RFC 5340
OSPF for IPv6
Pages: 94
Proposed Standard
→ Errata
Obsoletes: 2740
Updated by: 6845 6860 7503 8362 9454
Proposed Standard
→ Errata
Obsoletes: 2740
Updated by: 6845 6860 7503 8362 9454
Part 2 of 4 – Pages 13 to 40
4. Implementation Details
When going from IPv4 to IPv6, the basic OSPF mechanisms remain unchanged from those documented in [OSPFV2]. These mechanisms are briefly outlined in Section 4 of [OSPFV2]. Both IPv6 and IPv4 have a link-state database composed of LSAs and synchronized between adjacent routers. Initial synchronization is performed through the Database Exchange process, which includes the exchange of Database Description, Link State Request, and Link State Update packets. Thereafter, database synchronization is maintained via flooding, utilizing Link State Update and Link State Acknowledgment packets. Both IPv6 and IPv4 use OSPF Hello packets to discover and maintain neighbor relationships, as well as to elect Designated Routers and Backup Designated Routers on broadcast and NBMA links. The decision as to which neighbor relationships become adjacencies, and the basic ideas behind inter-area routing, importing external information in AS-external-LSAs, and the various routing calculations are also the same. In particular, the following IPv4 OSPF functionality described in [OSPFV2] remains completely unchanged for IPv6: o Both IPv4 and IPv6 use OSPF packet types described in Section 4.3 of [OSPFV2], namely: Hello, Database Description, Link State Request, Link State Update, and Link State Acknowledgment packets. While in some cases (e.g., Hello packets) their format has changed somewhat, the functions of the various packet types remain the same.
o The system requirements for an OSPF implementation remain
unchanged, although OSPF for IPv6 requires an IPv6 protocol stack
(from the network layer on down) since it runs directly over the
IPv6 network layer.
o The discovery and maintenance of neighbor relationships, and the
selection and establishment of adjacencies, remain the same. This
includes election of the Designated Router and Backup Designated
Router on broadcast and NBMA links. These mechanisms are
described in Sections 7, 7.1, 7.2, 7.3, 7.4, and 7.5 of [OSPFV2].
o The link types (or equivalently, interface types) supported by
OSPF remain unchanged, namely: point-to-point, broadcast, NBMA,
point-to-multipoint, and virtual links.
o The interface state machine, including the list of OSPF interface
states and events, and the Designated Router and Backup Designated
Router election algorithm remain unchanged. These are described
in Sections 9.1, 9.2, 9.3, and 9.4 of [OSPFV2].
o The neighbor state machine, including the list of OSPF neighbor
states and events, remains unchanged. The neighbor state machine
is described in Sections 10.1, 10.2, 10.3, and 10.4 of [OSPFV2].
o Aging of the link-state database, as well as flushing LSAs from
the routing domain through the premature aging process, remains
unchanged from the description in Sections 14 and 14.1 of
[OSPFV2].
However, some OSPF protocol mechanisms have changed as previously
described in Section 2 herein. These changes are explained in detail
in the following subsections, making references to the appropriate
sections of [OSPFV2].
The following subsections provide a recipe for turning an IPv4 OSPF
implementation into an IPv6 OSPF implementation.
4.1. Protocol Data Structures
The major OSPF data structures are the same for both IPv4 and IPv6:
areas, interfaces, neighbors, the link-state database, and the
routing table. The top-level data structures for IPv6 remain those
listed in Section 5 of [OSPFV2], with the following modifications:
o All LSAs with known LS type and AS flooding scope appear in the
top-level data structure, instead of belonging to a specific area
or link. AS-external-LSAs are the only LSAs defined by this
specification that have AS flooding scope. LSAs with unknown LS
type, U-bit set to 1 (flood even when unrecognized), and AS
flooding scope also appear in the top-level data structure.
4.1.1. The Area Data Structure
The IPv6 area data structure contains all elements defined for IPv4
areas in Section 6 of [OSPFV2]. In addition, all LSAs of known type
that have area flooding scope are contained in the IPv6 area data
structure. This always includes the following LSA types: router-
LSAs, network-LSAs, inter-area-prefix-LSAs, inter-area-router-LSAs,
and intra-area-prefix-LSAs. LSAs with unknown LS type, U-bit set to
1 (flood even when unrecognized), and area scope also appear in the
area data structure. NSSA-LSAs are also included in an NSSA area's
data structure.
4.1.2. The Interface Data Structure
In OSPF for IPv6, an interface connects a router to a link. The IPv6
interface structure modifies the IPv4 interface structure (as defined
in Section 9 of [OSPFV2]) as follows:
Interface ID
Every interface is assigned an Interface ID, which uniquely
identifies the interface with the router. For example, some
implementations MAY be able to use the MIB-II IfIndex ([INTFMIB])
as the Interface ID. The Interface ID appears in Hello packets
sent out the interface, the link-local-LSA originated by the
router for the attached link, and the router-LSA originated by the
router-LSA for the associated area. It will also serve as the
Link State ID for the network-LSA that the router will originate
for the link if the router is elected Designated Router.
The Interface ID for a virtual link is independent of the
Interface ID of the outgoing interface it traverses in the transit
area.
Instance ID
Every interface is assigned an Instance ID. This should default
to 0. It is only necessary to assign a value other than 0 on
those links that will contain multiple separate communities of
OSPF routers. For example, suppose that there are two communities
of routers on a given ethernet segment that you wish to keep
separate.
The first community is assigned an Instance ID of 0 and all the
routers in the first community will be assigned 0 as the Instance
ID for interfaces connected to the ethernet segment. An Instance
ID of 1 is assigned to the other routers' interfaces connected to
the ethernet segment. The OSPF transmit and receive processing
(see Section 4.2) will then keep the two communities separate.
List of LSAs with link-local scope
All LSAs with link-local scope and that were originated/flooded on
the link belong to the interface structure that connects to the
link. This includes the collection of the link's link-LSAs.
IP interface address
For IPv6, the IPv6 address appearing in the source of OSPF packets
sent on the interface is almost always a link-local address. The
one exception is for virtual links that MUST use one of the
router's own global IPv6 addresses as IP interface address.
List of link prefixes
A list of IPv6 prefixes can be configured for the attached link.
These will be advertised by the router in link-LSAs, so that they
can be advertised by the link's Designated Router in intra-area-
prefix-LSAs.
