RFC 1583
OSPF Version 2
12. Link State Advertisements Each router in the Autonomous System originates one or more link state advertisements. There are five distinct types of link state advertisements, which are described in Section 4.3. The collection of link state advertisements forms the link state or topological database. Each separate type of advertisement has a separate
Type Dest Area Path Type Cost Next Adv.
Hop(s) Router(s)
________________________________________________________________
N Ib 0 intra-area 16 RT3 *
N Ia 0 intra-area 21 RT3 *
BR RT3 0 intra-area 1 * *
BR RT10 0 intra-area 16 RT3 *
BR RT11 0 intra-area 19 RT3 *
________________________________________________________________
N N9-N11,H1 0 inter-area 20 RT3 RT11
Table 14: Changes resulting from an
additional virtual link.
function. Router links and network links advertisements describe
how an area's routers and networks are interconnected. Summary link
advertisements provide a way of condensing an area's routing
information. AS external advertisements provide a way of
transparently advertising externally-derived routing information
throughout the Autonomous System.
Each link state advertisement begins with a standard 20-byte header.
This link state advertisement header is discussed below.
12.1. The Link State Advertisement Header
The link state advertisement header contains the LS type, Link
State ID and Advertising Router fields. The combination of
these three fields uniquely identifies the link state
advertisement.
There may be several instances of an advertisement present in
the Autonomous System, all at the same time. It must then be
determined which instance is more recent. This determination is
made by examining the LS sequence, LS checksum and LS age
fields. These fields are also contained in the 20-byte link
state advertisement header.
Several of the OSPF packet types list link state advertisements.
When the instance is not important, an advertisement is referred
to by its LS type, Link State ID and Advertising Router (see
Link State Request Packets). Otherwise, the LS sequence number,
LS age and LS checksum fields must also be referenced.
A detailed explanation of the fields contained in the link state
advertisement header follows.
12.1.1. LS age
This field is the age of the link state advertisement in
seconds. It should be processed as an unsigned 16-bit
integer. It is set to 0 when the link state advertisement
is originated. It must be incremented by InfTransDelay on
every hop of the flooding procedure. Link state
advertisements are also aged as they are held in each
router's database.
The age of a link state advertisement is never incremented
past MaxAge. Advertisements having age MaxAge are not used
in the routing table calculation. When an advertisement's
age first reaches MaxAge, it is reflooded. A link state
advertisement of age MaxAge is finally flushed from the
database when it is no longer needed to ensure database
synchronization. For more information on the aging of link
state advertisements, consult Section 14.
The LS age field is examined when a router receives two
instances of a link state advertisement, both having
identical LS sequence numbers and LS checksums. An instance
of age MaxAge is then always accepted as most recent; this
allows old advertisements to be flushed quickly from the
routing domain. Otherwise, if the ages differ by more than
MaxAgeDiff, the instance having the smaller age is accepted
as most recent.[11] See Section 13.1 for more details.
12.1.2. Options
The Options field in the link state advertisement header
indicates which optional capabilities are associated with
the advertisement. OSPF's optional capabilities are
described in Section 4.5. There are currently two optional
capabilities defined; they are represented by the T-bit and
E-bit found in the Options field. The rest of the Options
field should be set to zero.
The E-bit represents OSPF's ExternalRoutingCapability. This
bit should be set in all advertisements associated with the
backbone, and all advertisements associated with non-stub
areas (see Section 3.6). It should also be set in all AS
external link advertisements. It should be reset in all
router links, network links and summary link advertisements
associated with a stub area. For all link state
advertisements, the setting of the E-bit is for
informational purposes only; it does not affect the routing
table calculation.
The T-bit represents OSPF's TOS routing capability. This
bit should be set in a router links advertisement if and
only if the router is capable of calculating separate routes
for each IP TOS (see Section 2.4). The T-bit should always
be set in network links advertisements. It should be set in
summary link and AS external link advertisements if and only
if the advertisement describes paths for all TOS values,
instead of just the TOS 0 path. Note that, with the T-bit
set, there may still be only a single metric in the
advertisement (the TOS 0 metric). This would mean that
paths for non-zero TOS exist, but are equivalent to the TOS
0 path. A link state advertisement's T-bit is examined when
calculating the routing table's non-zero TOS paths (see
Section 16.9).
12.1.3. LS type
The LS type field dictates the format and function of the
link state advertisement. Advertisements of different types
have different names (e.g., router links or network links).
