RFC 1583
OSPF Version 2
A.3.4 The Link State Request packet Link State Request packets are OSPF packet type 3. After exchanging Database Description packets with a neighboring router, a router may find that parts of its topological database are out of date. The Link State Request packet is used to request the pieces of the neighbor's database that are more up to date. Multiple Link State Request packets may need to be used. The sending of Link State Request packets is the last step in bringing up an adjacency. A router that sends a Link State Request packet has in mind the precise instance of the database pieces it is requesting, defined by LS sequence number, LS checksum, and LS age, although these fields are not specified in the Link State Request Packet itself. The router may receive even more recent instances in response. The sending of Link State Request packets is documented in Section 10.9. The reception of Link State Request packets is documented in Section 10.7. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Version # | 3 | Packet length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Router ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Area ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Checksum | AuType | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Authentication | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Authentication | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link State ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Advertising Router | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | Each advertisement requested is specified by its LS type, Link State ID, and Advertising Router. This uniquely identifies the advertisement, but not its instance. Link State Request packets are
understood to be requests for the most recent instance (whatever
that might be).
A.3.5 The Link State Update packet Link State Update packets are OSPF packet type 4. These packets implement the flooding of link state advertisements. Each Link State Update packet carries a collection of link state advertisements one hop further from its origin. Several link state advertisements may be included in a single packet. Link State Update packets are multicast on those physical networks that support multicast/broadcast. In order to make the flooding procedure reliable, flooded advertisements are acknowledged in Link State Acknowledgment packets. If retransmission of certain advertisements is necessary, the retransmitted advertisements are always carried by unicast Link State Update packets. For more information on the reliable flooding of link state advertisements, consult Section 13. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Version # | 4 | Packet length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Router ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Area ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Checksum | AuType | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Authentication | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Authentication | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | # advertisements | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +- +-+ | Link state advertisements | +- +-+ | ... | # advertisements The number of link state advertisements included in this update.
The body of the Link State Update packet consists of a list of link
state advertisements. Each advertisement begins with a common 20
byte header, the link state advertisement header. This header is
described in Section A.4.1. Otherwise, the format of each of the
five types of link state advertisements is different. Their formats
are described in Section A.4.
A.3.6 The Link State Acknowledgment packet Link State Acknowledgment Packets are OSPF packet type 5. To make the flooding of link state advertisements reliable, flooded advertisements are explicitly acknowledged. This acknowledgment is accomplished through the sending and receiving of Link State Acknowledgment packets. Multiple link state advertisements can be acknowledged in a single Link State Acknowledgment packet. Depending on the state of the sending interface and the source of the advertisements being acknowledged, a Link State Acknowledgment packet is sent either to the multicast address AllSPFRouters, to the multicast address AllDRouters, or as a unicast. The sending of Link State Acknowledgement packets is documented in Section 13.5. The reception of Link State Acknowledgement packets is documented in Section 13.7. The format of this packet is similar to that of the Data Description packet. The body of both packets is simply a list of link state advertisement headers. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Version # | 5 | Packet length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Router ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Area ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Checksum | AuType | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Authentication | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Authentication | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +- -+ | A | +- Link State Advertisement -+ | Header | +- -+ | | +- -+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... |
Each acknowledged link state advertisement is described by its link
state advertisement header. The link state advertisement header is
documented in Section A.4.1. It contains all the information
required to uniquely identify both the advertisement and the
advertisement's current instance.
A.4 Link state advertisement formats There are five distinct types of link state advertisements. Each link state advertisement begins with a standard 20-byte link state advertisement header. This header is explained in Section A.4.1. Succeeding sections then diagram the separate link state advertisement types. Each link state advertisement describes a piece of the OSPF routing domain. Every router originates a router links advertisement. In addition, whenever the router is elected Designated Router, it originates a network links advertisement. Other types of link state advertisements may also be originated (see Section 12.4). All link state advertisements are then flooded throughout the OSPF routing domain. The flooding algorithm is reliable, ensuring that all routers have the same collection of link state advertisements. (See Section 13 for more information concerning the flooding algorithm). This collection of advertisements is called the link state (or topological) database. From the link state database, each router constructs a shortest path tree with itself as root. This yields a routing table (see Section 11). For the details of the routing table build process, see Section 16.