In OSPF for IPv6, each router interface has a single metric
representing the cost of sending packets on the interface. In
addition, OSPF for IPv6 relies on the IP Authentication Header (see
[IPAUTH]) and the IP Encapsulating Security Payload (see [IPESP]) as
described in [OSPFV3-AUTH] to ensure integrity and authentication/
confidentiality of routing exchanges. For this reason, AuType and
Authentication key are not associated with IPv6 OSPF interfaces.
Interface states, events, and the interface state machine remain
unchanged from IPv4 as documented in Sections 9.1, 9.2, and 9.3 of
[OSPFV2] respectively. The Designated Router and Backup Designated
Router election algorithm also remains unchanged from the IPv4
election in Section 9.4 of [OSPFV2].
4.1.3. The Neighbor Data Structure
The neighbor structure performs the same function in both IPv6 and
IPv4. Namely, it collects all information required to form an
adjacency between two routers when such an adjacency becomes
necessary. Each neighbor structure is bound to a single OSPF
interface. The differences between the IPv6 neighbor structure and
the neighbor structure defined for IPv4 in Section 10 of [OSPFV2]
are:
Neighbor's Interface ID
The Interface ID that the neighbor advertises in its Hello packets
must be recorded in the neighbor structure. The router will
include the neighbor's Interface ID in the router's router-LSA
when either a) advertising a point-to-point or point-to-multipoint
link to the neighbor or b) advertising a link to a network where
the neighbor has become the Designated Router.
Neighbor IP address
The neighbor's IPv6 address contained as the source address in
OSPF for IPv6 packets. This will be an IPv6 link-local address
for all link types except virtual links.
Neighbor's Designated Router
The neighbor's choice of Designated Router is now encoded as a
Router ID instead of as an IP address.
Neighbor's Backup Designated Router
The neighbor's choice of Backup Designated Router is now encoded
as a Router ID instead of as an IP address.
Neighbor states, events, and the neighbor state machine remain
unchanged from IPv4 as documented in Sections 10.1, 10.2, and 10.3 of
[OSPFV2] respectively. The decision as to which adjacencies to form
also remains unchanged from the IPv4 logic documented in Section 10.4
of [OSPFV2].
4.2. Protocol Packet Processing
OSPF for IPv6 runs directly over IPv6's network layer. As such, it
is encapsulated in one or more IPv6 headers with the Next Header
field of the immediately encapsulating IPv6 header set to the value
89.
As for OSPF for IPv4, OSPF for IPv6 OSPF routing protocol packets are
sent along adjacencies only (with the exception of Hello packets,
which are used to discover the adjacencies). OSPF packet types and
functions are the same in both IPv4 and IPv6, encoded by the Type
field of the standard OSPF packet header.
4.2.1. Sending Protocol Packets
When an IPv6 router sends an OSPF routing protocol packet, it fills
in the fields of the standard OSPF for IPv6 packet header (see
Appendix A.3.1) as follows:
Version #
Set to 3, the version number of the protocol as documented in this
specification.
Type
The type of OSPF packet, such as Link State Update or Hello
packet.
Packet length
The length of the entire OSPF packet in bytes, including the
standard OSPF packet header.
Router ID
The identity of the router itself (who is originating the packet).
Area ID
The OSPF area for the interface on which the packet is being sent.
Instance ID
The OSPF Instance ID associated with the interface out of which
the packet is being sent.
Checksum
The standard IPv6 Upper-Layer checksum (as described in Section
8.1 of [IPV6]) covering the entire OSPF packet and prepended IPv6
pseudo-header (see Appendix A.3.1).
Selection of OSPF routing protocol packets' IPv6 source and
destination addresses is performed identically to the IPv4 logic in
Section 8.1 of [OSPFV2]. The IPv6 destination address is chosen from
among the addresses AllSPFRouters, AllDRouters, and the Neighbor IP
address associated with the other end of the adjacency (which in
IPv6, for all links except virtual links, is an IPv6 link-local
address).
The sending of Link State Request packets and Link State
Acknowledgment packets remains unchanged from the IPv4 procedures
documented in Sections 10.9 and 13.5 of [OSPFV2] respectively.
Sending Hello packets is documented in Section 4.2.1.1, and the
sending of Database Description packets in Section 4.2.1.2. The
sending of Link State Update packets is documented in Section 4.5.2.
4.2.1.1. Sending Hello Packets
IPv6 changes the way OSPF Hello packets are sent in the following
ways (compare to Section 9.5 of [OSPFV2]):
o Before the Hello packet is sent on an interface, the interface's
Interface ID MUST be copied into the Hello packet.
o The Hello packet no longer contains an IP network mask since OSPF
for IPv6 runs per-link instead of per-subnet.
o The choice of Designated Router and Backup Designated Router is
now indicated within Hellos by their Router IDs instead of by
their IP interface addresses. Advertising the Designated Router
(or Backup Designated Router) as 0.0.0.0 indicates that the
Designated Router (or Backup Designated Router) has not yet been
chosen.
o The Options field within Hello packets has moved around, getting
larger in the process. More Options bits are now possible. Those
that MUST be set correctly in Hello packets are as follows. The
E-bit is set if and only if the interface attaches to a regular
area, i.e., not a stub or NSSA area. Similarly, the N-bit is set
if and only if the interface attaches to an NSSA area (see
[NSSA]). Finally, the DC-bit is set if and only if the router
wishes to suppress the sending of future Hellos over the interface
(see [DEMAND]). Unrecognized bits in the Hello packet's Options
field should be cleared.
Sending Hello packets on NBMA networks proceeds for IPv6 in exactly
the same way as for IPv4, as documented in Section 9.5.1 of [OSPFV2].
4.2.1.2. Sending Database Description Packets
The sending of Database Description packets differs from Section 10.8
of [OSPFV2] in the following ways:
o The Options field within Database Description packets has moved
around, getting larger in the process. More Options bits are now
possible. Those that MUST be set correctly in Database
Description packets are as follows. The DC-bit is set if and only
if the router wishes to suppress the sending of Hellos over the
interface (see [DEMAND]). Unrecognized bits in the Database
Description packet's Options field should be cleared.
