13. Maintaining the forwarding cache
A MOSPF router may, for resource reasons, limit the size of its
forwarding cache. At any time cache entries can be purged to make
room for newer entries, since the purged entries can always be
rebuilt when necessary. This memo does not specify an algorithm to
select which entries to purge. However, care should be taken to
ensure that any particular entry is not continually rebuilt and then
purged again (i.e., thrashing should be avoided).
The building of the forwarding cache has been previously described
in Section 12. There are events that force one or more forwarding
cache entries to be deleted; these events are described below. Note
that deleted cache entries will be rebuilt on an as-needed basis.
o When the internal topology of the MOSPF system changes, all
forwarding cache entries must be deleted. This is because
internal topology changes may invalidate the previously
calculated datagram shortest-path trees. Since the multicast
routing calculation depends on the result of the unicast routing
calculations, the forwarding cache should be cleared after the
unicast routing table is rebuilt. Internal topology changes are
indicated when both a) a new instance of either a router-LSA or
a network-LSA is received and b) the contents of the new
advertisement (other than the LS age, LS sequence number and LS
checksum fields) are different from the previous instance. This
covers routers and links going up or down, routers that change
from being multicast-incapable to being multicast-capable, etc.
o When a Type 3 summary-link-LSA (network summary) changes, those
forwarding cache entries specifying datagram sources belonging
to the range of addresses described by the updated summary-
link-LSA must be deleted. See Sections 12.2.3 and 12.2.5.
o Suppose that the content of an AS external-link-LSA changes. If
the AS external-link-LSA describes an external network N, then
all forwarding cache entries specifying an external source
network that is contained in N or that contains N (i.e.,
external sources that are a subset or a superset of N) must be
o When membership in a multicast group changes, all forwarding
cache entries for the particular group must be deleted. Group
membership changes are indicated when either a) the content of a
group-membership-LSA changes or b) an entry in the local group
database (see Section 8.4) changes.
o When the cost to an AS boundary router or to a forwarding
address specified by one or more AS external-link-LSAs changes,
all forwarding cache entries specifying an external network as
datagram source must be deleted. In this case, potentially all
inter-AS datagram shortest-path trees have been invalidated. The
forwarding cache entries should be deleted after the new best
cost to the AS boundary router/forwarding address has been
14. Other additions to the OSPF specification
MOSPF requires some modifications to the base OSPF protocol. All
these modifications are backward-compatible. A router running MOSPF
will still interoperate with an OSPF router when forwarding unicast
traffic. Most of the modifications have been described earlier in
this document. This section collects together those changes which
have yet to be mentioned, organizing them by the affected Section of
14.1. The Designated Router
This functionality is described in Section 7.3 of [OSPF]. In
OSPF, a network's Designated Router has two specialized roles.
First, it originates the network's network-LSA. Second, it
controls the flooding on the network, in that all of the routers
on the network synchronize with the Designated Router (and the
Backup Designated Router) only. For these reasons, when one
or more of the network's routers are running MOSPF, the
Designated Router should be running MOSPF also. This can be
ensured by assigning all non-multicast routers the Router
Priority of 0.
In MOSPF, the Designated Router also has the additional
responsibility of monitoring the network's multicast group
membership. This is done by periodically sending Host Membership
Queries, and receiving Host Membership Reports in response (see
Section 9). This is yet another reason why the Designated Router
must be multicast-capable.
14.2. Sending Hello packets
This functionality is described in Section 9.5 of [OSPF]. A
MOSPF router sets the MC-bit in the Options field of its Hello
packets. This indicates that the router is multicast-capable; it
does not necessarily indicate the state of the sending
interface's IPMulticastForwarding parameter (see Section B.2).
Setting the MC-bit in Hellos is done strictly for informational
purposes. Neighbors receiving the router's Hello packets do not
act on the state of the MC-bit. A neighbor's multicast-
capability is learned instead during the Database Exchange
Process (see Section 14.4).
