The OSPF Opaque LSA Option
draft-ietf-ospf-opaque-05
The information below is for an old version of the document that is already published as an RFC.
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draft-ietf-ospf-opaque-05
Internet-Draft Opaque May 1998
Expiration Date: November 1998 FORE Systems
File name: draft-ietf-ospf-opaque-05.txt
The OSPF Opaque LSA Option
Rob Coltun
FORE Systems
(703) 245-4543
rcoltun@fore.com
Status Of This Memo
This document is an Internet-Draft. Internet-Drafts are working docu-
ments of the Internet Engineering Task Force (IETF), its areas, and
its working groups. Note that other groups may also distribute work-
ing documents as Internet-Drafts.
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Table Of Contents
1.0 Abstract ................................................. 3
2.0 Overview ................................................. 3
2.1 Organization Of This Document ............................ 3
2.2 Acknowledgments .......................................... 4
3.0 The Opaque LSA ........................................... 4
3.1 Flooding Opaque LSAs ..................................... 5
3.2 Modifications To The Neighbor State Machine .............. 6
4.0 Protocol Data Structures ................................. 8
4.1 Additions To The OSPF Neighbor Structure ................. 8
5.0 Management Considerations ................................ 8
6.0 Security Considerations .................................. 10
7.0 References ............................................... 12
Appendix A: OSPF Data Formats ................................ 13
A.1 The Options Field ........................................ 13
A.2 Opaque LSA ............................................... 15
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1.0 Abstract
This memo defines enhancements to the OSPF protocol to support a new
class of link-state advertisements (LSA) called Opaque LSAs. Opaque
LSAs provide a generalized mechanism to allow for the future extensi-
bility of OSPF. Opaque LSAs consist of a standard LSA header followed
by application-specific information. The information field may be
used directly by OSPF or by other applications. Standard OSPF link-
state database flooding mechanisms are used to distribute Opaque LSAs
to all or some limited portion of the OSPF topology.
2.0 Overview
Over the last several years the OSPF routing protocol [OSPF] has been
widely deployed throughout the Internet. As a result of this deploy-
ment and the evolution of networking technology, OSPF has been
extended to support many options; this evolution will obviously con-
tinue.
This memo defines enhancements to the OSPF protocol to support a new
class of link-state advertisements (LSA) called Opaque LSAs. Opaque
LSAs provide a generalized mechanism to allow for the future extensi-
bility of OSPF. The information contained in Opaque LSAs may be used
directly by OSPF or indirectly by some application wishing to distri-
bute information throughout the OSPF domain. For example, the OSPF
LSA may be used by routers to distribute IP to link-layer address
resolution information (see [ARA] for more information). The exact
use of Opaque LSAs is beyond the scope of this draft.
Opaque LSAs consist of a standard LSA header followed by a 32-bit
aligned application-specific information field. Like any other LSA,
the Opaque LSA uses the link-state database distribution mechanism for
flooding this information throughout the topology. The link-state
type field of the Opaque LSA identifies the LSA's range of topological
distribution. This range is referred to as the Flooding Scope.
It is envisioned that an implementation of the Opaque option provides
an application interface for 1) encapsulating application-specific
information in a specific Opaque type, 2) sending and receiving
application-specific information, and 3) if required, informing the
application of the change in validity of previously received informa-
tion when topological changes are detected.
2.1 Organization Of This Document
This document first defines the three types of Opaque LSAs followed by
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a description of OSPF packet processing. The packet processing sec-
tions include modifications to the flooding procedure and to the
neighbor state machine. Appendix A then gives the packet formats.
2.2 Acknowledgments
The author would like to thank Dennis Ferguson, Acee Lindem, John Moy,
Sandra Murphy, Man-Kit Yeung, Zhaohui "Jeffrey" Zhang and the rest of
the OSPF Working Group for the ideas and support they have given to
this project.