4.2.2. Receiving Protocol Packets
Whenever a router receives an OSPF protocol packet, it is marked with
the interface on which it was received. For routers that have
virtual links configured, it may not be immediately obvious with
which interface to associate the packet. For example, consider the
Router RT11 depicted in Figure 6 of [OSPFV2]. If RT11 receives an
OSPF protocol packet on its interface to Network N8, it may want to
associate the packet with the interface to Area 2, or with the
virtual link to Router RT10 (which is part of the backbone). In the
following, we assume that the packet is initially associated with the
non-virtual link.
In order for the packet to be passed to OSPF for processing, the
following tests must be performed on the encapsulating IPv6 headers:
o The packet's IP destination address MUST be one of the IPv6
unicast addresses associated with the receiving interface (this
includes link-local addresses), one of the IPv6 multicast
addresses AllSPFRouters or AllDRouters, or an IPv6 global address
(for virtual links).
o The Next Header field of the immediately encapsulating IPv6 header
MUST specify the OSPF protocol (89).
o Any encapsulating IP Authentication Headers (see [IPAUTH]) and the
IP Encapsulating Security Payloads (see [IPESP]) MUST be processed
and/or verified to ensure integrity and authentication/
confidentiality of OSPF routing exchanges. This is described in
[OSPFV3-AUTH].
After processing the encapsulating IPv6 headers, the OSPF packet
header is processed. The fields specified in the header must match
those configured for the receiving OSPFv3 interface. If they do not,
the packet SHOULD be discarded:
o The version number field MUST specify protocol version 3.
o The IPv6 Upper-Layer checksum (as described in Section 8.1 of
[IPV6]), covering the entire OSPF packet and prepended IPv6
pseudo-header, must be verified (see Appendix A.3.1).
o The Area ID and Instance ID found in the OSPF header must be
verified. If both of the following cases fail, the packet should
be discarded. The Area ID and Instance ID specified in the header
must either:
1. Match one of the Area ID(s) and Interface Instance ID(s) for
the receiving link. Unlike IPv4, the IPv6 source address is
not restricted to lie within the same IPv6 subnet as the
receiving link. IPv6 OSPF runs per-link instead of per-IP-
subnet.
2. Match the backbone area and other criteria for a configured
virtual link. The receiving router must be an ABR (Area
Border Router) and the Router ID specified in the packet (the
source router) must be the other end of a configured virtual
link. Additionally, the receiving link must have an OSPFv3
interface that attaches to the virtual link's configured
transit area and the Instance ID must match the virtual link's
Instance ID. If all of these checks succeed, the packet is
accepted and is associated with the virtual link (and the
backbone area).
o Locally originated packets SHOULD NOT be processed by OSPF except
for support of multiple interfaces attached to the same link as
described in Section 4.9. Locally originated packets have a
source address equal to one of the router's local addresses.
o Packets whose IPv6 destination is AllDRouters should only be
accepted if the state of the receiving OSPFv3 interface is DR or
Backup (see Section 9.1 [OSPFV2]).
After header processing, the packet is further processed according to
its OSPF packet type. OSPF packet types and functions are the same
for both IPv4 and IPv6.
If the packet type is Hello, it should then be further processed by
the Hello packet processing as described in Section 4.2.2.1. All
other packet types are sent/received only on adjacencies. This means
that the packet must have been sent by one of the router's active
neighbors. The neighbor is identified by the Router ID appearing in
the received packet's OSPF header. Packets not matching any active
neighbor are discarded.
The receive processing of Database Description packets, Link State
Request packets, and Link State Acknowledgment packets is almost
identical to the IPv4 procedures documented in Sections 10.6, 10.7,
and 13.7 of [OSPFV2] respectively with the exceptions noted below.
o LSAs with unknown LS types in Database Description packets that
have an acceptable flooding scope are processed the same as LSAs
with known LS types. In OSPFv2 [OSPFV2], these would result in
the adjacency being brought down with a SequenceMismatch event.
The receiving of Hello packets is documented in Section 4.2.2.1 and
the receiving of Link State Update packets is documented in
Section 4.5.1.
4.2.2.1. Receiving Hello Packets
The receive processing of Hello packets differs from Section 10.5 of
[OSPFV2] in the following ways:
o On all link types (e.g., broadcast, NBMA, point-to-point, etc.),
neighbors are identified solely by their OSPF Router ID. For all
link types except virtual links, the Neighbor IP address is set to
the IPv6 source address in the IPv6 header of the received OSPF
Hello packet.
o There is no longer a Network Mask field in the Hello packet.
o The neighbor's choice of Designated Router and Backup Designated
Router is now encoded as an OSPF Router ID instead of an IP
interface address.
4.3. The Routing table Structure
The routing table used by OSPF for IPv4 is defined in Section 11 of
[OSPFV2]. For IPv6, there are analogous routing table entries: there
are routing table entries for IPv6 address prefixes and also for AS
boundary routers. The latter routing table entries are only used to
hold intermediate results during the routing table build process (see
Section 4.8).
Also, to hold the intermediate results during the shortest-path
calculation for each area, there is a separate routing table for each
area holding the following entries:
o An entry for each router in the area. Routers are identified by
their OSPF Router ID. These routing table entries hold the set of
shortest paths through a given area to a given router, which in
turn allows calculation of paths to the IPv6 prefixes advertised
by that router in intra-area-prefix-LSAs. If the router is also
an area border router, these entries are also used to calculate
paths for inter-area address prefixes. If in addition the router
is the other endpoint of a virtual link, the routing table entry
describes the cost and viability of the virtual link.
o An entry for each transit link in the area. Transit links have
associated network-LSAs. Both the transit link and the network-
LSA are identified by a combination of the Designated Router's
Interface ID on the link and the Designated Router's OSPF Router
ID. These routing table entries allow later calculation of paths
to IP prefixes advertised for the transit link in intra-area-
prefix-LSAs.
The fields in the IPv4 OSPF routing table (see Section 11 of
[OSPFV2]) remain valid for IPv6: optional capabilities (routers
only), path type, cost, type 2 cost, link state origin, and for each
of the equal cost paths to the destination, the next-hop and
advertising routers.
For IPv6, the link-state origin field in the routing table entry is
the router-LSA or network-LSA that has directly or indirectly
produced the routing table entry. For example, if the routing table
entry describes a route to an IPv6 prefix, the link state origin is
the router-LSA or network-LSA that is listed in the body of the
intra-area-prefix-LSA that has produced the route (see
Appendix A.4.10).