All advertisement types, except the AS external link
advertisements (LS type = 5), are flooded throughout a
single area only. AS external link advertisements are
flooded throughout the entire Autonomous System, excepting
stub areas (see Section 3.6). Each separate advertisement
type is briefly described below in Table 15.
12.1.4. Link State ID
This field identifies the piece of the routing domain that
is being described by the advertisement. Depending on the
advertisement's LS type, the Link State ID takes on the
values listed in Table 16.
Actually, for Type 3 summary link (LS type = 3)
advertisements and AS external link (LS type = 5)
advertisements, the Link State ID may additionally have one
or more of the destination network's "host" bits set. For
example, when originating an AS external link for the
network 10.0.0.0 with mask of 255.0.0.0, the Link State ID
LS Type Advertisement description
__________________________________________________
1 These are the router links
advertisements. They describe the
collected states of the router's
interfaces. For more information,
consult Section 12.4.1.
__________________________________________________
2 These are the network links
advertisements. They describe the set
of routers attached to the network. For
more information, consult
Section 12.4.2.
__________________________________________________
3 or 4 These are the summary link
advertisements. They describe
inter-area routes, and enable the
condensation of routing information at
area borders. Originated by area border
routers, the Type 3 advertisements
describe routes to networks while the
Type 4 advertisements describe routes to
AS boundary routers.
__________________________________________________
5 These are the AS external link
advertisements. Originated by AS
boundary routers, they describe routes
to destinations external to the
Autonomous System. A default route for
the Autonomous System can also be
described by an AS external link
advertisement.
Table 15: OSPF link state advertisements.
LS Type Link State ID
_______________________________________________
1 The originating router's Router ID.
2 The IP interface address of the
network's Designated Router.
3 The destination network's IP address.
4 The Router ID of the described AS
boundary router.
5 The destination network's IP address.
Table 16: The advertisement's Link State ID.
can be set to anything in the range 10.0.0.0 through
10.255.255.255 inclusive (although 10.0.0.0 should be used
whenever possible). The freedom to set certain host bits
allows a router to originate separate advertisements for two
networks having the same address but different masks. See
Appendix F for details.
When the link state advertisement is describing a network
(LS type = 2, 3 or 5), the network's IP address is easily
derived by masking the Link State ID with the network/subnet
mask contained in the body of the link state advertisement.
When the link state advertisement is describing a router (LS
type = 1 or 4), the Link State ID is always the described
router's OSPF Router ID.
When an AS external advertisement (LS Type = 5) is
describing a default route, its Link State ID is set to
DefaultDestination (0.0.0.0).
12.1.5. Advertising Router
This field specifies the OSPF Router ID of the
advertisement's originator. For router links
advertisements, this field is identical to the Link State ID
field. Network link advertisements are originated by the
network's Designated Router. Summary link advertisements
are originated by area border routers. AS external link
advertisements are originated by AS boundary routers.
12.1.6. LS sequence number
The sequence number field is a signed 32-bit integer. It is
used to detect old and duplicate link state advertisements.
The space of sequence numbers is linearly ordered. The
larger the sequence number (when compared as signed 32-bit
integers) the more recent the advertisement. To describe to
sequence number space more precisely, let N refer in the
discussion below to the constant 2**31.
The sequence number -N (0x80000000) is reserved (and
unused). This leaves -N + 1 (0x80000001) as the smallest
(and therefore oldest) sequence number. A router uses this
sequence number the first time it originates any link state
advertisement. Afterwards, the advertisement's sequence
number is incremented each time the router originates a new
instance of the advertisement. When an attempt is made to
increment the sequence number past the maximum value of N -
1 (0x7fffffff), the current instance of the advertisement
must first be flushed from the routing domain. This is done
by prematurely aging the advertisement (see Section 14.1)
and reflooding it. As soon as this flood has been
acknowledged by all adjacent neighbors, a new instance can
be originated with sequence number of -N + 1 (0x80000001).
The router may be forced to promote the sequence number of
one of its advertisements when a more recent instance of the
advertisement is unexpectedly received during the flooding
process. This should be a rare event. This may indicate
that an out-of-date advertisement, originated by the router
itself before its last restart/reload, still exists in the
Autonomous System. For more information see Section 13.4.
12.1.7. LS checksum
This field is the checksum of the complete contents of the
advertisement, excepting the LS age field. The LS age field
is excepted so that an advertisement's age can be
incremented without updating the checksum. The checksum
used is the same that is used for ISO connectionless
datagrams; it is commonly referred to as the Fletcher
checksum. It is documented in Annex B of [RFC 905]. The
link state advertisement header also contains the length of
the advertisement in bytes; subtracting the size of the LS
age field (two bytes) yields the amount of data to checksum.