3.0 The Opaque LSA
Opaque LSAs are types 9, 10 and 11 link-state advertisements. Opaque
LSAs consist of a standard LSA header followed by a 32-bit aligned
application-specific information field. Standard link-state database
flooding mechanisms are used for distribution of Opaque LSAs. The
range of topological distribution (i.e., the flooding scope) of an
Opaque LSA is identified by its link-state type. This section docu-
ments the flooding of Opaque LSAs.
The flooding scope associated with each Opaque link-state type is
defined as follows.
o Link-state type 9 denotes a link-local scope. Type-9 Opaque
LSAs are not flooded beyond the local (sub)network.
o Link-state type 10 denotes an area-local scope. Type-10 Opaque
LSAs are not flooded beyond the borders of their associated area.
o Link-state type 11 denotes that the LSA is flooded throughout
the Autonomous System (AS). The flooding scope of type-11 LSAs
are equivalent to the flooding scope of AS-external (type-5)
LSAs. Specifically type-11 Opaque LSAs are 1) flooded throughout
all transit areas, 2) not flooded into stub areas from the back-
bone and 3) not originated by routers into their connected stub
areas. As with type-5 LSAs, if a type-11 Opaque LSA is received
in a stub area from a neighboring router within the stub area the
LSA is rejected.
The link-state ID of the Opaque LSA is divided into an Opaque type
field (the first 8 bits) and a type-specific ID (the remaining 24
bits). Opaque type values in the range of 0-127 are reserved for
definition by the IANA (iana@ISI.EDU) whereas Opaque type values in
the range of 128-255 are reserved for private and experimental use.
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The packet format of the Opaque LSA is given in Appendix A.
The responsibility for proper handling of the Opaque LSA's flooding
scope is placed on both the sender and receiver of the LSA. The
receiver must always store a valid received Opaque LSA in its link-
state database. The receiver must not accept Opaque LSAs that violate
the flooding scope (e.g., a type-11 (domain-wide) Opaque LSA is not
accepted in a stub area). The flooding scope effects both the build-
ing of the Database Summary List during the initial synchronization of
the link-state database and the flooding procedure.
The following describes the modifications to these procedures that are
necessary to insure conformance to the Opaque LSA's Scoping Rules.
3.1 Flooding Opaque LSAs
The flooding of Opaque LSAs must follow the rules of Flooding Scope as
specified in this section. Section 13 of [OSPF] describes the OSPF
flooding procedure. The following describes the Opaque LSA's type-
specific flooding restrictions.
o If the Opaque LSA is type 9 (the flooding scope is link-local)
and the interface that the LSA was received on is not the same as
the target interface (e.g., the interface associated with a par-
ticular target neighbor), the Opaque LSA must not be flooded out
that interface (or to that neighbor). An implementation should
keep track of the IP interface associated with each Opaque LSA
having a link-local flooding scope.
o If the Opaque LSA is type 10 (the flooding scope is area-local)
and the area associated with Opaque LSA (upon reception) is not
the same as the area associated with the target interface, the
Opaque LSA must not be flooded out the interface. An implementa-
tion should keep track of the OSPF area associated with each
Opaque LSA having an area-local flooding scope.
o If the Opaque LSA is type 11 (the LSA is flooded throughout the
AS) and the target interface is associated with a stub area the
Opaque LSA must not be flooded out the interface. A type-11
Opaque LSA that is received on an interface associated with a
stub area must be discarded and not acknowledged (the neighboring
router has flooded the LSA in error).
When opaque-capable routers and non-opaque-capable OSPF routers are
mixed together in a routing domain, the Opaque LSAs are not flooded to
the non-opaque-capable routers. As a general design principle,
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optional OSPF advertisements are only flooded to those routers that
understand them.