4.3.1. Routing Table Lookup
Routing table lookup (i.e., determining the best matching routing table entry during IP forwarding) is the same for IPv6 as for IPv4.4.4. Link State Advertisements
For IPv6, the OSPF LSA header has changed slightly, with the LS type field expanding and the Options field being moved into the body of appropriate LSAs. Also, the formats of some LSAs have changed somewhat (namely, router-LSAs, network-LSAs, AS-external-LSAs, and NSSA-LSAs), while the names of other LSAs have been changed (type 3 and 4 summary-LSAs are now inter-area-prefix-LSAs and inter-area- router-LSAs respectively) and additional LSAs have been added (link- LSAs and intra-area-prefix-LSAs). Type of Service (TOS) has been removed from the OSPFv2 specification [OSPFV2] and is not encoded within OSPF for IPv6's LSAs. These changes will be described in detail in the following subsections.4.4.1. The LSA Header
In both IPv4 and IPv6, all OSPF LSAs begin with a standard 20-byte LSA header. However, the contents of this 20-byte header have changed in IPv6. The LS age, Advertising Router, LS Sequence Number, LS checksum, and length fields within the LSA header remain unchanged, as documented in Sections 12.1.1, 12.1.5, 12.1.6, 12.1.7, and A.4.1 of [OSPFV2], respectively. However, the following fields have changed for IPv6: Options The Options field has been removed from the standard 20-byte LSA header and moved into the body of router-LSAs, network-LSAs, inter-area-router-LSAs, and link-LSAs. The size of the Options field has increased from 8 to 24 bits, and some of the bit definitions have changed (see Appendix A.2). Additionally, a separate PrefixOptions field, 8 bits in length, is attached to each prefix advertised within the body of an LSA. LS type The size of the LS type field has increased from 8 to 16 bits, with high-order bit encoding the handling of unknown types and the next two bits encoding flooding scope. See Appendix A.4.2.1 for the current coding of the LS type field.
Link State ID
The Link State ID remains at 32 bits in length. However, except
for network-LSAs and link-LSAs, the Link State ID has shed any
addressing semantics. For example, an IPv6 router originating
multiple AS-external-LSAs could start by assigning the first a
Link State ID of 0.0.0.1, the second a Link State ID of 0.0.0.2,
and so on. Instead of the IPv4 behavior of encoding the network
number within the AS-external-LSA's Link State ID, the IPv6 Link
State ID simply serves as a way to differentiate multiple LSAs
originated by the same router.
For network-LSAs, the Link State ID is set to the Designated
Router's Interface ID on the link. When a router originates a
link-LSA for a given link, its Link State ID is set equal to the
router's Interface ID on the link.
4.4.2. The Link-State Database
In IPv6, as in IPv4, individual LSAs are identified by a combination
of their LS type, Link State ID, and Advertising Router fields.
Given two instances of an LSA, the most recent instance is determined
by examining the LSAs' LS sequence number, using LS checksum and LS
age as tiebreakers (see Section 13.1 of [OSPFV2]).
In IPv6, the link-state database is split across three separate data
structures. LSAs with AS flooding scope are contained within the
top-level OSPF data structure (see Section 4.1) as long as either
their LS type is known or their U-bit is 1 (flood even when
unrecognized); this includes the AS-external-LSAs. LSAs with area
flooding scope are contained within the appropriate area structure
(see Section 4.1.1) as long as either their LS type is known or their
U-bit is 1 (flood even when unrecognized); this includes router-LSAs,
network-LSAs, inter-area-prefix-LSAs, inter-area-router-LSAs, NSSA-
LSAs, and intra-area-prefix-LSAs. LSAs with an unknown LS type, the
U-bit set to 0, and/or link-local flooding scope are contained within
the appropriate interface structure (see Section 4.1.2); this
includes link-LSAs.
To look up or install an LSA in the database, you first examine the
LS type and the LSA's context (i.e., the area or link to which the
LSA belongs). This information allows you to find the correct
database of LSAs where you then search based on the LSA's type, Link
State ID, and Advertising Router.
4.4.3. Originating LSAs
The process of reoriginating an LSA in IPv6 is the same as in IPv4: the LSA's LS sequence number is incremented, its LS age is set to 0, its LS checksum is calculated, and the LSA is added to the link state database and flooded on the appropriate interfaces. The list of events causing LSAs to be reoriginated for IPv4 is given in Section 12.4 of [OSPFV2]. The following events and/or actions are added for IPv6: o The state or interface ID of one of the router's interfaces changes. The router may need to (re)originate or flush its link- LSA and one or more router-LSAs and/or intra-area-prefix-LSAs. If the router is the Designated Router, the router may also need to (re)originate and/or flush the network-LSA corresponding to the interface. o The identity of a link's Designated Router changes. The router may need to (re)originate or flush the link's network-LSA and one or more router-LSAs and/or intra-area-prefix-LSAs. o A neighbor transitions to/from "Full" state. The router may need to (re)originate or flush the link's network-LSA and one or more router-LSAs and/or intra-area-prefix-LSAs. o The Interface ID of a neighbor changes. This may cause a new instance of a router-LSA to be originated for the associated area. o A new prefix is added to an attached link, or a prefix is deleted (both through configuration). This causes the router to reoriginate its link-LSA for the link or, if it is the only router attached to the link, causes the router to reoriginate an intra- area-prefix-LSA. o A new link-LSA is received, causing the link's collection of prefixes to change. If the router is the Designated Router for the link, it originates a new intra-area-prefix-LSA. o A new link-LSA is received, causing the logical OR of LSA options advertised by adjacent routers on the link to change. If the router is the Designated Router for the link, it originates a new network-LSA. Detailed construction of the seven required IPv6 LSA types is supplied by the following subsections. In order to display example LSAs, the network map in Figure 15 of [OSPFV2] has been reworked to show IPv6 addressing, resulting in Figure 1. The OSPF cost of each
interface is displayed in Figure 1. The assignment of IPv6 prefixes to network links is shown in Table 1. A single area address range has been configured for Area 1, so that outside of Area 1 all of its prefixes are covered by a single route to 2001:0db8:c001::/48. The OSPF interface IDs and the link-local addresses for the router interfaces in Figure 1 are given in Table 2. .......................................... . Area 1. . + . . | . . | 3+---+1 . . N1 |--|RT1|-----+ . . | +---+ \ . . | \ ______ . . + \/ \ 1+---+ . * N3 *------|RT4|------ . + /\_______/ +---+ . | / | . . | 3+---+1 / | . . N2 |--|RT2|-----+ 1| . . | +---+ +---+ . . | |RT3|---------------- . + +---+ . . |2 . . | . . +------------+ . . N4 . .......................................... Figure 1: Area 1 with IP Addresses Shown Network IPv6 prefix ----------------------------------- N1 2001:0db8:c001:0200::/56 N2 2001:0db8:c001:0300::/56 N3 2001:0db8:c001:0100::/56 N4 2001:0db8:c001:0400::/56 Table 1: IPv6 Link Prefixes for Sample Network