The checksum is used to detect data corruption of an
advertisement. This corruption can occur while an
advertisement is being flooded, or while it is being held in
a router's memory. The LS checksum field cannot take on the
value of zero; the occurrence of such a value should be
considered a checksum failure. In other words, calculation
of the checksum is not optional.
The checksum of a link state advertisement is verified in
two cases: a) when it is received in a Link State Update
Packet and b) at times during the aging of the link state
database. The detection of a checksum failure leads to
separate actions in each case. See Sections 13 and 14 for
more details.
Whenever the LS sequence number field indicates that two
instances of an advertisement are the same, the LS checksum
field is examined. If there is a difference, the instance
with the larger LS checksum is considered to be most
recent.[12] See Section 13.1 for more details.
12.2. The link state database
A router has a separate link state database for every area to
which it belongs. The link state database has been referred to
elsewhere in the text as the topological database. All routers
belonging to the same area have identical topological databases
for the area.
The databases for each individual area are always dealt with
separately. The shortest path calculation is performed
separately for each area (see Section 16). Components of the
area topological database are flooded throughout the area only.
Finally, when an adjacency (belonging to Area A) is being
brought up, only the database for Area A is synchronized between
the two routers.
The area database is composed of router links advertisements,
network links advertisements, and summary link advertisements
(all listed in the area data structure). In addition, external
routes (AS external advertisements) are included in all non-stub
area databases (see Section 3.6).
An implementation of OSPF must be able to access individual
pieces of an area database. This lookup function is based on an
advertisement's LS type, Link State ID and Advertising
Router.[13] There will be a single instance (the most up-to-
date) of each link state advertisement in the database. The
database lookup function is invoked during the link state
flooding procedure (Section 13) and the routing table
calculation (Section 16). In addition, using this lookup
function the router can determine whether it has itself ever
originated a particular link state advertisement, and if so,
with what LS sequence number.
A link state advertisement is added to a router's database when
either a) it is received during the flooding process (Section
13) or b) it is originated by the router itself (Section 12.4).
A link state advertisement is deleted from a router's database
when either a) it has been overwritten by a newer instance
during the flooding process (Section 13) or b) the router
originates a newer instance of one of its self-originated
advertisements (Section 12.4) or c) the advertisement ages out
and is flushed from the routing domain (Section 14). Whenever a
link state advertisement is deleted from the database it must
also be removed from all neighbors' Link state retransmission
lists (see Section 10).
12.3. Representation of TOS
All OSPF link state advertisements (with the exception of
network links advertisements) specify metrics. In router links
advertisements, the metrics indicate the costs of the described
interfaces. In summary link and AS external link
advertisements, the metric indicates the cost of the described
path. In all of these advertisements, a separate metric can be
specified for each IP TOS. The encoding of TOS in OSPF link
state advertisements is specified in Table 17. That table
relates the OSPF encoding to the IP packet header's TOS field
(defined in [RFC 1349]). The OSPF encoding is expressed as a
decimal integer, and the IP packet header's TOS field is
expressed in the binary TOS values used in [RFC 1349].
OSPF encoding RFC 1349 TOS values
___________________________________________
0 0000 normal service
2 0001 minimize monetary cost
4 0010 maximize reliability
6 0011
8 0100 maximize throughput
10 0101
12 0110
14 0111
16 1000 minimize delay
18 1001
20 1010
22 1011
24 1100
26 1101
28 1110
30 1111
Table 17: Representing TOS in OSPF.
Each OSPF link state advertisement must specify the TOS 0
metric. Other TOS metrics, if they appear, must appear in order
of increasing TOS encoding. For example, the TOS 8 (maximize
throughput) metric must always appear before the TOS 16
(minimize delay) metric when both are specified. If a metric
for some non-zero TOS is not specified, its cost defaults to the
cost for TOS 0, unless the T-bit is reset in the advertisement's
Options field (see Section 12.1.2 for more details).
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
A router originates a router links advertisement for each
area that it belongs to. Such an advertisement describes
the collected states of the router's links to the area. The
advertisement is flooded throughout the particular area, and
no further.
....................................
. 192.1.2 Area 1 .
. + .
. | .
. | 3+---+1 .
. N1 |--|RT1|-----+ .
. | +---+ .
. | _______N3 .
. + / . 1+---+
. * 192.1.1 *------|RT4|
. + /_______/ . +---+
. | / | .
. | 3+---+1 / | .