14.3. The Neighbor state machine
This functionality is described in Section 10.3 of [OSPF]. When
a neighbor enters state Exchange, the neighbor Database summary
list is initialized (see the OSPF neighbor FSM entry for State:
ExStart and Event: NegotiationDone). This list describes of the
portion of the router's link state database that needs to be
synchronized with the neighbor. Group-membership-LSAs are
included in the neighbor Database summary list if and only if
the neighbor is multicast-capable. The neighbor's multicast
capability is learned by examining the neighbor's Database
Description packets (see Section 14.4).
14.4. Receiving Database Description packets
This functionality is described in Section 10.6 of [OSPF]. A
neighbor's multicast-capability is learned through received
Database Description packets. When the Database Description
packet is received that transitions the neighbor from ExStart to
Exchange, the state of the MC-bit in the packet's Options field
is examined. The neighbor is multicast-capable if and only if
the MC-bit is set.
The neighbor's multicast capability controls whether group-
membership-LSAs are summarized to the neighbor during the
Database Exchange process (see Section 14.3), and whether
group-membership-LSAs are flooded to the neighbor during the
flooding process (see Section 10.2).
14.5. Sending Database Description packets
This functionality is described in Section 10.8 of [OSPF]. A
MOSPF router sets the MC-bit in the Options field of its
Database Description packets. This indicates to its adjacent
neighbors that the router is multicast-capable; it does not
necessarily indicate the state of the sending interface's
IPMulticastForwarding parameter (see Section B.2).
When a router goes from being multicast-capable to multicast-
incapable, or vice-versa, it must indicate this fact to its
adjacent neighbors by restarting the Database Description
process (i.e., rolling back the state of all adjacent neighbors
14.6. Originating Router-LSAs
This functionality is described in Section 12.4.1 of [OSPF]. A
MOSPF router sets the MC-bit in the Options field of its
router-LSA. This allows the router to be included in datagram
shortest-path trees (see Step 5a of Section 12.2).
In addition, MOSPF has introduced a new flag in the router-LSA's
rtype field: the W-bit. When the W-bit is set, the router is
included on all datagram shortest-path trees, regardless of
multicast group (see Section 12.2.6). Such a router is called a
wild-card multicast receiver. The router sets the W-bit when it
wishes to receive all multicast datagrams, regardless of
destination. This will sometimes be true of inter-area multicast
forwarders (see Section 3.1), and inter-AS multicast forwarders
(see Section 4).
A router must originate a new instance of its router-LSA
whenever an event occurs that would invalidate the LSA's current
contents. In particular, if the router's multicast capability or
its ability to function as either an inter-area or inter-AS
multicast forwarder changes, its router-LSA must be
14.7. Originating Network-LSAs
This functionality is described in Section 12.4.2 of [OSPF]. In
OSPF, a transit network's network-LSA is originated by the
network's Designated Router. The Designated Router sets the MC-
bit in the Options field of the network-LSA if and only if both
a) the Designated Router is multicast-capable (i.e., running
MOSPF) and b) the Designated Router's interface's
IPMulticastForwarding parameter has been set to a value other
than disabled (see Section B.2). When the network-LSA has the
MC-bit set, the network can be included in datagram shortest-
path trees (see Section 12.2.6).
It is intended that all routers attached to a common network
agree on the network's IPMulticastForwarding capability.
However, this agreement is not enforced. When there are
disagreements, incorrect routing of multicast datagrams can
14.8. Originating Summary-link-LSAs
This functionality is described in Section 12.4.3 of [OSPF].
Inter-area multicast forwarders always set the MC-bit in the
Options field of their summary-link-LSAs, regardless of whether
the path described by the summary-link-LSA is actually
multicast-capable. Indeed, it is possible that there is no
multicast-capable path to the described destination. All other
area border routers (ones that are not inter-area multicast
forwarders) clear the MC-bit in the Options field of their
If its MC-bit is clear, the summary-link-LSA will not be used
when initializing the candidate list in Sections 12.2.2, 12.2.3
14.9. Originating AS external-link-LSAs
This functionality is described in Section 12.4.4 of [OSPF].
Unlike in summary-link-LSAs, an inter-AS multicast forwarder
should clear the MC-bit in the Options field of one of its AS
external-link-LSAs if it is known that there is no multicast-
capable path from the described destination to the router
itself. This knowledge may possibly be obtained, for example,
from an inter-AS multicast routing algorithm (see Section 4).