An opaque-capable router learns of its neighbor's opaque capability at
the beginning of the "Database Exchange Process" (see Section 10.6 of
[OSPF], receiving Database Description packets from a neighbor in
state ExStart). A neighbor is opaque-capable if and only if it sets
the O-bit in the Options field of its Database Description packets;
the O-bit is not set in packets other than Database Description pack-
ets. Then, in the next step of the Database Exchange process, Opaque
LSAs are included in the Database summary list that is sent to the
neighbor (see Sections 3.2 below and 10.3 of [OSPF]) if and only if
the neighbor is opaque capable.
When flooding Opaque-LSAs to adjacent neighbors, a opaque-capable
router looks at the neighbor's opaque capability. Opaque LSAs are
only flooded to opaque-capable neighbors. To be more precise, in Sec-
tion 13.3 of [OSPF], Opaque LSAs are only placed on the link-state
retransmission lists of opaque-capable neighbors. However, when send-
ing Link State Update packets as multicasts, a non-opaque-capable
neighbor may (inadvertently) receive Opaque LSAs. The non-opaque-
capable router will then simply discard the LSA (see Section 13 of
[OSPF], receiving LSAs having unknown LS types).
3.2 Modifications To The Neighbor State Machine
The state machine as it exists in section 10.3 of [OSPF] remains
unchanged except for the action associated with State: ExStart, Event:
NegotiationDone which is where the Database summary list is built. To
incorporate the Opaque LSA in OSPF this action is changed to the fol-
lowing.
State(s): ExStart
Event: NegotiationDone
New state: Exchange
Action: The router must list the contents of its entire area
link-state database in the neighbor Database summary
list. The area link-state database consists of the
Router LSAs, Network LSAs, Summary LSAs and types 9
and 10 Opaque LSAs contained in the area structure,
along with AS External and type-11 Opaque LSAs
contained in the global structure. AS External and
type-11 Opaque LSAs are omitted from a virtual
neighbor's Database summary list. AS External LSAs
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and type-11 Opaque LSAs are omitted from the
Database summary list if the area has been
configured as a stub area (see Section 3.6 of [OSPF]).
Type-9 Opaque LSAs are omitted from the Database
summary list if the interface associated with the
neighbor is not the interface associated with the
Opaque LSA (as noted upon reception).
Any advertisement whose age is equal to MaxAge is
omitted from the Database summary list. It is
instead added to the neighbor's link-state
retransmission list. A summary of the Database
summary list will be sent to the neighbor in
Database Description packets. Each Database
Description Packet has a DD sequence number, and is
explicitly acknowledged. Only one Database
Description Packet is allowed to be outstanding at
any one time. For more detail on the sending and
receiving of Database Description packets, see
Sections 10.6 and 10.8 of [OSPF].
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4.0 Protocol data structures
The Opaque option is described herein in terms of its operation on
various protocol data structures. These data structures are included
for explanatory uses only, and are not intended to constrain an imple-
mentation. In addition to the data structures listed below, this
specification references the various data structures (e.g., OSPF
neighbors) defined in [OSPF].
In an OSPF router, the following item is added to the list of global
OSPF data structures described in Section 5 of [OSPF]:
o Opaque capability. Indicates whether the router is running the
Opaque option (i.e., capable of storing Opaque LSAs). Such a
router will continue to inter-operate with non-opaque-capable
OSPF routers.
4.1 Additions To The OSPF Neighbor Structure
The OSPF neighbor structure is defined in Section 10 of [OSPF]. In an
opaque-capable router, the following items are added to the OSPF
neighbor structure:
o Neighbor Options. This field was already defined in the OSPF
specification. However, in opaque-capable routers there is a new
option which indicates the neighbor's Opaque capability. This new
option is learned in the Database Exchange process through recep-
tion of the neighbor's Database Description packets, and deter-
mines whether Opaque LSAs are flooded to the neighbor. For a more
detailed explanation of the flooding of the Opaque LSA see sec-
tion 3 of this document.
5.0 Management Considerations
This section identifies the current OSPF MIB [OSPFMIB] capabilities
that are applicable to the Opaque option and lists the additional
management information which is required for its support.
