o Equal to the value that appears for vertex W on the
candidate list, calculate the set of next hops that
result from using the advertised link. Input to
this calculation is the destination (W), and its
parent (V). This calculation is shown in Section
16.1.1. This set of hops should be added to the
next hop values that appear for W on the candidate
o Less than the value that appears for vertex W on the
candidate list, or if W does not yet appear on the
candidate list, then set the entry for W on the
candidate list to indicate a distance of D from the
root. Also calculate the list of next hops that
result from using the advertised link, setting the
next hop values for W accordingly. The next hop
calculation is described in Section 16.1.1; it takes
as input the destination (W) and its parent (V).
(3) If at this step the candidate list is empty, the shortest-
path tree (of transit vertices) has been completely built
and this stage of the procedure terminates. Otherwise,
choose the vertex belonging to the candidate list that is
closest to the root, and add it to the shortest-path tree
(removing it from the candidate list in the process). Note
that when there is a choice of vertices closest to the root,
network vertices must be chosen before router vertices in
order to necessarily find all equal-cost paths. This is
consistent with the tie-breakers that were introduced in the
modified Dijkstra algorithm used by OSPF's Multicast routing
(4) Possibly modify the routing table. For those routing table
entries modified, the associated area will be set to Area A,
the path type will be set to intra-area, and the cost will
be set to the newly discovered shortest path's calculated
If the newly added vertex is an area border router (call it
ABR), a routing table entry is added whose destination type
is "area border router". The Options field found in the
associated router links advertisement is copied into the
routing table entry's Optional capabilities field. If in
addition ABR is the endpoint of one of the calculating
router's configured virtual links that uses Area A as its
Transit area: the virtual link is declared up, the IP
address of the virtual interface is set to the IP address of
the outgoing interface calculated above for ABR, and the
virtual neighbor's IP address is set to the ABR interface
address (contained in ABR's router links advertisement) that
points back to the root of the shortest-path tree;
equivalently, this is the interface that points back to
ABR's parent vertex on the shortest-path tree (similar to
the calculation in Section 16.1.1).
If the newly added vertex is an AS boundary router, the
routing table entry of type "AS boundary router" for the
destination is located. Since routers can belong to more
than one area, it is possible that several sets of intra-
area paths exist to the AS boundary router, each set using a
different area. However, the AS boundary router's routing
table entry must indicate a set of paths which utilize a
single area. The area leading to the routing table entry is
selected as follows: The area providing the shortest path is
always chosen; if more than one area provides paths with the
same minimum cost, the area with the largest OSPF Area ID
(when considered as an unsigned 32-bit integer) is chosen.
Note that whenever an AS boundary router's routing table
entry is added/modified, the Options found in the associated
router links advertisement is copied into the routing table
entry's Optional capabilities field.
If the newly added vertex is a transit network, the routing
table entry for the network is located. The entry's
Destination ID is the IP network number, which can be
obtained by masking the Vertex ID (Link State ID) with its
associated subnet mask (found in the body of the associated
network links advertisement). If the routing table entry
already exists (i.e., there is already an intra-area route
to the destination installed in the routing table), multiple
vertices have mapped to the same IP network. For example,
this can occur when a new Designated Router is being
established. In this case, the current routing table entry
should be overwritten if and only if the newly found path is
just as short and the current routing table entry's Link
State Origin has a smaller Link State ID than the newly
added vertex' link state advertisement.
If there is no routing table entry for the network (the
usual case), a routing table entry for the IP network should
be added. The routing table entry's Link State Origin
should be set to the newly added vertex' link state
(5) Iterate the algorithm by returning to Step 2.
The stub networks are added to the tree in the procedure's
second stage. In this stage, all router vertices are again
examined. Those that have been determined to be unreachable in
the above first phase are discarded. For each reachable router
vertex (call it V), the associated router links advertisement is
found in the link state database. Each stub network link
appearing in the advertisement is then examined, and the
following steps are executed:
(1) Calculate the distance D of stub network from the root. D
is equal to the distance from the root to the router vertex
(calculated in stage 1), plus the stub network link's
advertised cost. Compare this distance to the current best
cost to the stub network. This is done by looking up the
stub network's current routing table entry. If the
calculated distance D is larger, go on to examine the next
stub network link in the advertisement.
(2) If this step is reached, the stub network's routing table
entry must be updated. Calculate the set of next hops that
would result from using the stub network link. This
calculation is shown in Section 16.1.1; input to this
calculation is the destination (the stub network) and the
parent vertex (the router vertex). If the distance D is the
same as the current routing table cost, simply add this set
of next hops to the routing table entry's list of next hops.
In this case, the routing table already has a Link State
Origin. If this Link State Origin is a router links
advertisement whose Link State ID is smaller than V's Router
ID, reset the Link State Origin to V's router links
Otherwise D is smaller than the routing table cost.