If the inter-AS multicast forwarder is unsure of whether a
multicast-capable path exists between the described destination
and the router itself, the MC-bit should be set in the AS
external-link-LSA. All other AS boundary routers (ones that are
not inter-AS multicast forwarders) clear the MC-bit in the
Options field of their AS external-link-LSAs.
If its MC-bit is clear, the AS external-link-LSA will not be
used when initializing the candidate list in Section 12.2.4.
When multicast connectivity to an external destination exists,
but no unicast connectivity, an AS external-link-LSA can be
originated having its MC-bit set and specifying a cost of
LSInfinity. Such an AS external-link-LSA will still be used by
the multicast routing calculation (see Section 12.2.4). As a
result, when a MOSPF router wishes to stop advertising an AS
external destination, it must use the premature aging procedure
specified in Section 14.1 of [OSPF], rather than simply setting
the AS external-link-LSA's cost to LSInfinity.
14.10. Next step in the flooding procedure
This functionality is described in Section 13.3 of [OSPF].
Group-membership-LSAs are specific to a OSPF single area, and
are flooded to multicast-capable routers only. When flooding a
group-membership-LSA, Section 13.3 of the OSPF specification is
modified as follows: 1) The list of interfaces examined during
flooding (called the eligible interfaces in Section 13.3 of
[OSPF]) is the set of all interfaces attaching to Area A (the
area that the group-membership-LSA is received from), just as
for router-LSAs, network-LSAs and summary-link-LSAs. 2) When
examining each interface, a group-membership-LSA is added to a
neighbor's link state retransmission list if and only if both a)
Step 1d of [OSPF]'s Section 13.3 is reached for the neighbor and
b) the neighbor is multicast-capable. The neighbor's multicast
capability is discovered during the Database Exchange process
(see Section 14.4).
Note that, since on broadcast networks Link State Update packets
are sent initially as multicasts, non-multicast routers may
receive group-membership-LSAs. However, non-multicast routers
will simply drop the group-membership-LSAs, for reasons of
unrecognized LS type (see Step 2 of [OSPF]'s Section 13). Link
State acknowledgments for group-membership-LSAs are not expected
from non-multicast routers, and group-membership-LSAs will never
be retransmitted to non-multicast routers, since the LSAs are
not added to these routers' link state retransmission lists (see
For more information on flooding group-membership-LSAs, see
14.11. Virtual links
This functionality is described in Section 15 of [OSPF]. When a
MOSPF router (i.e., multicast-capable router) is both an area
border router and an endpoint of a virtual link whose other
endpoint is also multicast capable, the router must then also be
an inter-area multicast forwarder. This is necessary to ensure
that multicast datagrams will flow through the virtual link's
transit area, from one endpoint to the other. When the
[Bharath-Kumar] Bharath-Kumar, K. and J. Jaffe, "Routing to Multiple
Destinations in Computer Networks", IEEE
Transactions on Communications, COM-31, March
[Deering] Deering, S., "Multicast Routing in Internetworks and
Extended LANs", SIGCOMM Summer 1988 Proceedings,
[Deering2] Deering, S., "Multicast Routing in a Datagram
Internetwork", Stanford Technical Report, STAN-CS-
92-1415, Department of Computer Science, Stanford
University, December 1991.
[OSPF] Moy, J., "OSPF Version 2", RFC 1583, Proteon, Inc.,
[RFC 1075] Waitzman, D., Partridge, C., and S. Deering,
"Distance Vector Multicast Routing Protocol", RFC
1075, BBN STC, Stanford University, November 1988.
[RFC 1112] Deering, S., "Host Extensions for IP Multicasting",
STD 5, RFC 1112, Stanford University, May 1988.
[RFC 1209] Piscitello, D., and J. Lawrence, "Transmission of IP
Datagrams over the SMDS Service", RFC 1209, Bell
Communications Research, March 1991.
