Appendix C. Configurable Constants
In router-LSAs, the Link State ID field is set to the router's OSPF
Router ID. Router-LSAs are flooded throughout a single area only.
bit V
When set, the router is an endpoint of one or more fully
adjacent virtual links having the described area as 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 in this LSA. 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 Link Data field. For links to stub networks this
field specifies the network's IP address mask. For other link types
the Link Data field specifies the router interface's IP address.
Type
A quick description of the router link. One of the following.
Note that host routes are classified as links to stub networks
with network mask of 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 an LSA (i.e., another router or a transit
network) the Link ID is equal to the neighboring LSA's Link
State ID. This provides the key for looking up the neighboring
LSA in the link state database during the routing table
calculation. 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
Value again depends on the link's Type field. For connections to
stub networks, Link Data specifies the network's IP address
mask. For unnumbered point-to-point connections, it specifies
the interface's MIB-II [Ref8] ifIndex value. For the other link
types it specifies the router interface's IP 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 link metric (referred to as the TOS 0
metric in [Ref9]). For example, if no additional TOS metrics
are given, this field is set to 0.
metric
The cost of using this router link.
Additional TOS-specific information may also be included, for
backward compatibility with previous versions of the OSPF
specification ([Ref9]). Within each link, and for each desired TOS,
TOS TOS-specific link information may be encoded as follows:
TOS IP Type of Service that this metric refers to. The encoding of
TOS in OSPF LSAs is described in Section 12.3.
TOS metric
TOS-specific metric information.
A.4.3 Network-LSAs
Network-LSAs are the Type 2 LSAs. A network-LSA is originated for
each broadcast and NBMA network in the area which supports two or
more routers. The network-LSA is originated by the network's
Designated Router. The LSA describes all routers attached to the
network, including the Designated Router itself. The LSA'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. This
is why metric fields need not be specified in the network-LSA. For
details concerning the construction of network-LSAs, 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 LSA header's length field.
A.4.4 Summary-LSAs
Summary-LSAs are the Type 3 and 4 LSAs. These LSAs are originated
by area border routers. Summary-LSAs describe inter-area
destinations. For details concerning the construction of summary-
LSAs, see Section 12.4.3.
Type 3 summary-LSAs are used when the destination is an IP network.
In this case the LSA'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 E for details). When the
destination is an AS boundary router, a Type 4 summary-LSA 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 summary-LSAs 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOS | TOS metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
For stub areas, Type 3 summary-LSAs 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 summary-LSA's Link State ID
is always set to DefaultDestination (0.0.0.0) and the Network Mask
is set to 0.0.0.0.
Network Mask
For Type 3 summary-LSAs, 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
summary-LSAs.
metric
The cost of this route. Expressed in the same units as the
interface costs in the router-LSAs.
Additional TOS-specific information may also be included, for
backward compatibility with previous versions of the OSPF
specification ([Ref9]). For each desired TOS, TOS-specific
information is encoded as follows:
TOS IP Type of Service that this metric refers to. The encoding of
TOS in OSPF LSAs is described in Section 12.3.
TOS metric
TOS-specific metric information.
A.4.5 AS-external-LSAs
AS-external-LSAs are the Type 5 LSAs. These LSAs are originated by
AS boundary routers, and describe destinations external to the AS.
For details concerning the construction of AS-external-LSAs, see
Section 12.4.3.
AS-external-LSAs usually describe a particular external destination.
For these LSAs 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 E for details). AS-
external-LSAs 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| 0 | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forwarding address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External Route Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| TOS | TOS metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forwarding address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External Route Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
Network Mask
The IP address mask for the advertised destination. For
example, when advertising a class A network the mask 0xff000000
would be used.
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 it is expressed in the same units as the link state metric
(i.e., the same units as interface cost).
metric
The cost of this route. Interpretation depends on the external
type indication (bit E above).
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 LSA's originator (i.e.,
the responsible AS boundary router).
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.
Additional TOS-specific information may also be included, for
backward compatibility with previous versions of the OSPF
specification ([Ref9]). For each desired TOS, TOS-specific
information is encoded as follows:
TOS The Type of Service that the following fields concern. The
encoding of TOS in OSPF LSAs is described in Section 12.3.
bit E
For backward-compatibility with [Ref9].
TOS metric
TOS-specific metric information.
Forwarding address
For backward-compatibility with [Ref9].
External Route Tag
For backward-compatibility with [Ref9].
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
LSA. If the LS age field of one of the router's self-originated
LSAs reaches the value LSRefreshTime, a new instance of the LSA
is originated, even though the contents of the LSA (apart from
the LSA header) will be the same. The value of LSRefreshTime is
set to 30 minutes.
MinLSInterval
The minimum time between distinct originations of any particular
LSA. The value of MinLSInterval is set to 5 seconds.
MinLSArrival
For any particular LSA, the minimum time that must elapse
between reception of new LSA instances during flooding. LSA
instances received at higher frequencies are discarded. The
value of MinLSArrival is set to 1 second.
MaxAge
The maximum age that an LSA can attain. When an LSA's LS age
field reaches MaxAge, it is reflooded in an attempt to flush the
LSA from the routing domain (See Section 14). LSAs of age MaxAge
are not used in the routing table calculation. The value of
MaxAge is set to 1 hour.
CheckAge
When the age of an LSA in the link state database hits a
multiple of CheckAge, the LSA'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 an LSA is flooded
throughout the AS. Most of this time is accounted for by the
LSAs 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 an
LSA is unreachable. Used in summary-LSAs and AS-external-LSAs 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-
LSAs and in stub areas' type 3 summary-LSAs. Its value is the
IP address 0.0.0.0. Its associated Network Mask is also always
0.0.0.0.
InitialSequenceNumber
The value used for LS Sequence Number when originating the first
instance of any LSA. Its value is the signed 32-bit integer
0x80000001.
MaxSequenceNumber
The maximum value that LS Sequence Number can attain. Its value
is the signed 32-bit integer 0x7fffffff.