Router Interface Interface ID link-local address ------------------------------------------------------- RT1 to N1 1 fe80:0001::RT1 to N3 2 fe80:0002::RT1 RT2 to N2 1 fe80:0001::RT2 to N3 2 fe80:0002::RT2 RT3 to N3 1 fe80:0001::RT3 to N4 2 fe80:0002::RT3 RT4 to N3 1 fe80:0001::RT4 Table 2: OSPF Interface IDs and Link-Local Addresses Figure 14.4.3.1. LSA Options
The Options field in LSAs should be coded as follows. The V6-bit should be set unless the router will not participate in transit IPv6 routing. The E-bit should be clear if and only if the attached area is an OSPF stub or OSPF NSSA area. The E-bit should always be set in AS scoped LSAs. The N-bit should be set if and only if the attached area is an OSPF NSSA area. The R-bit should be set unless the router will not participate in any transit routing. The DC-bit should be set if and only if the router can correctly process the DoNotAge bit when it appears in the LS age field of LSAs (see [DEMAND]). All unrecognized bits in the Options field should be cleared. The V6-bit and R-bit are only examined in Router-LSAs during the SPF computation. In other LSA types containing options, they are set for informational purposes only.4.4.3.2. Router-LSAs
The LS type of a router-LSA is set to the value 0x2001. Router-LSAs have area flooding scope. A router MAY originate one or more router- LSAs for a given area. Each router-LSA contains an integral number of interface descriptions. Taken together, the collection of router- LSAs originated by the router for an area describes the collected states of all the router's interfaces attached to the area. When multiple router-LSAs are used, they are distinguished by their Link State ID fields. To the left of the Options field, the router capability bits V, E, and B should be set according to Section 12.4.1 of [OSPFV2]. Each of the router's interfaces to the area is then described by appending "link descriptions" to the router-LSA. Each link description is 16 bytes long, consisting of five fields: (link) Type,
Metric, Interface ID, Neighbor Interface ID, and Neighbor Router ID (see Appendix A.4.3). Interfaces in the state "Down" or "Loopback" are not described (although looped back interfaces can contribute prefixes to intra-area-prefix-LSAs), nor are interfaces without any full adjacencies described (except in the case of multiple Standby Interfaces as described in Section 4.9). All other interfaces to the area add zero, one, or more link descriptions. The number and content of these depend on the interface type. Within each link description, the Metric field is always set to the interface's output cost, and the Interface ID field is set to the interface's OSPF Interface ID. Point-to-point interfaces If the neighboring router is fully adjacent, add a Type 1 link description (point-to-point). The Neighbor Interface ID field is set to the Interface ID advertised by the neighbor in its Hello packets, and the Neighbor Router ID field is set to the neighbor's Router ID. Broadcast and NBMA interfaces If the router is fully adjacent to the link's Designated Router or if the router itself is the Designated Router and is fully adjacent to at least one other router, add a single Type 2 link description (transit network). The Neighbor Interface ID field is set to the Interface ID advertised by the Designated Router in its Hello packets, and the Neighbor Router ID field is set to the Designated Router's Router ID. Virtual links If the neighboring router is fully adjacent, add a Type 4 link description (virtual). The Neighbor Interface ID field is set to the Interface ID advertised by the neighbor in its Hello packets, and the Neighbor Router ID field is set to the neighbor's Router ID. Note that the output cost of a virtual link is calculated during the routing table calculation (see Section 4.7). Point-to-Multipoint interfaces For each fully adjacent neighbor associated with the interface, add a separate Type 1 link description (point-to-point) with the Neighbor Interface ID field set to the Interface ID advertised by the neighbor in its Hello packets and the Neighbor Router ID field set to the neighbor's Router ID. As an example, consider the router-LSA that router RT3 would originate for Area 1 in Figure 1. Only a single interface must be described, namely, that which connects to the transit network N3. It assumes that RT4 has been elected the Designated Router of Network N3.
; RT3's router-LSA for Area 1
LS age = 0 ;newly (re)originated
LS type = 0x2001 ;router-LSA
Link State ID = 0 ;first fragment
Advertising Router = 192.0.2.3 ;RT3's Router ID
bit E = 0 ;not an AS boundary router
bit B = 1 ;area border router
Options = (V6-bit|E-bit|R-bit)
Type = 2 ;connects to N3
Metric = 1 ;cost to N3
Interface ID = 1 ;RT3's Interface ID on N3
Neighbor Interface ID = 1 ;RT4's Interface ID on N3
Neighbor Router ID = 192.0.2.4 ; RT4's Router ID
RT3's router-LSA for Area 1
For example, if another router was added to Network N4, RT3 would
have to advertise a second link description for its connection to
(the now transit) network N4. This could be accomplished by
reoriginating the above router-LSA, this time with two link
descriptions. Or, a separate router-LSA could be originated with a
separate Link State ID (e.g., using a Link State ID of 1) to describe
the connection to N4.
Host routes for stub networks no longer appear in the router-LSA.
Rather, they are included in intra-area-prefix-LSAs.
4.4.3.3. Network-LSAs
The LS type of a network-LSA is set to the value 0x2002. Network-
LSAs have area flooding scope. A network-LSA is originated for every
broadcast or NBMA link with an elected Designated Router that is
fully adjacent with at least one other router on the link. The
network-LSA is originated by the link's Designated Router and lists
all routers on the link with which it is fully adjacent.
The procedure for originating network-LSAs in IPv6 is the same as the
IPv4 procedure documented in Section 12.4.2 of [OSPFV2], with the
following exceptions:
o An IPv6 network-LSA's Link State ID is set to the Interface ID of
the Designated Router on the link.
o IPv6 network-LSAs do not contain a Network Mask. All addressing
information formerly contained in the IPv4 network-LSA has now
been consigned to intra-Area-Prefix-LSAs originated by the link's
Designated Router.
o The Options field in the network-LSA is set to the logical OR of
the Options fields contained within the link's associated link-
LSAs corresponding to fully adjacent neighbors. In this way, the
network link exhibits a capability when at least one fully
adjacent neighbor on the link requests that the capability be
advertised.