Opaque LSAs are types 9, 10 and 11 link-state advertisements. The
link-state ID of the Opaque LSA is divided into an Opaque type field
(the first 8 bits) and a type-specific ID (the remaining 24 bits).
The packet format of the Opaque LSA is given in Appendix A. The range
of topological distribution (i.e., the flooding scope) of an Opaque
LSA is identified by its link-state type.
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o Link-State type 9 Opaque LSAs have a link-local scope. Type-9
Opaque LSAs are flooded on a single local (sub)network but are
not flooded beyond the local (sub)network.
o Link-state type 10 Opaque LSAs have an area-local scope. Type-
10 Opaque LSAs are flooded throughout a single area but are not
flooded beyond the borders of the associated area.
o Link-state type 11 Opaque LSAs have an Autonomous-System-wide
scope. The flooding scope of type-11 LSAs are equivalent to the
flooding scope of AS-external (type-5) LSAs.
The OSPF MIB provides a number of objects that can be used to manage
and monitor an OSPF router's Link-State Database. The ones that are
relevant to the Opaque option are as follows.
The ospfGeneralGroup defines two objects for keeping track of
newly originated and newly received LSAs (ospfOriginateNewLsas
and ospfRxNewLsas respectively).
The OSPF MIB defines a set of optional traps. The ospfOrigina-
teLsa trap signifies that a new LSA has been originated by a
router and the ospfMaxAgeLsa trap signifies that one of the LSAs
in the router's link-state database has aged to MaxAge.
The ospfAreaTable describes the configured parameters and cumula-
tive statistics of the router's attached areas. This table
includes a count of the number of LSAs contained in the area's
link-state database (ospfAreaLsaCount), and a sum of the LSA's LS
checksums contained in this area (ospfAreaLsaCksumSum). This sum
can be used to determine if there has been a change in a router's
link-state database, and to compare the link-state database of
two routers.
The ospfLsdbTable describes the OSPF Process's link-state data-
base (excluding AS-external LSAs). Entries in this table are
indexed with an Area ID, a link-state type, a link-state ID and
the originating router's Router ID.
The management objects that are needed to support the Opaque option
are as follows.
An Opaque-option-enabled object is needed to indicate if the
Opaque option is enabled on the router.
The origination and reception of new Opaque LSAs should be
reflected in the counters ospfOriginateNewLsas and ospfRxNewLsas
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(inclusive for types 9, 10 and 11 Opaque LSAs).
If the OSPF trap option is supported, the origination of new
Opaque LSAs and purging of MaxAge Opaque LSAs should be reflected
in the ospfOriginateLsa and ospfMaxAgeLsa traps (inclusive for
types 9, 10 and 11 Opaque LSAs).
The number of type-10 Opaque LSAs should be reflected in
ospfAreaLsaCount; the checksums of type-10 Opaque LSAs should be
included in ospfAreaLsaChksumSum.
Type-10 Opaque LSAs should be included in the ospfLsdbTable.
Note that this table does not include a method of examining the
Opaque type field (in the Opaque option this is a sub-field of
the link-state ID).
Up until now, LSAs have not had a link-local scope so there is no
method of requesting the number of, or examining the LSAs that
are associated with a specific OSPF interface. A new group of
management objects are required to support type-9 Opaque LSAs.
These objects should include a count of type-9 Opaque LSAs, a
checksum sum and a table for displaying the link-state database
for type-9 Opaque LSAs on a per-interface basis. Entries in this
table should be indexed with an Area ID, interface's IP address,
Opaque type, link-state ID and the originating router's Router
ID.
Prior to the introduction of type-11 Opaque LSAs, AS-External
(type-5) LSAs have been the only link-state types which have an
Autonomous-System-wide scope. A new group of objects are
required to support type-11 Opaque LSAs. These objects should
include a count of type-11 Opaque LSAs, a type-11 checksum sum
and a table for displaying the type-11 link-state database.