[RFC 1340] Reynolds, J. and J. Postel, "Assigned Numbers", STD
2, RFC 1340, USC/Information Sciences Institute,
[RFC 1390] Katz, D., "Transmission of IP and ARP over FDDI
Networks", STD 36, RFC 1390, cisco Systems, Inc.,
Actually, OSPF allows a separate link cost to be configured for
each TOS. MOSPF then potentially calculates separate paths for each
TOS. For details, see Section 6.2.
We also assume in this section that the pictured multi-access
networks provide data-link multicast/broadcast services.
Note that if N3 were a non-broadcast network, Router RT3 would
send separate copies of the datagram to routers RT1 and RT2. Since
the IGMP protocol is not defined on non-broadcast networks, there
could in this case be no Group B member attached to Network N3.
However the multicast datagram would still be delivered to the Group
B members attached to networks N1 and N2.
Actually, in MOSPF there is a separate forwarding cache entry for
each combination of source, destination and TOS. For a discussion of
TOS-based multicast routing, see Section 6.2.
The discussion in this section omits mention of the Backup
Designated Router's role in the IGMP protocol. While the Backup
Designated Router does not send IGMP Host Membership Queries, it
does listen to IGMP Host Membership Reports, building "shadow" local
group database entries in the process. These entries do not lead to
group-membership-LSAs, nor do they influence delivery of multicast
datagrams, but are merely maintained to ease the transition from
Backup Designated Router to Designated Router, should the Designated
Router fail. See Sections 2.3.4, 9 and 10 for details.
One might imagine building all possible datagram shortest-path
trees up front. However, this might be expensive, both in router CPU
time and in router memory. It is hoped that building the datagram
shortest-path trees on demand and caching the results will ease
demands on router resources by spreading out the calculations over a
longer period of time.
It is possible that, due to the existence of alternate paths,
several different shortest-path trees are available. MOSPF depends
on all routers constructing the exact same shortest path tree. For
that reason, tie-breaking schemes have been implemented during tree
construction to ensure that identical trees result. See Section 12
for more details.
Note that the expanding ring search yields the nearest server in
terms of hop count, but not necessarily in terms of the OSPF metric.
This means that in MOSPF, just as in OSPF, the only kind of link
state advertisement that can be flooded between areas is the AS
A router indicates that it is a wild-card multicast receiver by
setting the appropriate flag in its router-LSA. See Section 14.6 for
This is not quite true. As we shall see, any inter-AS multicast
forwarders belonging to the backbone are designated as wild-card
multicast receivers. See Section 4.
It is possible that through the operation of an inter-AS
multicast routing protocol, Router RT7 knows that it does not have
multicast connectivity to Network N15 (even though it has unicast
connectivity). In this case, RT7 would not advertise the external
link to N15 as being multicast capable.
Synchronization of the IPMulticastForwarding interface parameter
is not enforced by the MOSPF protocol, since it is not included in
the contents of a MOSPF router's Hello packets.
Actually, when multiple IP networks have been assigned to the
same physical network, the first thing that needs to be done is to
associate an IP network with the received Host Membership Report.
This is done in the same way that a receiving interface is
associated with a received multicast datagram; see Section 11.1.
For this reason when a transit network has both MOSPF routers
and non-multicast OSPF routers attached, care should be taken to
ensure that a MOSPF router is elected Designated Router. This can be
accomplished through proper setting of the routers' configured
Note that just because these advertisements exist in the link
state database, it does not mean that the Group G members are
reachable. Reachability does not enter into the building of the
transit vertex list, in order to simplify the calculation. This is a
trade-off. As a result, some multicast datagrams may be forwarded
further than necessary, when the described Group G members actually
Since the Designated Router controls flooding on the network,
this is another reason to ensure that a MOSPF router is elected as
In other words, group-membership-LSAs will never be
retransmitted to non-multicast routers.
This last step will not be necessary if the configuration
guidelines presented in Section 6.5 are followed.
The TOS 0 routing table entry is examined regardless of the TOS
specified by the multicast datagram.
It is assumed that a MOSPF router that wants to stop advertising
a route to an external destination will use the premature aging
procedure specified in Section 14.1 of [OSPF], rather than setting
the AS external-link-LSA's cost to LSInfinity.