. N2 |--|RT2|-----+ 1| .
. | +---+ +---+8 . 6+---+
. | |RT3|----------------|RT6|
. + +---+ . +---+
. 192.1.3 |2 . 18.10.0.6|7
. | . |
. +------------+ .
. 192.1.4 (N4) .
....................................
Figure 15: Area 1 with IP addresses shown
The format of a router links advertisement is shown in
Appendix A (Section A.4.2). The first 20 bytes of the
advertisement consist of the generic link state
advertisement header that was discussed in Section 12.1.
Router links advertisements have LS type = 1. The router
indicates whether it is willing to calculate separate routes
for each IP TOS by setting (or resetting) the T-bit of the
link state advertisement's Options field.
A router also indicates whether it is an area border router,
or an AS boundary router, by setting the appropriate bits
(bit B and bit E, respectively) in its router links
advertisements. This enables paths to those types of routers
to be saved in the routing table, for later processing of
summary link advertisements and AS external link
advertisements. Bit B should be set whenever the router is
actively attached to two or more areas, even if the router
is not currently attached to the OSPF backbone area. Bit E
should never be set in a router links advertisement for a
stub area (stub areas cannot contain AS boundary routers).
In addition, the router sets bit V in its router links
advertisement for Area A if and only if it is the endpoint
of an active virtual link using Area A as its Transit area.
This enables the other routers attached to Area A to
discover whether the area supports any virtual links (i.e.,
is a transit area).
The router links advertisement then describes the router's
working connections (i.e., interfaces or links) to the area.
Each link is typed according to the kind of attached
network. Each link is also labelled with its Link ID. This
Link ID gives a name to the entity that is on the other end
of the link. Table 18 summarizes the values used for the
Type and Link ID fields.
Link type Description Link ID
__________________________________________________
1 Point-to-point Neighbor Router ID
link
2 Link to transit Interface address of
network Designated Router
3 Link to stub IP network number
network
4 Virtual link Neighbor Router ID
Table 18: Link descriptions in the
router links advertisement.
In addition, the Link Data field is specified for each link.
This field gives 32 bits of extra information for the link.
For links to transit networks, numbered links to routers and
virtual links, this field specifies the IP interface address
of the associated router interface (this is needed by the
routing table calculation, see Section 16.1.1). For links
to stub networks, this field specifies the network's IP
address mask. For unnumbered point-to-point networks, the
Link Data field should be set to the unnumbered interface's
MIB-II [RFC 1213] ifIndex value.
Finally, the cost of using the link for output (possibly
specifying a different cost for each Type of Service) is
specified. The output cost of a link is configurable. It
must always be non-zero.
To further describe the process of building the list of link
descriptions, suppose a router wishes to build a router
links advertisement for Area A. The router examines its
collection of interface data structures. For each
interface, the following steps are taken:
o If the attached network does not belong to Area A, no
links are added to the advertisement, and the next
interface should be examined.
o Else, if the state of the interface is Down, no links
are added.
o Else, if the state of the interface is Point-to-Point,
then add links according to the following:
- If the neighboring router is fully adjacent, add a
Type 1 link (point-to-point) if this is an interface
to a point-to-point network, or add a Type 4 link
(virtual link) if this is a virtual link. The Link
ID should be set to the Router ID of the neighboring
router. For virtual links and numbered point-to-
point networks, the Link Data should specify the IP
interface address. For unnumbered point-to-point
networks, the Link Data field should specify the
interface's MIB-II [RFC 1213] ifIndex value.
- If this is a numbered point-to-point network (i.e,
not a virtual link and not an unnumbered point-to-
point network) and the neighboring router's IP
address is known, add a Type 3 link (stub network)
whose Link ID is the neighbor's IP address, whose
Link Data is the mask 0xffffffff indicating a host
route, and whose cost is the interface's configured
output cost.
o Else if the state of the interface is Loopback, add a
Type 3 link (stub network) as long as this is not an
interface to an unnumbered serial line. The Link ID
should be set to the IP interface address, the Link Data
set to the mask 0xffffffff (indicating a host route),
and the cost set to 0.
o Else if the state of the interface is Waiting, add a
Type 3 link (stub network) whose Link ID is the IP
network number of the attached network and whose Link
Data is the attached network's address mask.
o Else, there has been a Designated Router selected for
the attached network. If the router is fully adjacent
to the Designated Router, or if the router itself is
Designated Router and is fully adjacent to at least one
other router, add a single Type 2 link (transit network)
whose Link ID is the IP interface address of the
attached network's Designated Router (which may be the
router itself) and whose Link Data is the router's own
IP interface address. Otherwise, add a link as if the
interface state were Waiting (see above).