Overwrite the current routing table entry by setting the
routing table entry's cost to D, and by setting the entry's
list of next hops to the newly calculated set. Set the
routing table entry's Link State Origin to V's router links
advertisement. Then go on to examine the next stub network
For all routing table entries added/modified in the second
stage, the associated area will be set to Area A and the path
type will be set to intra-area. When the list of reachable
router links is exhausted, the second stage is completed. At
this time, all intra-area routes associated with Area A have
The specification does not require that the above two stage
method be used to calculate the shortest path tree. However, if
another algorithm is used, an identical tree must be produced.
For this reason, it is important to note that links between
transit vertices must be bidirectional in ordered to be included
in the above tree. It should also be mentioned that more
efficient algorithms exist for calculating the tree; for
example, the incremental SPF algorithm described in [BBN].
16.1.1. The next hop calculation
This section explains how to calculate the current set of
next hops to use for a destination. Each next hop consists
of the outgoing interface to use in forwarding packets to
the destination together with the next hop router (if any).
The next hop calculation is invoked each time a shorter path
to the destination is discovered. This can happen in either
stage of the shortest-path tree calculation (see Section
16.1). In stage 1 of the shortest-path tree calculation a
shorter path is found as the destination is added to the
candidate list, or when the destination's entry on the
candidate list is modified (Step 2d of Stage 1). In stage 2
a shorter path is discovered each time the destination's
routing table entry is modified (Step 2 of Stage 2).
The set of next hops to use for the destination may be
recalculated several times during the shortest-path tree
calculation, as shorter and shorter paths are discovered.
In the end, the destination's routing table entry will
always reflect the next hops resulting from the absolute
Input to the next hop calculation is a) the destination and
b) its parent in the current shortest path between the root
(the calculating router) and the destination. The parent is
always a transit vertex (i.e., always a router or a transit
If there is at least one intervening router in the current
shortest path between the destination and the root, the
destination simply inherits the set of next hops from the
parent. Otherwise, there are two cases. In the first case,
the parent vertex is the root (the calculating router
itself). This means that the destination is either a
directly connected network or directly connected router.
The next hop in this case is simply the OSPF interface
connecting to the network/router; no next hop router is
required. If the connecting OSPF interface in this case is a
virtual link, the setting of the next hop should be deferred
until the calculation in Section 16.3.
In the second case, the parent vertex is a network that
directly connects the calculating router to the destination
router. The list of next hops is then determined by
examining the destination's router links advertisement. For
each link in the advertisement that points back to the
parent network, the link's Link Data field provides the IP
address of a next hop router. The outgoing interface to use
can then be derived from the next hop IP address (or it can
be inherited from the parent network).
16.2. Calculating the inter-area routes
The inter-area routes are calculated by examining summary link
advertisements. If the router has active attachments to
multiple areas, only backbone summary link advertisements are
examined. Routers attached to a single area examine that area's
summary links. In either case, the summary links examined below
are all part of a single area's link state database (call it
Summary link advertisements are originated by the area border
routers. Each summary link advertisement in Area A is
considered in turn. Remember that the destination described by
a summary link advertisement is either a network (Type 3 summary
link advertisements) or an AS boundary router (Type 4 summary
link advertisements). For each summary link advertisement:
(1) If the cost specified by the advertisement is LSInfinity, or
if the advertisement's LS age is equal to MaxAge, then
examine the the next advertisement.
(2) If the advertisement was originated by the calculating
router itself, examine the next advertisement.
(3) If the collection of destinations described by the summary
link advertisement falls into one of the router's configured
area address ranges (see Section 3.5) and the particular
area address range is active, the summary link advertisement
should be ignored. Active means that there are one or more
reachable (by intra-area paths) networks contained in the
area range. In this case, all addresses in the area range
are assumed to be either reachable via intra-area paths, or
else to be unreachable by any other means.
(4) Else, call the destination described by the advertisement N
(for Type 3 summary links, N's address is obtained by
masking the advertisement's Link State ID with the
network/subnet mask contained in the body of the
advertisement), and the area border originating the
advertisement BR. Look up the routing table entry for BR
having Area A as its associated area. If no such entry
exists for router BR (i.e., BR is unreachable in Area A), do
nothing with this advertisement and consider the next in the
list. Else, this advertisement describes an inter-area path
to destination N, whose cost is the distance to BR plus the
cost specified in the advertisement. Call the cost of this
inter-area path IAC.
(5) Next, look up the routing table entry for the destination N.
(The entry's Destination Type is either Network or AS
boundary router.) If no entry exists for N or if the
entry's path type is "type 1 external" or "type 2 external",
then install the inter-area path to N, with associated area
Area A, cost IAC, next hop equal to the list of next hops to
router BR, and Advertising router equal to BR.
(6) Else, if the paths present in the table are intra-area
paths, do nothing with the advertisement (intra-area paths
are always preferred).
(7) Else, the paths present in the routing table are also
inter-area paths. Install the new path through BR if it is
cheaper, overriding the paths in the routing table.
Otherwise, if the new path is the same cost, add it to the
list of paths that appear in the routing table entry.