This preference ordering is used in Step 5c of Section 12.2.
No attempt is made to match the links' two halves. See Step 5d.
However, a summary-link-LSA is eligible for matching even if the
MC-bit in its Options field is clear.
Costs may have both a Type 2 and a Type 1 component; the Type 2
component is always most significant.
This case mirrors the SourceIntraArea candidate list
initialization in Section 12.2.1.
This case mirrors the SourceInterArea1 candidate list
initialization in Section 12.2.2.
This case mirrors the SourceInterArea2 candidate list
initialization in Section 12.2.3.
Note that selecting the upstream node in this manner enforces
the inter-area routing architecture outlined in Section 3.1. Namely,
the multicast datagram is forwarded from the source area, over the
backbone and then into the non-backbone areas. This is similar to
the "hub and spoke" architecture for unicast forwarding described in
Section 3.2 of [OSPF].
This procedure seems backwards. One would expect that the TOS X
datagram tree would be built first. However, the SPF calculation
must ensure that all routers participating in the forwarding of that
datagram, both TOS-capable and non-TOS-capable, build the same tree.
Since it is known that the non-TOS-capable routers will use the TOS
0 tree, the only safe way to use the TOS X tree is when you are
guaranteed that the non-TOS-capable routers will decline to forward
the datagram. This guarantee is clearly met when there are only
TOS-capable routers on the TOS 0 datagram tree.
Indeed, there will also be those cases where the router, not
being on a particular datagram shortest-path tree, will never have
to calculate the particular tree, since the router will not receive
the datagram in the first place.
Group-membership-LSAs are not processed by non-multicast routers
(see Section 10.2). Also, if the Designated Router was not running
the multicast extensions, multicast datagrams would not be forwarded
over the network because its network-LSA would have its MC-bit clear
(see Step 5a in Section 12.2).
A. Data Formats
This section documents the format of MOSPF protocol packets and link
state advertisements (LSAs). All changes and additions made to the
OSPF Version 2 data formats have been made in a backward-compatible
manner. In other words, multicast routers running MOSPF can
interoperate with (non-multicast) OSPF Version 2 routers when
forwarding regular (unicast) IP data traffic.
The MOSPF packet formats are the same as for OSPF Version 2
(described in Appendix A of [OSPF]). One additional option has been
added to the Options field that appears in OSPF Hello packets,
Database Description packets and all link state advertisements. This
new option indicates a router's/network's multicast capability, and
is documented in Section A.1. The presence of this new option is
ignored by all non-multicast routers.
To support MOSPF, one of OSPF's link state advertisements has been
modified, and a new link state advertisement has been added. The
format of the router-LSA has been modified (see Section A.2) to
include a new flag indicating whether the router is a wild-card
multicast receiver. A new link state advertisement, called the
group-membership-LSA, has been added to pinpoint multicast group
members in the link state database. This new advertisement is
neither flooded nor processed by non-multicast routers. The group-
membership-LSA is documented in Section A.3.
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
capabilities, 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 LSA types to a neighbor
because of its reduced functionality. Lastly, listing capabilities
in LSAs allows routers to route traffic around reduced functionality
routers, by excluding them from parts of the routing table
Three capabilities are currently defined. For each capability, the
effect of the capability's appearance (or lack of appearance) in
Hello packets, Database Description packets and link state
advertisements is specified below. For example, the
ExternalRoutingCapability (below called the E-bit) has meaning only
in OSPF Hello packets.
| * | * | * | * | * |MC | E | T |
The OSPF Options field
o T-bit. This describes the router's TOS capability. If the T-bit
is reset, then the router supports only a single TOS (TOS 0).