As an example, assuming that Router RT4 has been elected the
Designated Router of Network N3 in Figure 1, the following network-
LSA is originated:
; Network-LSA for Network N3
LS age = 0 ;newly (re)originated
LS type = 0x2002 ;network-LSA
Link State ID = 1 ;RT4's Interface ID on N3
Advertising Router = 192.0.2.4 ;RT4's Router ID
Options = (V6-bit|E-bit|R-bit)
Attached Router = 192.0.2.4 ;Router ID
Attached Router = 192.0.2.1 ;Router ID
Attached Router = 192.0.2.2 ;Router ID
Attached Router = 192.0.2.3 ;Router ID
Network-LSA for Network N3
4.4.3.4. Inter-Area-Prefix-LSAs
The LS type of an inter-area-prefix-LSA is set to the value 0x2003.
Inter-area-prefix-LSAs have area flooding scope. In IPv4, inter-
area-prefix-LSAs were called type 3 summary-LSAs. Each inter-area-
prefix-LSA describes a prefix external to the area, yet internal to
the Autonomous System.
The procedure for originating inter-area-prefix-LSAs in IPv6 is the
same as the IPv4 procedure documented in Sections 12.4.3 and 12.4.3.1
of [OSPFV2], with the following exceptions:
o The Link State ID of an inter-area-prefix-LSA has lost all of its
addressing semantics and simply serves to distinguish multiple
inter-area-prefix-LSAs that are originated by the same router.
o The prefix is described by the PrefixLength, PrefixOptions, and
Address Prefix fields embedded within the LSA body. Network Mask
is no longer specified.
o The NU-bit in the PrefixOptions field should be clear.
o Link-local addresses MUST never be advertised in inter-area-
prefix-LSAs.
As an example, the following shows the inter-area-prefix-LSA that
Router RT4 originates into the OSPF backbone area, condensing all of
Area 1's prefixes into the single prefix 2001:0db8:c001::/48. The
cost is set to 4, which is the maximum cost of all of the individual
component prefixes. The prefix is padded out to an even number of
32-bit words, so that it consumes 64 bits of space instead of 48
bits.
; Inter-area-prefix-LSA for Area 1 addresses
; originated by Router RT4 into the backbone
LS age = 0 ;newly (re)originated
LS type = 0x2003 ;inter-area-prefix-LSA
Advertising Router = 192.0.2.4 ;RT4's ID
Metric = 4 ;maximum to components
PrefixLength = 48
PrefixOptions = 0
Address Prefix = 2001:0db8:c001 ;padded to 64-bits
Inter-area-prefix-LSA for Area 1 addresses originated
by Router
RT4 into the backbone
4.4.3.5. Inter-Area-Router-LSAs
The LS type of an inter-area-router-LSA is set to the value 0x2004.
Inter-area-router-LSAs have area flooding scope. In IPv4, inter-
area-router-LSAs were called type 4 summary-LSAs. Each inter-area-
router-LSA describes a path to a destination OSPF router (i.e., an AS
Boundary Router (ASBR)) that is external to the area yet internal to
the Autonomous System.
The procedure for originating inter-area-router-LSAs in IPv6 is the
same as the IPv4 procedure documented in Section 12.4.3 of [OSPFV2],
with the following exceptions:
o The Link State ID of an inter-area-router-LSA is no longer the
destination router's OSPF Router ID and now simply serves to
distinguish multiple inter-area-router-LSAs that are originated by
the same router. The destination router's Router ID is now found
in the body of the LSA.
o The Options field in an inter-area-router-LSA should be set equal
to the Options field contained in the destination router's own
router-LSA. The Options field thus describes the capabilities
supported by the destination router.
As an example, consider the OSPF Autonomous System depicted in Figure
6 of [OSPFV2]. Router RT4 would originate into Area 1 the following
inter-area-router-LSA for destination router RT7.
; inter-area-router-LSA for AS boundary router RT7
; originated by Router RT4 into Area 1
LS age = 0 ;newly (re)originated
LS type = 0x2004 ;inter-area-router-LSA
Advertising Router = 192.0.2.4 ;RT4's ID
Options = (V6-bit|E-bit|R-bit) ;RT7's capabilities
Metric = 14 ;cost to RT7
Destination Router ID = Router RT7's ID
Inter-area-router-LSA for AS boundary router RT7 originated by Router
RT4 into Area 1
4.4.3.6. AS-External-LSAs
The LS type of an AS-external-LSA is set to the value 0x4005. AS-
external-LSAs have AS flooding scope. Each AS-external-LSA describes
a path to a prefix external to the Autonomous System.
The procedure for originating AS-external-LSAs in IPv6 is the same as
the IPv4 procedure documented in Section 12.4.4 of [OSPFV2], with the
following exceptions:
o The Link State ID of an AS-external-LSA has lost all of its
addressing semantics and simply serves to distinguish multiple AS-
external-LSAs that are originated by the same router.
o The prefix is described by the PrefixLength, PrefixOptions, and
Address Prefix fields embedded within the LSA body. Network Mask
is no longer specified.
o The NU-bit in the PrefixOptions field should be clear.
o Link-local addresses can never be advertised in AS-external-LSAs.
o The forwarding address is present in the AS-external-LSA if and
only if the AS-external-LSA's bit F is set.
o The external route tag is present in the AS-external-LSA if and
only if the AS-external-LSA's bit T is set.
o The capability for an AS-external-LSA to reference another LSA has
been supported through the inclusion of the Referenced LS Type
field and the optional Referenced Link State ID field (the latter
present if and only if the Referenced LS Type is non-zero). This
capability is for future use; the Referenced LS Type should be set
to 0, and received non-zero values for this field should be
ignored until its use is defined.
As an example, consider the OSPF Autonomous System depicted in Figure
6 of [OSPFV2]. Assume that RT7 has learned its route to N12 via BGP
and that it wishes to advertise a Type 2 metric into the AS. Also
assume that the IPv6 prefix for N12 is the value 2001:0db8:0a00::/40.