A.4.1 The Link State Advertisement header All link state advertisements begin with a common 20 byte header. This header contains enough information to uniquely identify the advertisement (LS type, Link State ID, and Advertising Router). Multiple instances of the link state advertisement may exist in the routing domain at the same time. It is then necessary to determine which instance is more recent. This is accomplished by examining the LS age, LS sequence number and LS checksum fields that are also contained in the link state advertisement header. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS age | Options | LS type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link State ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Advertising Router | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS checksum | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ LS age The time in seconds since the link state advertisement was originated. Options The optional capabilities supported by the described portion of the routing domain. OSPF's optional capabilities are documented in Section A.2. LS type The type of the link state advertisement. Each link state type has a separate advertisement format. The link state types are as follows (see Section 12.1.3 for further explanation):
LS Type Description
___________________________________
1 Router links
2 Network links
3 Summary link (IP network)
4 Summary link (ASBR)
5 AS external link
Link State ID
This field identifies the portion of the internet environment
that is being described by the advertisement. The contents of
this field depend on the advertisement's LS type. For example,
in network links advertisements the Link State ID is set to the
IP interface address of the network's Designated Router (from
which the network's IP address can be derived). The Link State
ID is further discussed in Section 12.1.4.
Advertising Router
The Router ID of the router that originated the link state
advertisement. For example, in network links advertisements
this field is set to the Router ID of the network's Designated
Router.
LS sequence number
Detects old or duplicate link state advertisements. Successive
instances of a link state advertisement are given successive LS
sequence numbers. See Section 12.1.6 for more details.
LS checksum
The Fletcher checksum of the complete contents of the link state
advertisement, including the link state advertisement header but
excepting the LS age field. See Section 12.1.7 for more details.
length
The length in bytes of the link state advertisement. This
includes the 20 byte link state advertisement header.
A.4.2 Router links advertisements Router links advertisements are the Type 1 link state advertisements. Each router in an area originates a router links advertisement. The advertisement describes the state and cost of the router's links (i.e., interfaces) to the area. All of the router's links to the area must be described in a single router links advertisement. For details concerning the construction of router links advertisements, see Section 12.4.1. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS age | Options | 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link State ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Advertising Router | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS checksum | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 |V|E|B| 0 | # links | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | # TOS | TOS 0 metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TOS | 0 | metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TOS | 0 | metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | In router links advertisements, the Link State ID field is set to the router's OSPF Router ID. The T-bit is set in the advertisement's Option field if and only if the router is able to
calculate a separate set of routes for each IP TOS. Router links
advertisements are flooded throughout a single area only.
bit V
When set, the router is an endpoint of an active virtual link
that is using the described area as a Transit area (V is for
virtual link endpoint).
bit E
When set, the router is an AS boundary router (E is for
external)
bit B
When set, the router is an area border router (B is for border)
# links
The number of router links described by this advertisement.
This must be the total collection of router links (i.e.,
interfaces) to the area.
The following fields are used to describe each router link (i.e.,
interface). Each router link is typed (see the below Type field).
The Type field indicates the kind of link being described. It may
be a link to a transit network, to another router or to a stub
network. The values of all the other fields describing a router
link depend on the link's Type. For example, each link has an
associated 32-bit data field. For links to stub networks this field
specifies the network's IP address mask. For other link types the
Link Data specifies the router's associated IP interface address.
Type
A quick description of the router link. One of the following.
Note that host routes are classified as links to stub networks
whose network mask is 0xffffffff.
Type Description
__________________________________________________
1 Point-to-point connection to another router
2 Connection to a transit network
3 Connection to a stub network
4 Virtual link
Link ID
Identifies the object that this router link connects to. Value
depends on the link's Type. When connecting to an object that
also originates a link state advertisement (i.e., another router
or a transit network) the Link ID is equal to the neighboring
advertisement's Link State ID. This provides the key for
looking up said advertisement in the link state database. See
Section 12.2 for more details.
Type Link ID
______________________________________
1 Neighboring router's Router ID
2 IP address of Designated Router
3 IP network/subnet number
4 Neighboring router's Router ID
Link Data
Contents again depend on the link's Type field. For connections
to stub networks, it specifies the network's IP address mask.
For unnumbered point-to-point connections, it specifies the
interface's MIB-II [RFC 1213] ifIndex value. For the other link
types it specifies the router's associated IP interface address.
This latter piece of information is needed during the routing
table build process, when calculating the IP address of the next
hop. See Section 16.1.1 for more details.
# TOS
The number of different TOS metrics given for this link, not
counting the required metric for TOS 0. For example, if no
additional TOS metrics are given, this field should be set to 0.