Such a router is also said to be incapable of TOS-routing. The
absence of the T-bit in a router links advertisement causes the
router to be skipped when building a non-zero TOS shortest-path
tree. In other words, routers incapable of TOS routing will be
avoided as much as possible when forwarding data traffic
requesting a non-zero TOS. The absence of the T-bit in a summary
link advertisement or an AS external link advertisement
indicates that the advertisement is describing a TOS 0 route
only (and not routes for non-zero TOS).
o E-bit. AS external link advertisements are not flooded
into/through OSPF stub areas. The E-bit ensures that all members
of a stub area agree on that area's configuration. The E-bit is
meaningful only in OSPF Hello packets. When the E-bit is reset
in the Hello packet sent out a particular interface, it means
that the router will neither send nor receive AS external link
state advertisements on that interface (in other words, the
interface connects to a stub area). Two routers will not become
neighbors unless they agree on the state of the E-bit.
o MC-bit. The MC-bit describes the multicast capability of the
various pieces of the OSPF routing domain. When calculating the
path of multicast datagrams, only those link state
advertisements having their MC-bit set are used. In addition, a
router uses the MC-bit in its Database Description packets to
tell adjacent neighbors whether the router will participate in
the flooding of the new group-membership-LSAs.
An OSPF router originates a router-LSA into each of its attached
areas. The router-LSA describes the state and cost of the router's
interfaces to the area. The contents of the router-LSA are described
in detail in Section A.4.2 of [OSPF]. There are flags in the
router-LSA that indicate whether the router is either a) an area
border router or b) an AS boundary router or c) the endpoint of a
virtual link. One more flag has been added to the router-LSA for
MOSPF; it is called bit W below. This flag indicates whether the
router wishes to receive all multicast datagrams regardless of
destination (i.e., is a wild-card multicast receiver).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| LS age | Options | 1 |
| Link State ID |
| Advertising Router |
| LS sequence number |
| LS checksum | length |
| rtype | 0 | # links |
| Link ID | P
| Link Data | R
| Type | # TOS | TOS 0 metric | #
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ L
# | TOS | 0 | metric | I
T +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ N
O | ... | K
S +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ S
| | TOS | 0 | metric | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| ... |
The router LSA
| * | * | * | * | W | V | E | B |
The rtype field
The following defines the flags found in the rtype field. Each flag
classifies the router by function:
o bit B. When set, the router is an area border router (B is for
border). These routers forward unicast data traffic between OSPF
o bit E. When set, the router is an AS boundary router (E is for
external). These routers forward unicast data traffic between
o bit V. When set, the router is an endpoint of an active virtual
link (V is for virtual) which uses the described area as its
o bit W. When set, the router is a wild-card multicast receiver.
These routers receive all multicast datagrams, regardless of
destination. Inter-area multicast forwarders and inter-AS
multicast forwarders are sometimes wild-card multicast receivers
(see Sections 3 and 4).
Group-membership-LSAs are the Type 6 link state advertisements.
Group-membership-LSAs are specific to a particular OSPF area. They
are never flooded beyond their area of origination. A router's
group-membership-LSA for Area A indicates its directly attached
networks which belong to Area A and contain members of a particular
multicast group. A router originates a group-membership-LSA for
multicast group D when the following conditions are met for at least
one directly attached network: 1) the router has been elected
Designated Router for the network and 2) at least one host on the
network has joined Group D via the IGMP protocol.
A router may also originate a group-membership-LSA for Group D if
the router itself has internal applications belonging to Group D. In
addition, area border routers originate group-membership-LSAs into
the backbone area when there are group members in the router's
attached non-backbone areas. See Section 10 for more information
concerning the origination of group-membership-LSAs.
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 | 6 |
| Link State ID = Destination Group |
| Advertising Router |
| LS sequence number |
| LS checksum | length |
| Vertex type |
| Vertex ID |
| ... |
The group-membership-LSA consists of the standard 20-byte link state
header (see Section A.4.1 of [OSPF]) followed by a list of transit
vertices to label with the multicast destination. The
advertisement's Link State ID is set to the destination multicast
group address. There is no metric associated with the advertisement.
Each transit vertex is specified by its Vertex type and Vertex ID
(see Section 12.1 for an explanation of this terminology):
o Vertex type. Set equal to 1 for a router, and 2 for a transit
network. Note that the only router that may be included in the
list is the Advertising Router itself.
o Vertex ID. For router vertices, this field indicates the
router's OSPF Router ID. For transit network vertices, this
field indicates the IP address of the network's Designated
Router. Note that the link state advertisement associated with
the transit vertex is the LSA whose LS type = Vertex type, Link
State ID = Vertex ID and Advertising Router = the group-
membership-LSA's Advertising Router.