RT7 would then originate the following AS-external-LSA for the
external network N12. Note that within the AS-external-LSA, N12's
prefix occupies 64 bits of space in order to maintain 32-bit
alignment.
; AS-external-LSA for Network N12,
; originated by Router RT7
LS age = 0 ;newly (re)originated
LS type = 0x4005 ;AS-external-LSA
Link State ID = 123 ;LSA type/scope unique identifier
Advertising Router = Router RT7's ID
bit E = 1 ;Type 2 metric
bit F = 0 ;no forwarding address
bit T = 1 ;external route tag included
Metric = 2
PrefixLength = 40
PrefixOptions = 0
Referenced LS Type = 0 ;no Referenced Link State ID
Address Prefix = 2001:0db8:0a00 ;padded to 64-bits
External Route Tag = as per BGP/OSPF interaction
AS-external-LSA for Network N12, originated by Router RT7
4.4.3.7. NSSA-LSAs
The LS type of an NSSA-LSA is set to the value 0x2007. NSSA-LSAs
have area flooding scope. Each NSSA-LSA describes a path to a prefix
external to the Autonomous System whose flooding scope is restricted
to a single NSSA area.
The procedure for originating NSSA-LSAs in IPv6 is the same as the
IPv4 procedure documented in [NSSA], with the following exceptions:
o The Link State ID of an NSSA-LSA has lost all of its addressing
semantics and simply serves to distinguish multiple NSSA-LSAs that
are originated by the same router in the same area.
o The prefix is described by the PrefixLength, PrefixOptions, and
Address Prefix fields embedded within the LSA body. Network Mask
is no longer specified.
o The NU-bit in the PrefixOptions field should be clear.
o Link-local addresses can never be advertised in NSSA-LSAs.
o The forwarding address is present in the NSSA-LSA if and only if
the NSSA-LSA's bit F is set.
o The external route tag is present in the NSSA-LSA if and only if
the NSSA-LSA's bit T is set.
o The capability for an NSSA-LSA to reference another LSA has been
supported through the inclusion of the Referenced LS Type field
and the optional Referenced Link State ID field (the latter
present if and only if the Referenced LS Type is non-zero). This
capability is for future use; the Referenced LS Type should be set
to 0, and received non-zero values for this field should be
ignored until its use is defined.
An example of an NSSA-LSA would only differ from an AS-external-LSA
in that the LS type would be 0x2007 rather than 0x4005.
4.4.3.8. Link-LSAs
The LS type of a link-LSA is set to the value 0x0008. Link-LSAs have
link-local flooding scope. A router originates a separate link-LSA
for each attached link that supports two or more (including the
originating router itself) routers. Link-LSAs SHOULD NOT be
originated for virtual links.
Link-LSAs have three purposes:
1. They provide the router's link-local address to all other routers
attached to the link.
2. They inform other routers attached to the link of a list of IPv6
prefixes to associate with the link.
3. They allow the router to advertise a collection of Options bits
in the network-LSA originated by the Designated Router on a
broadcast or NBMA link.
A link-LSA for a given Link L is built in the following fashion:
o The Link State ID is set to the router's Interface ID on Link L.
o The Router Priority of the router's interface to Link L is
inserted into the link-LSA.
o The link-LSA's Options field is set to reflect the router's
capabilities. On multi-access links, the Designated Router will
logically OR the link-LSA Options fields for all fully adjacent
neighbors in Link L's network-LSA.
o The router inserts its link-local address on Link L into the link-
LSA. This information will be used when the other routers on Link
L do their next-hop calculations (see Section 4.8.2).
o Each IPv6 address prefix that has been configured on Link L is
added to the link-LSA by specifying values for the PrefixLength,
PrefixOptions, and Address Prefix fields.
After building a link-LSA for a given link, the router installs the
link-LSA into the associated interface data structure and floods the
link-LSA on the link. All other routers on the link will receive the
link-LSA, but they will not flood the link-LSA on other links.
If LinkLSASuppression is configured for the interface and the
interface type is not broadcast or NBMA, origination of the link-LSA
may be suppressed. This implies that other routers on the link will
ascertain the router's next-hop address using a mechanism other than
the link-LSA (see Section 4.8.2). Refer to Appendix C.3 for a
description of the LinkLSASuppression interface configuration
parameter.
As an example, consider the link-LSA that RT3 will build for N3 in
Figure 1. Suppose that the prefix 2001:0db8:c001:0100::/56 has been
configured within RT3 for N3. This will result in the following
link-LSA that RT3 will flood only on N3. Note that not all routers
on N3 need be configured with the prefix; those not configured will
learn the prefix when receiving RT3's link-LSA.
; RT3's link-LSA for N3
LS age = 0 ;newly (re)originated
LS type = 0x0008 ;link-LSA
Link State ID = 1 ;RT3's Interface ID on N3
Advertising Router = 192.0.2.3 ;RT3's Router ID
Rtr Priority = 1 ;RT3's N3 Router Priority
Options = (V6-bit|E-bit|R-bit)
Link-local Interface Address = fe80:0001::RT3
# prefixes = 1
PrefixLength = 56
PrefixOptions = 0
Address Prefix = 2001:0db8:c001:0100 ;pad to 64-bits
RT3's link-LSA for N3
4.4.3.9. Intra-Area-Prefix-LSAs
The LS type of an intra-area-prefix-LSA is set to the value 0x2009.
Intra-area-prefix-LSAs have area flooding scope. An intra-area-
prefix-LSA has one of two functions. It either associates a list of
IPv6 address prefixes with a transit network link by referencing a
network-LSA, or associates a list of IPv6 address prefixes with a
router by referencing a router-LSA. A stub link's prefixes are
associated with its attached router.
A router MAY originate multiple intra-area-prefix-LSAs for a given
area. Each intra-area-prefix-LSA has a unique Link State ID and
contains an integral number of prefix descriptions.