Entries in this table should be indexed with the Opaque type,
link-state ID and the originating router's Router ID. The type-
11 link-state database table will allow type-11 LSAs to be
displayed once for the router rather than once in each non-stub
area.
6.0 Security Considerations
There are two types of issues that need be addressed when looking at
protecting routing protocols from misconfigurations and malicious
attacks. The first is authentication and certification of routing
protocol information. The second is denial of service attacks result-
ing from repetitive origination of the same router advertisement or
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origination a large number of distinct advertisements resulting in
database overflow. Note that both of these concerns exist indepen-
dently of a router's support for the Opaque option.
To address the authentication concerns, OSPF protocol exchanges are
authenticated. OSPF supports multiple types of authentication; the
type of authentication in use can be configured on a per network seg-
ment basis. One of OSPF's authentication types, namely the Crypto-
graphic authentication option, is believed to be secure against pas-
sive attacks and provide significant protection against active
attacks. When using the Cryptographic authentication option, each
router appends a "message digest" to its transmitted OSPF packets.
Receivers then use the shared secret key and received digest to verify
that each received OSPF packet is authentic.
The quality of the security provided by the Cryptographic authentica-
tion option depends completely on the strength of the message digest
algorithm (MD5 is currently the only message digest algorithm speci-
fied), the strength of the key being used, and the correct implementa-
tion of the security mechanism in all communicating OSPF implementa-
tions. It also requires that all parties maintain the secrecy of the
shared secret key. None of the standard OSPF authentication types
provide confidentiality. Nor do they protect against traffic analysis.
For more information on the standard OSPF security mechanisms, see
Sections 8.1, 8.2, and Appendix D of [OSPF].
[DIGI] describes the extensions to OSPF required to add digital signa-
ture authentication to Link State data and to provide a certification
mechanism for router data. [DIGI] also describes the added LSA pro-
cessing and key management as well as a method for migration from, or
co-existence with, standard OSPF V2.
Repetitive origination of advertisements are addressed by OSPF by man-
dating a limit on the frequency that new instances of any particular
LSA can be originated and accepted during the flooding procedure. The
frequency at which new LSA instances may be originated is set equal to
once every MinLSInterval seconds, whose value is 5 seconds (see Sec-
tion 12.4 of [OSPF]). The frequency at which new LSA instances are
accepted during flooding is once every MinLSArrival seconds, whose
value is set to 1 (see Section 13, Appendix B and G.5 of [OSPF]).
Proper operation of the OSPF protocol requires that all OSPF routers
maintain an identical copy of the OSPF link-state database. However,
when the size of the link-state database becomes very large, some
routers may be unable to keep the entire database due to resource
shortages; we term this "database overflow". When database overflow
is anticipated, the routers with limited resources can be accommodated
by configuring OSPF stub areas and NSSAs. [OVERFLOW] details a way of
Coltun [Page 11]
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gracefully handling unanticipated database overflows.
7.0 References
[ARA] Coltun, R., Heinanen, J., "The OSPF Address Resolution
Advertisement Option", work in progress.
[DEMD] Moy, J., "Extending OSPF to Support Demand Circuits", RFC 1793,
Cascade, April 1995.
[DIGI] S. Murphy, M. Badger, B. Wellington, "OSPF with Digital Signatures",
RFC 2154, Trusted Information Systems, June 1997.
[MOSPF] Moy, J., "Multicast Extensions to OSPF", RFC 1584, Proteon,
Inc., March 1994.
[NSSA] Coltun, R., Fuller, V., "The OSPF NSSA Option", RFC 1587,
March 1994.
[OSPF] Moy, J., "OSPF Version 2", RFC 2328, Cascade, April 1998.
[OSPFMIB] F. Baker, R. Coltun, "OSPF Version 2 Management Information
Base", RFC 1850, November 1995.
[OVERFLOW] Moy, J., "OSPF Database Overflow", RFC 1765,
Cascade, March 1995.