TOS 0 metric
The cost of using this router link for TOS 0.
For each link, separate metrics may be specified for each Type of
Service (TOS). The metric for TOS 0 must always be included, and
was discussed above. Metrics for non-zero TOS are described below.
The encoding of TOS in OSPF link state advertisements is described
in Section 12.3. Note that the cost for non-zero TOS values that
are not specified defaults to the TOS 0 cost. Metrics must be
listed in order of increasing TOS encoding. For example, the metric
for TOS 16 must always follow the metric for TOS 8 when both are
specified.
TOS IP Type of Service that this metric refers to. The encoding of
TOS in OSPF link state advertisements is described in Section
12.3.
metric
The cost of using this outbound router link, for traffic of the
specified TOS.
A.4.3 Network links advertisements Network links advertisements are the Type 2 link state advertisements. A network links advertisement is originated for each transit network in the area. A transit network is a multi- access network that has more than one attached router. The network links advertisement is originated by the network's Designated Router. The advertisement describes all routers attached to the network, including the Designated Router itself. The advertisement's Link State ID field lists the IP interface address of the Designated Router. The distance from the network to all attached routers is zero, for all Types of Service. This is why the TOS and metric fields need not be specified in the network links advertisement. For details concerning the construction of network links advertisements, see Section 12.4.2. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS age | Options | 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link State ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Advertising Router | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS checksum | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Network Mask | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Attached Router | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | Network Mask The IP address mask for the network. For example, a class A network would have the mask 0xff000000. Attached Router The Router IDs of each of the routers attached to the network. Actually, only those routers that are fully adjacent to the Designated Router are listed. The Designated Router includes
itself in this list. The number of routers included can be
deduced from the link state advertisement header's length field.
A.4.4 Summary link advertisements Summary link advertisements are the Type 3 and 4 link state advertisements. These advertisements are originated by area border routers. A separate summary link advertisement is made for each destination (known to the router) which belongs to the AS, yet is outside the area. For details concerning the construction of summary link advertisements, see Section 12.4.3. Type 3 link state advertisements are used when the destination is an IP network. In this case the advertisement's Link State ID field is an IP network number (if necessary, the Link State ID can also have one or more of the network's "host" bits set; see Appendix F for details). When the destination is an AS boundary router, a Type 4 advertisement is used, and the Link State ID field is the AS boundary router's OSPF Router ID. (To see why it is necessary to advertise the location of each ASBR, consult Section 16.4.) Other than the difference in the Link State ID field, the format of Type 3 and 4 link state advertisements is identical. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS age | Options | 3 or 4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link State ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Advertising Router | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS checksum | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Network Mask | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TOS | metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | For stub areas, Type 3 summary link advertisements can also be used to describe a (per-area) default route. Default summary routes are used in stub areas instead of flooding a complete set of external routes. When describing a default summary route, the advertisement's Link State ID is always set to DefaultDestination (0.0.0.0) and the Network Mask is set to 0.0.0.0.
Separate costs may be advertised for each IP Type of Service. The
encoding of TOS in OSPF link state advertisements is described in
Section 12.3. Note that the cost for TOS 0 must be included, and is
always listed first. If the T-bit is reset in the advertisement's
Option field, only a route for TOS 0 is described by the
advertisement. Otherwise, routes for the other TOS values are also
described; if a cost for a certain TOS is not included, its cost
defaults to that specified for TOS 0.
Network Mask
For Type 3 link state advertisements, this indicates the
destination network's IP address mask. For example, when
advertising the location of a class A network the value
0xff000000 would be used. This field is not meaningful and must
be zero for Type 4 link state advertisements.
For each specified Type of Service, the following fields are
defined. The number of TOS routes included can be calculated from
the link state advertisement header's length field. Values for TOS
0 must be specified; they are listed first. Other values must be
listed in order of increasing TOS encoding. For example, the cost
for TOS 16 must always follow the cost for TOS 8 when both are
specified.
TOS The Type of Service that the following cost concerns. The
encoding of TOS in OSPF link state advertisements is described
in Section 12.3.
metric
The cost of this route. Expressed in the same units as the
interface costs in the router links advertisements.
