DD sequence number. Neighbor conversations in this state or
greater are called adjacencies.
Exchange
In this state the router is describing its entire link state
database by sending Database Description packets to the
neighbor. Each Database Description Packet has a DD
sequence number, and is explicitly acknowledged. Only one
Database Description Packet is allowed outstanding at any
one time. In this state, Link State Request Packets may
also be sent asking for the neighbor's more recent LSAs.
All adjacencies in Exchange state or greater are used by the
flooding procedure. In fact, these adjacencies are fully
capable of transmitting and receiving all types of OSPF
routing protocol packets.
Loading
In this state, Link State Request packets are sent to the
neighbor asking for the more recent LSAs that have been
discovered (but not yet received) in the Exchange state.
Full
In this state, the neighboring routers are fully adjacent.
These adjacencies will now appear in router-LSAs and
network-LSAs.
10.2. Events causing neighbor state changes
State changes can be effected by a number of events. These
events are shown in the labels of the arcs in Figures 12 and 13.
The label definitions are as follows:
HelloReceived
An Hello packet has been received from the neighbor.
Start
This is an indication that Hello Packets should now be sent
to the neighbor at intervals of HelloInterval seconds. This
event is generated only for neighbors associated with NBMA
networks.
2-WayReceived
Bidirectional communication has been realized between the
two neighboring routers. This is indicated by the router
seeing itself in the neighbor's Hello packet.
NegotiationDone
The Master/Slave relationship has been negotiated, and DD
sequence numbers have been exchanged. This signals the
start of the sending/receiving of Database Description
packets. For more information on the generation of this
event, consult Section 10.8.
ExchangeDone
Both routers have successfully transmitted a full sequence
of Database Description packets. Each router now knows what
parts of its link state database are out of date. For more
information on the generation of this event, consult Section
10.8.
BadLSReq
A Link State Request has been received for an LSA not
contained in the database. This indicates an error in the
Database Exchange process.
Loading Done
Link State Updates have been received for all out-of-date
portions of the database. This is indicated by the Link
state request list becoming empty after the Database
Exchange process has completed.
AdjOK?
A decision must be made as to whether an adjacency should be
established/maintained with the neighbor. This event will
start some adjacencies forming, and destroy others.
The following events cause well developed neighbors to revert to
lesser states. Unlike the above events, these events may occur
when the neighbor conversation is in any of a number of states.
SeqNumberMismatch
A Database Description packet has been received that either
a) has an unexpected DD sequence number, b) unexpectedly has
the Init bit set or c) has an Options field differing from
the last Options field received in a Database Description
packet. Any of these conditions indicate that some error
has occurred during adjacency establishment.
1-Way
An Hello packet has been received from the neighbor, in
which the router is not mentioned. This indicates that
communication with the neighbor is not bidirectional.
KillNbr
This is an indication that all communication with the
neighbor is now impossible, forcing the neighbor to
revert to Down state.
InactivityTimer
The inactivity Timer has fired. This means that no Hello
packets have been seen recently from the neighbor. The
neighbor reverts to Down state.
LLDown
This is an indication from the lower level protocols that
the neighbor is now unreachable. For example, on an X.25
network this could be indicated by an X.25 clear indication
with appropriate cause and diagnostic fields. This event
forces the neighbor into Down state.
10.3. The Neighbor state machine
A detailed description of the neighbor state changes follows.
Each state change is invoked by an event (Section 10.2). This
event may produce different effects, depending on the current
state of the neighbor. For this reason, the state machine below
is organized by current neighbor state and received event. Each
entry in the state machine describes the resulting new neighbor
state and the required set of additional actions.
When a neighbor's state changes, it may be necessary to rerun
the Designated Router election algorithm. This is determined by
whether the interface NeighborChange event is generated (see
Section 9.2). Also, if the Interface is in DR state (the router
is itself Designated Router), changes in neighbor state may
cause a new network-LSA to be originated (see Section 12.4).
When the neighbor state machine needs to invoke the interface
state machine, it should be done as a scheduled task (see
Section 4.4). This simplifies things, by ensuring that neither
state machine will be executed recursively.
State(s): Down
Event: Start
New state: Attempt
Action: Send an Hello Packet to the neighbor (this neighbor
is always associated with an NBMA network) and start
the Inactivity Timer for the neighbor. The timer's
later firing would indicate that communication with
the neighbor was not attained.
State(s): Attempt
Event: HelloReceived
New state: Init
Action: Restart the Inactivity Timer for the neighbor, since
the neighbor has now been heard from.
State(s): Down
Event: HelloReceived
New state: Init
Action: Start the Inactivity Timer for the neighbor. The
timer's later firing would indicate that the
neighbor is dead.
State(s): Init or greater
Event: HelloReceived
New state: No state change.
Action: Restart the Inactivity Timer for the neighbor, since
the neighbor has again been heard from.
State(s): Init
Event: 2-WayReceived
New state: Depends upon action routine.
Action: Determine whether an adjacency should be established
with the neighbor (see Section 10.4). If not, the
new neighbor state is 2-Way.
Otherwise (an adjacency should be established) the
neighbor state transitions to ExStart. Upon
entering this state, the router increments the DD
sequence number in the neighbor data structure. If
this is the first time that an adjacency has been
attempted, the DD sequence number should be assigned
some unique value (like the time of day clock). It
then declares itself master (sets the master/slave
bit to master), and starts sending Database
Description Packets, with the initialize (I), more
(M) and master (MS) bits set. This Database
Description Packet should be otherwise empty. This
Database Description Packet should be retransmitted
at intervals of RxmtInterval until the next state is
entered (see Section 10.8).