B. Configurable Constants
This section documents the configurable parameters used by OSPF's
multicast routing extensions. These parameters are in addition to
the configurable constants used by the base OSPF protocol
(documented in Appendix C of [OSPF]). An implementation of MOSPF
must provide the ability to set these parameters, either through
network management or some other means.
B.1 Global parameters
The following parameters apply to the router as a whole.
o Multicast capability. An indication of whether the router is
running MOSPF. If the router is running MOSPF, it will
perform the algorithms as set forth in this specification.
Otherwise, the router is still able to run the basic OSPF
algorithm (as set forth in [OSPF]), and will be able to
interoperate with multicast capable routers (see Section
6.1) when forwarding regular (unicast) IP data traffic.
o Inter-area multicast forwarder. This parameter indicates
whether the router will forward multicast datagrams between
OSPF areas. Such a router summarizes group membership
information to the backbone, and acts as a wild-card
multicast receiver in all its attached non-backbone areas
(see Section 3.1). Not all multicast-capable area border
routers need be configured as inter-area multicast
forwarders. However, whenever both ends of a virtual link
are multicast-capable, they must both be configured as
inter-area multicast forwarders (see Section 14.11). By
default, all multicast-capable area border routers are
configured as inter-area multicast forwarders.
o Inter-AS multicast forwarder. This parameter indicates
whether the router forwards multicast datagrams between
Autonomous Systems. Such a router acts as a wild-card
multicast receiver in all attached areas (see Section 4). It
is also assumed that an inter-AS multicast forwarder runs
some kind of inter-AS multicast routing algorithm.
B.2 Router interface parameters
The following parameters can be configured separately for each
of the router's OSPF interfaces. Remember that an OSPF interface
is the connection between the router and one of its attached IP
networks. Note that the IPMulticastForwarding parameter is
really a description of the attached network. As such, it should
be configured identically on all routers attached to a common
network; otherwise incorrect routing of multicast datagrams may
o IPMulticastForwarding. This configurable parameter indicates
whether IP multicasts should be forwarded over the attached
network, and if so, how the forwarding should be done. The
parameter can assume one of three possible values: disabled,
data-link multicast and data-link unicast. When set to
disabled, IP multicast datagrams will not be forwarded out
the interface. When set to data-link multicast, IP multicast
datagrams will be forwarded as data-link multicasts. When
set to data-link unicast, IP multicast datagrams will be
forwarded as data-link unicasts. The default value for this
parameter is data-link multicast. The other two settings are
for use in the special circumstances described in Sections
6.3 and 6.4. When set to disabled or to data-link unicast,
IGMP group membership is not monitored on the attached
o IGMPPollingInterval. The number of seconds between IGMP Host
Membership Queries sent out this interface. A multicast-
capable router sends IGMP Host Membership Queries only when
it has been elected Designated Router for the attached
network. See [RFC 1112] for a discussion of this parameter's
o IGMP timeout. If no IGMP Host Membership Reports have been
heard on an attached network for a particular multicast
group A after this period of time, the entry [Group A,
attached network] is deleted from the router's local group
database. See Section 9 for more information.
C. Sample datagram shortest-path trees
In MOSPF, all routers must calculate exactly the same datagram
shortest-path trees. In order to ensure this in internetworks having
redundant links, a number of tie-breakers were defined in the MOSPF
routing table calculation (see Steps 4 and 5c of Section 12.2, and
Sections 12.2.4 and 12.2.7). This section illustrates the use of
these tie-breakers on a sample topology.
Three different examples are given. All examples use the same
physical topology and the same set of OSPF interface costs (see the
left side of Figure 14). The source of the datagram is always Host
H1 on the network at the top of the figure (188.8.131.52), and the
destination group members are the two hosts labelled with Group Ma
at the bottom of the figure. The first case shows an example of
intra-area multicast, while the remaining two cases show the
influence of OSPF areas on the path of a multicast datagram.