Unless otherwise specified, the cost of each link generated
by the above procedure is equal to the output cost of the
associated interface. Note that in the case of serial
lines, multiple links may be generated by a single
interface.
After consideration of all the router interfaces, host links
are added to the advertisement by examining the list of
attached hosts. A host route is represented as a Type 3
link (stub network) whose Link ID is the host's IP address
and whose Link Data is the mask of all ones (0xffffffff).
As an example, consider the router links advertisements
generated by Router RT3, as pictured in Figure 6. The area
containing Router RT3 (Area 1) has been redrawn, with actual
network addresses, in Figure 15. Assume that the last byte
of all of RT3's interface addresses is 3, giving it the
interface addresses 192.1.1.3 and 192.1.4.3, and that the
other routers have similar addressing schemes. In addition,
assume that all links are functional, and that Router IDs
are assigned as the smallest IP interface address.
RT3 originates two router links advertisements, one for Area
1 and one for the backbone. Assume that Router RT4 has been
selected as the Designated router for network 192.1.1.0.
RT3's router links advertisement for Area 1 is then shown
below. It indicates that RT3 has two connections to Area 1,
the first a link to the transit network 192.1.1.0 and the
second a link to the stub network 192.1.4.0. Note that the
transit network is identified by the IP interface of its
Designated Router (i.e., the Link ID = 192.1.1.4 which is
the Designated Router RT4's IP interface to 192.1.1.0).
Note also that RT3 has indicated that it is capable of
calculating separate routes based on IP TOS, through setting
the T-bit in the Options field. It has also indicated that
it is an area border router.
; RT3's router links advertisement for Area 1
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 1 ;indicates router links
Link State ID = 192.1.1.3 ;RT3's Router ID
Advertising Router = 192.1.1.3 ;RT3's Router ID
bit E = 0 ;not an AS boundary router
bit B = 1 ;area border router
#links = 2
Link ID = 192.1.1.4 ;IP address of Desig. Rtr.
Link Data = 192.1.1.3 ;RT3's IP interface to net
Type = 2 ;connects to transit network
# other metrics = 0
TOS 0 metric = 1
Link ID = 192.1.4.0 ;IP Network number
Link Data = 0xffffff00 ;Network mask
Type = 3 ;connects to stub network
# other metrics = 0
TOS 0 metric = 2
Next RT3's router links advertisement for the backbone is
shown. It indicates that RT3 has a single attachment to the
backbone. This attachment is via an unnumbered point-to-
point link to Router RT6. RT3 has again indicated that it
is TOS-capable, and that it is an area border router.
; RT3's router links advertisement for the backbone
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 1 ;indicates router links
Link State ID = 192.1.1.3 ;RT3's router ID
Advertising Router = 192.1.1.3 ;RT3's router ID
bit E = 0 ;not an AS boundary router
bit B = 1 ;area border router
#links = 1
Link ID = 18.10.0.6 ;Neighbor's Router ID
Link Data = 0.0.0.3 ;MIB-II ifIndex of P-P link
Type = 1 ;connects to router
# other metrics = 0
TOS 0 metric = 8
Even though Router RT3 has indicated that it is TOS-capable
in the above examples, only a single metric (the TOS 0
metric) has been specified for each interface. Different
metrics can be specified for each TOS. The encoding of TOS
in OSPF link state advertisements is described in Section
12.3.
As an example, suppose the point-to-point link between
Routers RT3 and RT6 in Figure 15 is a satellite link. The
AS administrator may want to encourage the use of the line
for high bandwidth traffic. This would be done by setting
the metric artificially low for the appropriate TOS value.
Router RT3 would then originate the following router links
advertisement for the backbone (TOS 8 = maximize
throughput):
; RT3's router links advertisement for the backbone
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 1 ;indicates router links
Link State ID = 192.1.1.3 ;RT3's Router ID
Advertising Router = 192.1.1.3
bit E = 0 ;not an AS boundary router
bit B = 1 ;area border router
#links = 1
Link ID = 18.10.0.6 ;Neighbor's Router ID
Link Data = 0.0.0.3 ;MIB-II ifIndex of P-P link
Type = 1 ;connects to router
# other metrics = 1
TOS 0 metric = 8
TOS = 8 ;maximize throughput
metric = 1 ;traffic preferred
12.4.2. Network links
A network links advertisement is generated for every transit
multi-access network. (A transit network is a network
having two or more attached routers). The network links
advertisement describes all the routers that are attached to
the network.