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Appendix A: OSPF Data formats
This appendix describes the format of the Options Field followed by
the packet format of the Opaque LSA.
A.1 The Options Field
The OSPF Options field is present in OSPF Hello packets, Database
Description packets and all link-state advertisements. The Options
field enables OSPF routers to support (or not support) optional capa-
bilities, and to communicate their capability level to other OSPF
routers. Through this mechanism routers of differing capabilities can
be mixed within an OSPF routing domain.
When used in Hello packets, the Options field allows a router to
reject a neighbor because of a capability mismatch. Alternatively,
when capabilities are exchanged in Database Description packets a
router can choose not to forward certain link-state advertisements to
a neighbor because of its reduced functionality. Lastly, listing
capabilities in link-state advertisements allows routers to forward
traffic around reduced functionality routers by excluding them from
parts of the routing table calculation.
Six bits of the OSPF Options field have been assigned, although only
the O-bit is described completely by this memo. Each bit is described
briefly below. Routers should reset (i.e., clear) unrecognized bits in
the Options field when sending Hello packets or Database Description
packets and when originating link-state advertisements. Conversely,
routers encountering unrecognized Option bits in received Hello Pack-
ets, Database Description packets or link-state advertisements should
ignore the capability and process the packet/advertisement normally.
+------------------------------------+
| * | O | DC | EA | N/P | MC | E | * |
+------------------------------------+
The Options Field
E-bit
This bit describes the way AS-external-LSAs are flooded, as
described in Sections 3.6, 9.5, 10.8 and 12.1.2 of [OSPF].
MC-bit
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This bit describes whether IP multicast datagrams are forwarded
according to the specifications in [MOSPF].
N/P-bit
This bit describes the handling of Type-7 LSAs, as specified in
[NSSA].
DC-bit
This bit describes the router's handling of demand circuits, as
specified in [DEMD].
EA-bit
This bit describes the router's willingness to receive and for-
ward External-Attributes-LSAs, as specified in [EAL].
O-bit
This bit describes the router's willingness to receive and for-
ward Opaque-LSAs as specified in this document.
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A.2 Opaque LSA
Opaque LSAs are Type 9, 10 and 11 link-state advertisements. These
advertisements may be used directly by OSPF or indirectly by some
application wishing to distribute information throughout the OSPF
domain. The function of the Opaque LSA option is to provide for
future extensibility of OSPF.
Opaque LSAs contain some number of octets (of application-specific
data) padded to 32-bit alignment. Like any other LSA, the Opaque LSA
uses the link-state database distribution mechanism for flooding this
information throughout the topology. However, the Opaque LSA has a
flooding scope associated with it so that the scope of flooding may be
link-local (type 9), area-local (type 10) or the entire OSPF routing
domain (type 11). Section 3 of this document describes the flooding
procedures for the Opaque LSA.
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 | 9, 10 or 11 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque Type | Opaque ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| Opaque Information |
+ +
| ... |
Link-State Type
The link-state type of the Opaque LSA identifies the LSA's range
of topological distribution. This range is referred to as the
Flooding Scope. The following explains the flooding scope of
each of the link-state types.
o A value of 9 denotes a link-local scope. Opaque LSAs with a
link-local scope are not flooded beyond the local (sub)network.
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o A value of 10 denotes an area-local scope. Opaque LSAs with a
area-local scope are not flooded beyond the area that they are
originated into.
o A value of 11 denotes that the LSA is flooded throughout the
Autonomous System (e.g., has the same scope as type-5 LSAs).
Opaque LSAs with AS-wide scope are not flooded into stub areas.
Syntax Of The Opaque LSA's Link-State ID
The link-state ID of the Opaque LSA is divided into an Opaque
Type field (the first 8 bits) and an Opaque ID (the remaining 24
bits). Opaque type values in the range of 0-127 are reserved for
definition by the IANA (iana@ISI.EDU) whereas Opaque type values
in the range of 128-255 are reserved for private and experimental
use.
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