C.1 An intra-area tree
The datagram shortest-path tree resulting from the intra-area case
is shown on the right of Figure 14. The root of the tree is the
source network (184.108.40.206), and the leaves are the two routers (RT4
and RT3) directly attached to the stub networks containing Group Ma
There are equal-cost paths available to both group members. For the
group member on the left, the path could go either through network
10.1.0.0 or through network 10.2.0.0. By the tie-breaking rules, the
path through 10.2.0.0 is chosen since it has the larger IP network
number (see Step 5c of Section 12.2).
For the group member on the right, the path could go either over
Network 10.2.0.0 or over the serial line connecting routers RT2 and
RT3. The path over Network 10.2.0.0 is chosen after executing two
tie-breaking rules. First, Network 10.2.0.0 is placed on the
shortest-path tree before Router RT3 since networks are always
chosen over routers (see Step 4 of Section 12.2). Then, given a
Net 220.127.116.11 |
+----------+ |1 |1
| Network | 8+---+ +---+ o 18.104.22.168
| 10.1.0.0 |------|RT1| |RT2| |
+----------+ +---+ +---+ 0|
| |8 8| |
8| +----------+ |8 o RT1
+---+10 | Network | 10+---+ |
|RT4|-------| 10.2.0.0 |----|RT3| 8|
+---+ +----------+ +---+ |
|3 |3 o 10.2.0.0
| | / \
+---------+ +-------+ 0/ \0
| | / \
+--+ +--+ o o
|Ma| |Ma| RT4 RT3
Figure 14: An intra-area tree
C.2 The effect of areas
In Figure 15 below, the previous diagram has been modified by the
inclusion of OSPF areas. The datagram source is now part of the OSPF
backbone (Area 0), while the rest of the topology is in Area 1. In
this case, since the datagram source and the group members belong to
different areas, reverse costs are used when building the tree (see
Step 5b of Section 12.2). This actually eliminates the equal cost
paths from the diagram, and leads to the Area 1 datagram shortest-
path tree on the right of Figure 15.
Net 22.214.171.124 |
..................... | |
. +----------+ . |1 |1 126.96.36.199
. | Network | 8+---+ +---+ o
. | 10.1.0.0 |------|RT1|........|RT2|... / \
. +----------+ +---+ +---+ . 1/ \1
. | |8 8| . / \
. 8| +----------+ |8 . o RT1 o RT2
. +---+10 | Network | 10+---+ . | \
. |RT4|-------| 10.2.0.0 |----|RT3| . 0| \8
. +---+ +----------+ +---+ . | \
. |3 |3 . o 10.1.0.0 o
. | | . | RT3
. +---------+ +-------+. 8|
. | | . |
. +--+ +--+ . o
. |Ma| |Ma| . RT4
. +--+ Area 1 +--+ .
Figure 15: The effect of areas
C.3 The effect of virtual links
In Figure 16 below, Network 10.1.0.0 has been configured as a
separate area (Area 1), while everything else belongs to the OSPF
backbone (Area 0). In addition, a virtual link has been configured
through Area 1, enhancing the backbone connectivity. In this case,
both the source and the group members belong to the same area, so
forward costs are used. However, since virtual links are preferred
over regular links (see Step 5c of Section 12.2), the backbone
datagram shortest-path tree uses Network 10.1.0.0 instead of
10.2.0.0 on the path to the left group member. This leads to the
tree on the right of Figure 16.
Net 188.8.131.52 |
. +----------+ . /1 |
. | Network |8. / |1
. | 10.1.0.0 |-+---+ +---+ o 184.108.40.206
. +----------+*|RT1| |RT2| |
. 8|*******+---+ +---+ 0|
.Area1 |*VL . \8 8| |
.....+---+...... +----------+ |8 o RT1
|RT4|10 | Network | 10+---+ / \
+---+-------| 10.2.0.0 |----|RT3| /8 \8
| +----------+ +---+ / \
|3 |3 o 10.1 o 10.2.0.0
| | | |
+---------+ +-------+ |0 |0
| | | |
+--+ +--+ o o
|Ma| |Ma| RT4 RT3
Figure 16: The effect of virtual links