The Designated Router for the network originates the
advertisement. The Designated Router originates the
advertisement only if it is fully adjacent to at least one
other router on the network. The network links
advertisement is flooded throughout the area that contains
the transit network, and no further. The networks links
advertisement lists those routers that are fully adjacent to
the Designated Router; each fully adjacent router is
identified by its OSPF Router ID. The Designated Router
includes itself in this list.
The Link State ID for a network links advertisement is the
IP interface address of the Designated Router. This value,
masked by the network's address mask (which is also
contained in the network links advertisement) yields the
network's IP address.
A router that has formerly been the Designated Router for a
network, but is no longer, should flush the network links
advertisement that it had previously originated. This
advertisement is no longer used in the routing table
calculation. It is flushed by prematurely incrementing the
advertisement's age to MaxAge and reflooding (see Section
14.1). In addition, in those rare cases where a router's
Router ID has changed, any network links advertisements that
were originated with the router's previous Router ID must be
flushed. Since the router may have no idea what it's
previous Router ID might have been, these network links
advertisements are indicated by having their Link State ID
equal to one of the router's IP interface addresses and
their Advertising Router not equal to the router's current
Router ID (see Section 13.4 for more details).
As an example of a network links advertisement, again
consider the area configuration in Figure 6. Network links
advertisements are originated for Network N3 in Area 1,
Networks N6 and N8 in Area 2, and Network N9 in Area 3.
Assuming that Router RT4 has been selected as the Designated
Router for Network N3, the following network links
advertisement is generated by RT4 on behalf of Network N3
(see Figure 15 for the address assignments):
; network links advertisement for Network N3
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 2 ;indicates network links
Link State ID = 192.1.1.4 ;IP address of Desig. Rtr.
Advertising Router = 192.1.1.4 ;RT4's Router ID
Network Mask = 0xffffff00
Attached Router = 192.1.1.4 ;Router ID
Attached Router = 192.1.1.1 ;Router ID
Attached Router = 192.1.1.2 ;Router ID
Attached Router = 192.1.1.3 ;Router ID
12.4.3. Summary links Each summary link advertisement describes a route to a single destination. Summary link advertisements are flooded throughout a single area only. The destination described is one that is external to the area, yet still belonging to the Autonomous System. Summary link advertisements are originated by area border routers. The precise summary routes to advertise into an area are determined by examining the routing table structure (see Section 11) in accordance with the algorithm described below. Note that only intra-area routes are advertised into the backbone, while both intra-area and inter-area routes are advertised into the other areas. To determine which routes to advertise into an attached Area A, each routing table entry is processed as follows. Remember that each routing table entry describes a set of equal-cost best paths to a particular destination: o Only Destination Types of network and AS boundary router are advertised in summary link advertisements. If the routing table entry's Destination Type is area border router, examine the next routing table entry. o AS external routes are never advertised in summary link advertisements. If the routing table entry has Path- type of type 1 external or type 2 external, examine the next routing table entry. o Else, if the area associated with this set of paths is the Area A itself, do not generate a summary link advertisement for the route.[14] o Else, if the next hops associated with this set of paths belong to Area A itself, do not generate a summary link advertisement for the route.[15] This is the logical equivalent of a Distance Vector protocol's split horizon logic. o Else, if the routing table cost equals or exceeds the value LSInfinity, a summary link advertisement cannot be generated for this route. o Else, if the destination of this route is an AS boundary router, generate a Type 4 link state advertisement for
the destination, with Link State ID equal to the AS
boundary router's Router ID and metric equal to the
routing table entry's cost. These advertisements should
not be generated if Area A has been configured as a stub
area.
o Else, the Destination type is network. If this is an
inter-area route, generate a Type 3 advertisement for
the destination, with Link State ID equal to the
network's address (if necessary, the Link State ID can
also have one or more of the network's host bits set;
see Appendix F for details) and metric equal to the
routing table cost.
o The one remaining case is an intra-area route to a
network. This means that the network is contained in
one of the router's directly attached areas. In
general, this information must be condensed before
appearing in summary link advertisements. Remember that
an area has been defined as a list of address ranges,
each range consisting of an [address,mask] pair and a
status indication of either Advertise or DoNotAdvertise.