A.4.5 AS external link advertisements AS external link advertisements are the Type 5 link state advertisements. These advertisements are originated by AS boundary routers. A separate advertisement is made for each destination (known to the router) which is external to the AS. For details concerning the construction of AS external link advertisements, see Section 12.4.3. AS external link advertisements usually describe a particular external destination. For these advertisements the Link State ID field specifies an IP network number (if necessary, the Link State ID can also have one or more of the network's "host" bits set; see Appendix F for details). AS external link advertisements are also used to describe a default route. Default routes are used when no specific route exists to the destination. When describing a default route, the Link State ID is always set to DefaultDestination (0.0.0.0) and the Network Mask is set to 0.0.0.0. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS age | Options | 5 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link State ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Advertising Router | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS checksum | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Network Mask | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |E| TOS | metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Forwarding address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | External Route Tag | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | Separate costs may be advertised for each IP Type of Service. The encoding of TOS in OSPF link state advertisements is described in Section 12.3. Note that the cost for TOS 0 must be included, and is
always listed first. If the T-bit is reset in the advertisement's
Option field, only a route for TOS 0 is described by the
advertisement. Otherwise, routes for the other TOS values are also
described; if a cost for a certain TOS is not included, its cost
defaults to that specified for TOS 0.
Network Mask
The IP address mask for the advertised destination. For
example, when advertising a class A network the mask 0xff000000
would be used.
For each specified Type of Service, the following fields are
defined. The number of TOS routes included can be calculated from
the link state advertisement header's length field. Values for TOS
0 must be specified; they are listed first. Other values must be
listed in order of increasing TOS encoding. For example, the cost
for TOS 16 must always follow the cost for TOS 8 when both are
specified.
bit E
The type of external metric. If bit E is set, the metric
specified is a Type 2 external metric. This means the metric is
considered larger than any link state path. If bit E is zero,
the specified metric is a Type 1 external metric. This means
that is is comparable directly (without translation) to the link
state metric.
Forwarding address
Data traffic for the advertised destination will be forwarded to
this address. If the Forwarding address is set to 0.0.0.0, data
traffic will be forwarded instead to the advertisement's
originator (i.e., the responsible AS boundary router).
TOS The Type of Service that the following cost concerns. The
encoding of TOS in OSPF link state advertisements is described
in Section 12.3.
metric
The cost of this route. Interpretation depends on the external
type indication (bit E above).
External Route Tag
A 32-bit field attached to each external route. This is not
used by the OSPF protocol itself. It may be used to communicate
information between AS boundary routers; the precise nature of
such information is outside the scope of this specification.
B. Architectural Constants Several OSPF protocol parameters have fixed architectural values. These parameters have been referred to in the text by names such as LSRefreshTime. The same naming convention is used for the configurable protocol parameters. They are defined in Appendix C. The name of each architectural constant follows, together with its value and a short description of its function. LSRefreshTime The maximum time between distinct originations of any particular link state advertisement. When the LS age field of one of the router's self-originated advertisements reaches the value LSRefreshTime, a new instance of the link state advertisement is originated, even though the contents of the advertisement (apart from the link state header) will be the same. The value of LSRefreshTime is set to 30 minutes. MinLSInterval The minimum time between distinct originations of any particular link state advertisement. The value of MinLSInterval is set to 5 seconds. MaxAge The maximum age that a link state advertisement can attain. When an advertisement's LS age field reaches MaxAge, it is reflooded in an attempt to flush the advertisement from the routing domain (See Section 14). Advertisements of age MaxAge are not used in the routing table calculation. The value of MaxAge must be greater than LSRefreshTime. The value of MaxAge is set to 1 hour. CheckAge When the age of a link state advertisement (that is contained in the link state database) hits a multiple of CheckAge, the advertisement's checksum is verified. An incorrect checksum at this time indicates a serious error. The value of CheckAge is set to 5 minutes. MaxAgeDiff The maximum time dispersion that can occur, as a link state advertisement is flooded throughout the AS. Most of this time is accounted for by the link state advertisements sitting on router output queues (and therefore not aging) during the flooding process. The value of MaxAgeDiff is set to 15 minutes.
LSInfinity
The metric value indicating that the destination described by a
link state advertisement is unreachable. Used in summary link
advertisements and AS external link advertisements as an
alternative to premature aging (see Section 14.1). It is defined
to be the 24-bit binary value of all ones: 0xffffff.
DefaultDestination
The Destination ID that indicates the default route. This route
is used when no other matching routing table entry can be found.
The default destination can only be advertised in AS external
link advertisements and in stub areas' type 3 summary link
advertisements. Its value is the IP address 0.0.0.0.
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