A link's Designated Router originates one or more intra-area-prefix-
LSAs to advertise the link's prefixes throughout the area. For a
link L, L's Designated Router builds an intra-area-prefix-LSA in the
following fashion:
o In order to indicate that the prefixes are to be associated with
the Link L, the fields Referenced LS Type, Referenced Link State
ID, and Referenced Advertising Router are set to the corresponding
fields in Link L's network-LSA (namely, LS type, Link State ID,
and Advertising Router respectively). This means that the
Referenced LS Type is set to 0x2002, the Referenced Link State ID
is set to the Designated Router's Interface ID on Link L, and the
Referenced Advertising Router is set to the Designated Router's
Router ID.
o Each link-LSA associated with Link L is examined (these are in the
Designated Router's interface structure for Link L). If the link-
LSA's Advertising Router is fully adjacent to the Designated
Router and the Link State ID matches the neighbor's interface ID,
the list of prefixes in the link-LSA is copied into the intra-
area-prefix-LSA that is being built. Prefixes having the NU-bit
and/or LA-bit set in their Options field SHOULD NOT be copied, nor
should link-local addresses be copied. Each prefix is described
by the PrefixLength, PrefixOptions, and Address Prefix fields.
Multiple prefixes having the same PrefixLength and Address Prefix
are considered to be duplicates. In this case, their
PrefixOptions fields should be logically OR'ed together, and a
single instance of the duplicate prefix should be included in the
intra-area-prefix-LSA. The Metric field for all prefixes is set
to 0.
o The "# prefixes" field is set to the number of prefixes that the
router has copied into the LSA. If necessary, the list of
prefixes can be spread across multiple intra-area-prefix-LSAs in
order to keep the LSA size small.
A router builds an intra-area-prefix-LSA to advertise prefixes for
its attached stub links, looped-back interfaces, and hosts. A Router
RTX would build its intra-area-prefix-LSA in the following fashion:
o In order to indicate that the prefixes are to be associated with
the Router RTX itself, RTX sets the Referenced LS Type to 0x2001,
the Referenced Link State ID to 0, and the Referenced Advertising
Router to RTX's own Router ID.
o Router RTX examines its list of interfaces to the area. If the
interface is in the state Down, its prefixes are not included. If
the interface has been reported in RTX's router-LSA as a Type 2
link description (link to transit network), prefixes that will be
included in the intra-area-prefix-LSA for the link are skipped.
However, any prefixes that would normally have the LA-bit set
SHOULD be advertised independent of whether or not the interface
is advertised as a transit link. If the interface type is point-
to-multipoint or the interface is in the state Loopback, the
global scope IPv6 addresses associated with the interface (if any)
are copied into the intra-area-prefix-LSA with the PrefixOptions
LA-bit set, the PrefixLength set to 128, and the metric set to 0.
Otherwise, the list of global prefixes configured in RTX for the
link are copied into the intra-area-prefix-LSA by specifying the
PrefixLength, PrefixOptions, and Address Prefix fields. The
Metric field for each of these prefixes is set to the interface's
output cost.
o RTX adds the IPv6 prefixes for any directly attached hosts
belonging to the area (see Appendix C.7) to the intra-area-prefix-
LSA.
o If RTX has one or more virtual links configured through the area,
it includes one of its global scope IPv6 interface addresses in
the LSA (if it hasn't already), setting the LA-bit in the
PrefixOptions field, the PrefixLength to 128, and the Metric to 0.
This information will be used later in the routing calculation so
that the two ends of the virtual link can discover each other's
IPv6 addresses.
o The "# prefixes" field is set to the number of prefixes that the
router has copied into the LSA. If necessary, the list of
prefixes can be spread across multiple intra-area-prefix-LSAs in
order to keep the LSA size small.
For example, the intra-area-prefix-LSA originated by RT4 for Network
N3 (assuming that RT4 is N3's Designated Router) and the intra-area-
prefix-LSA originated into Area 1 by Router RT3 for its own prefixes
are pictured below.
; RT4's Intra-area-prefix-LSA for network link N3
LS age = 0 ;newly (re)originated
LS type = 0x2009 ;Intra-area-prefix-LSA
Link State ID = 5 ;LSA type/scope unique identifier
Advertising Router = 192.0.2.4 ;RT4's Router ID
# prefixes = 1
Referenced LS Type = 0x2002 ;network-LSA reference
Referenced Link State ID = 1
Referenced Advertising Router = 192.0.2.4
PrefixLength = 56 ;N3's prefix
PrefixOptions = 0
Metric = 0
Address Prefix = 2001:0db8:c001:0100 ;pad
; RT3's Intra-area-prefix-LSA for its own prefixes
LS age = 0 ;newly (re)originated
LS type = 0x2009 ;Intra-area-prefix-LSA
Link State ID = 177 ;LSA type/scope unique identifier
Advertising Router = 192.0.2.3 ;RT3's Router ID
# prefixes = 1
Referenced LS Type = 0x2001 ;router-LSA reference
Referenced Link State ID = 0
Referenced Advertising Router = 192.0.2.3
PrefixLength = 56 ;N4's prefix
PrefixOptions = 0
Metric = 2 ;N4 interface cost
Address Prefix = 2001:0db8:c001:0400 ;pad
Intra-area-prefix-LSA for Network Link N3
When network conditions change, it may be necessary for a router to
move prefixes from one intra-area-prefix-LSA to another. For
example, if the router is the Designated Router for a link but the
link has no other attached routers, the link's prefixes are
advertised in an intra-area-prefix-LSA referring to the Designated
Router's router-LSA. When additional routers appear on the link, a
network-LSA is originated for the link and the link's prefixes are
moved to an intra-area-prefix-LSA referring to the network-LSA.
Note that in the intra-area-prefix-LSA, the Referenced Advertising
Router is always equal to the router that is originating the intra-
area-prefix-LSA (i.e., the LSA's Advertising Router). The reason the
Referenced Advertising Router field appears is that, even though it
is currently redundant, it may not be in the future. We may sometime
want to use the same LSA format to advertise address prefixes for
other protocol suites. In this case, the Designated Router may not
be running the other protocol suite, and so another of the link's routers may need to originate the intra-area-prefix-LSA. In that case, the Referenced Advertising Router and Advertising Router would be different.4.4.4. Future LSA Validation
It is expected that new LSAs will be defined that will not be processed during the Shortest Path First (SPF) calculation as described in Section 4.8, for example, OSPFv3 LSAs corresponding to information advertised in OSPFv2 using opaque LSAs [OPAQUE]. In general, the new information advertised in future LSAs should not be used unless the OSPFv3 router originating the LSA is reachable. However, depending on the application and the data advertised, this reachability validation MAY be done less frequently than every SPF calculation. To facilitate inter-area reachability validation, any OSPFv3 router originating AS scoped LSAs is considered an AS Boundary Router (ASBR).
(page 40 continued on part 3)