At most a single Type 3 advertisement is made for each
range. When the range's status indicates Advertise, a
Type 3 advertisement is generated with Link State ID
equal to the range's address (if necessary, the Link
State ID can also have one or more of the range's "host"
bits set; see Appendix F for details) and cost equal to
the smallest cost of any of the component networks. When
the range's status indicates DoNotAdvertise, the Type 3
advertisement is suppressed and the component networks
remain hidden from other areas.
By default, if a network is not contained in any
explicitly configured address range, a Type 3
advertisement is generated with Link State ID equal to
the network's address (if necessary, the Link State ID
can also have one or more of the network's "host" bits
set; see Appendix F for details) and metric equal to the
network's routing table cost.
If virtual links are being used to provide/increase
connectivity of the backbone, routing information
concerning the backbone networks should not be condensed
before being summarized into the virtual links' Transit
areas. Nor should the advertisement of backbone networks
into Transit areas be suppressed. In other words, the
backbone's configured ranges should be ignored when
originating summary links into Transit areas. The
existence of virtual links is determined during the
shortest path calculation for the Transit areas (see
Section 16.1).
If a router advertises a summary advertisement for a
destination which then becomes unreachable, the router must
then flush the advertisement from the routing domain by
setting its age to MaxAge and reflooding (see Section 14.1).
Also, if the destination is still reachable, yet can no
longer be advertised according to the above procedure (e.g.,
it is now an inter-area route, when it used to be an intra-
area route associated with some non-backbone area; it would
thus no longer be advertisable to the backbone), the
advertisement should also be flushed from the routing
domain.
For an example of summary link advertisements, consider
again the area configuration in Figure 6. Routers RT3, RT4,
RT7, RT10 and RT11 are all area border routers, and
therefore are originating summary link advertisements.
Consider in particular Router RT4. Its routing table was
calculated as the example in Section 11.3. RT4 originates
summary link advertisements into both the backbone and Area
1. Into the backbone, Router RT4 originates separate
advertisements for each of the networks N1-N4. Into Area 1,
Router RT4 originates separate advertisements for networks
N6-N8 and the AS boundary routers RT5,RT7. It also
condenses host routes Ia and Ib into a single summary link
advertisement. Finally, the routes to networks N9,N10,N11
and Host H1 are advertised by a single summary link
advertisement. This condensation was originally performed
by the router RT11.
These advertisements are illustrated graphically in Figures
7 and 8. Two of the summary link advertisements originated
by Router RT4 follow. The actual IP addresses for the
networks and routers in question have been assigned in
Figure 15.
; summary link advertisement for Network N1,
; originated by Router RT4 into the backbone
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 3 ;summary link to IP net
Link State ID = 192.1.2.0 ;N1's IP network number
Advertising Router = 192.1.1.4 ;RT4's ID
TOS = 0
metric = 4
; summary link advertisement for AS boundary router RT7
; originated by Router RT4 into Area 1
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 4 ;summary link to ASBR
Link State ID = Router RT7's ID
Advertising Router = 192.1.1.4 ;RT4's ID
TOS = 0
metric = 14
Summary link advertisements pertain to a single destination
(IP network or AS boundary router). However, for a single
destination there may be separate sets of paths, and
therefore separate routing table entries, for each Type of
Service. All these entries must be considered when building
the summary link advertisement for the destination; a single
advertisement must specify the separate costs (if they
exist) for each TOS. The encoding of TOS in OSPF link state
advertisements is described in Section 12.3.
Clearing the T-bit in the Options field of a summary link
advertisement indicates that there is a TOS 0 path to the
destination, but no paths for non-zero TOS. This can happen
when non-TOS-capable routers exist in the routing domain
(see Section 2.4).
12.4.4. Originating summary links into stub areas
The algorithm in Section 12.4.3 is optional when Area A is
an OSPF stub area. Area border routers connecting to a stub
area can originate summary link advertisements into the area
according to the above Section's algorithm, or can choose to
originate only a subset of the advertisements, possibly
under configuration control. The fewer advertisements
originated, the smaller the stub area's link state database,
further reducing the demands on its routers' resources.
However, omitting advertisements may also lead to sub-
optimal inter-area routing, although routing will continue
to function.
As specified in Section 12.4.3, Type 4 link state
advertisements (ASBR summary links) are never originated
into stub areas.
In a stub area, instead of importing external routes each
area border router originates a "default summary link" into
the area. The Link State ID for the default summary link is
set to DefaultDestination, and the metric set to the (per-
area) configurable parameter StubDefaultCost. Note that
StubDefaultCost need not be configured identically in all of
the stub area's area border routers.
12.4.5. AS external links
AS external link advertisements describe routes to
destinations external to the Autonomous System. Most AS
external link advertisements describe routes to specific
external destinations; in these cases the advertisement's
Link State ID is set to the destination network's IP address
(if necessary, the Link State ID can also have one or more
of the network's "host" bits set; see Appendix F for
details). However, a default route for the Autonomous
System can be described in an AS external link advertisement
by setting the advertisement's Link State ID to
DefaultDestination (0.0.0.0). AS external link
advertisements are originated by AS boundary routers. An AS
boundary router originates a single AS external link
advertisement for each external route that it has learned,
either through another routing protocol (such as EGP), or
through configuration information.
In general, AS external link advertisements are the only
type of link state advertisements that are flooded
throughout the entire Autonomous System; all other types of
link state advertisements are specific to a single area.
However, AS external link advertisements are not flooded
into/throughout stub areas (see Section 3.6). This enables
a reduction in link state database size for routers internal
to stub areas.
The metric that is advertised for an external route can be
one of two types. Type 1 metrics are comparable to the link
state metric. Type 2 metrics are assumed to be larger than
the cost of any intra-AS path. As with summary link
advertisements, if separate paths exist based on TOS,
separate TOS costs can be included in the AS external link
advertisement. The encoding of TOS in OSPF link state
advertisements is described in Section 12.3. If the T-bit
of the advertisement's Options field is clear, no non-zero
TOS paths to the destination exist.
If a router advertises an AS external link advertisement for
a destination which then becomes unreachable, the router
must then flush the advertisement from the routing domain by
setting its age to MaxAge and reflooding (see Section 14.1).
For an example of AS external link advertisements, consider
once again the AS pictured in Figure 6. There are two AS
boundary routers: RT5 and RT7. Router RT5 originates three
external link advertisements, for networks N12-N14. Router
RT7 originates two external link advertisements, for
networks N12 and N15. Assume that RT7 has learned its route
to N12 via EGP, and that it wishes to advertise a Type 2
metric to the AS. RT7 would then originate the following
advertisement for N12:
; AS external link advertisement for Network N12,
; originated by Router RT7
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 5 ;indicates AS external link
Link State ID = N12's IP network number
Advertising Router = Router RT7's ID
bit E = 1 ;Type 2 metric
TOS = 0
metric = 2
Forwarding address = 0.0.0.0
In the above example, the forwarding address field has been
set to 0.0.0.0, indicating that packets for the external
destination should be forwarded to the advertising OSPF
router (RT7). This is not always desirable. Consider the
example pictured in Figure 16. There are three OSPF routers
(RTA, RTB and RTC) connected to a common network. Only one
of these routers, RTA, is exchanging EGP information with
the non-OSPF router RTX. RTA must then originate AS
external link advertisements for those destinations it has
learned from RTX. By using the AS external link
advertisement's forwarding address field, RTA can specify
that packets for these destinations be forwarded directly to
RTX. Without this feature, Routers RTB and RTC would take
an extra hop to get to these destinations.
Note that when the forwarding address field is non-zero, it
should point to a router belonging to another Autonomous
System.
A forwarding address can also be specified for the default
route. For example, in figure 16 RTA may want to specify
that all externally-destined packets should by default be
forwarded to its EGP peer RTX. The resulting AS external
link advertisement is pictured below. Note that the Link
State ID is set to DefaultDestination.
; Default route, originated by Router RTA
; Packets forwarded through RTX
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 5 ;indicates AS external link
Link State ID = DefaultDestination ; default route
Advertising Router = Router RTA's ID
bit E = 1 ;Type 2 metric
TOS = 0
metric = 1
Forwarding address = RTX's IP address
In figure 16, suppose instead that both RTA and RTB exchange
EGP information with RTX. In this case, RTA and RTB would
originate the same set of AS external link advertisements.
These advertisements, if they specify the same metric, would
be functionally equivalent since they would specify the same
destination and forwarding address (RTX). This leads to a
clear duplication of effort. If only one of RTA or RTB
originated the set of external advertisements, the routing
would remain the same, and the size of the link state
database would decrease. However, it must be unambiguously
defined as to which router originates the advertisements
(otherwise neither may, or the identity of the originator
may oscillate). The following rule is thereby established:
if two routers, both reachable from one another, originate
functionally equivalent AS external advertisements (i.e.,
same destination, cost and non-zero forwarding address),
then the advertisement originated by the router having the
highest OSPF Router ID is used. The router having the lower
OSPF Router ID can then flush its advertisement. Flushing a
link state advertisement is discussed in Section 14.1.
(page 126 continued on part 6)