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 and summary-LSAs contained
in the area structure, along with the AS-external-
LSAs contained in the global structure. AS-
external-LSAs are omitted from a virtual neighbor's
Database summary list. AS-external-LSAs are omitted
from the Database summary list if the area has been
configured as a stub (see Section 3.6). LSAs whose
age is equal to MaxAge are 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 outstanding
at any one time. For more detail on the sending and
receiving of Database Description packets, see
Sections 10.8 and 10.6.
State(s): Exchange
Event: ExchangeDone
New state: Depends upon action routine.
Action: If the neighbor Link state request list is empty,
the new neighbor state is Full. No other action is
required. This is an adjacency's final state.
Otherwise, the new neighbor state is Loading. Start
(or continue) sending Link State Request packets to
the neighbor (see Section 10.9). These are requests
for the neighbor's more recent LSAs (which were
discovered but not yet received in the Exchange
state). These LSAs are listed in the Link state
request list associated with the neighbor.
State(s): Loading
Event: Loading Done
New state: Full
Action: No action required. This is an adjacency's final
state.
State(s): 2-Way
Event: AdjOK?
New state: Depends upon action routine.
Action: Determine whether an adjacency should be formed with
the neighboring router (see Section 10.4). If not,
the neighbor state remains at 2-Way. Otherwise,
transition the neighbor state to ExStart and perform
the actions associated with the above state machine
entry for state Init and event 2-WayReceived.
State(s): ExStart or greater
Event: AdjOK?
New state: Depends upon action routine.
Action: Determine whether the neighboring router should
still be adjacent. If yes, there is no state change
and no further action is necessary.
Otherwise, the (possibly partially formed) adjacency
must be destroyed. The neighbor state transitions
to 2-Way. The Link state retransmission list,
Database summary list and Link state request list
are cleared of LSAs.
State(s): Exchange or greater
Event: SeqNumberMismatch
New state: ExStart
Action: The (possibly partially formed) adjacency is torn
down, and then an attempt is made at
reestablishment. The neighbor state first
transitions to ExStart. The Link state
retransmission list, Database summary list and Link
state request list are cleared of LSAs. Then the
router increments the DD sequence number in the
neighbor data structure, declares itself master
(sets the master/slave bit to master), and starts
sending Database Description Packets, with the
initialize (I), more (M) and master (MS) bits set.
This Database Description Packet should be otherwise
empty (see Section 10.8).
State(s): Exchange or greater
Event: BadLSReq
New state: ExStart
Action: The action for event BadLSReq is exactly the same as
for the neighbor event SeqNumberMismatch. The
(possibly partially formed) adjacency is torn down,
and then an attempt is made at reestablishment. For
more information, see the neighbor state machine
entry that is invoked when event SeqNumberMismatch
is generated in state Exchange or greater.
State(s): Any state
Event: KillNbr
New state: Down
Action: The Link state retransmission list, Database summary
list and Link state request list are cleared of
LSAs. Also, the Inactivity Timer is disabled.
State(s): Any state
Event: LLDown
New state: Down
Action: The Link state retransmission list, Database summary
list and Link state request list are cleared of
LSAs. Also, the Inactivity Timer is disabled.
State(s): Any state
Event: InactivityTimer
New state: Down
Action: The Link state retransmission list, Database summary
list and Link state request list are cleared of
LSAs.
State(s): 2-Way or greater
Event: 1-WayReceived
New state: Init
Action: The Link state retransmission list, Database summary
list and Link state request list are cleared of
LSAs.
State(s): 2-Way or greater
Event: 2-WayReceived
New state: No state change.
Action: No action required.
State(s): Init
Event: 1-WayReceived
New state: No state change.
Action: No action required.
10.4. Whether to become adjacent
Adjacencies are established with some subset of the router's
neighbors. Routers connected by point-to-point networks,
Point-to-MultiPoint networks and virtual links always become
adjacent. On broadcast and NBMA networks, all routers become
adjacent to both the Designated Router and the Backup Designated
Router.
The adjacency-forming decision occurs in two places in the
neighbor state machine. First, when bidirectional communication
is initially established with the neighbor, and secondly, when
the identity of the attached network's (Backup) Designated
Router changes. If the decision is made to not attempt an
adjacency, the state of the neighbor communication stops at 2-
Way.
An adjacency should be established with a bidirectional neighbor
when at least one of the following conditions holds:
o The underlying network type is point-to-point
o The underlying network type is Point-to-MultiPoint
o The underlying network type is virtual link
o The router itself is the Designated Router
o The router itself is the Backup Designated Router
o The neighboring router is the Designated Router
o The neighboring router is the Backup Designated Router
10.5. Receiving Hello Packets
This section explains the detailed processing of a received
Hello Packet. (See Section A.3.2 for the format of Hello
packets.) The generic input processing of OSPF packets will
have checked the validity of the IP header and the OSPF packet
header. Next, the values of the Network Mask, HelloInterval,
and RouterDeadInterval fields in the received Hello packet must
be checked against the values configured for the receiving
interface. Any mismatch causes processing to stop and the
packet to be dropped. In other words, the above fields are
really describing the attached network's configuration. However,
there is one exception to the above rule: on point-to-point
networks and on virtual links, the Network Mask in the received
Hello Packet should be ignored.
The receiving interface attaches to a single OSPF area (this
could be the backbone). The setting of the E-bit found in the
Hello Packet's Options field must match this area's
ExternalRoutingCapability. If AS-external-LSAs are not flooded
into/throughout the area (i.e, the area is a "stub") the E-bit
must be clear in received Hello Packets, otherwise the E-bit
must be set. A mismatch causes processing to stop and the
packet to be dropped. The setting of the rest of the bits in
the Hello Packet's Options field should be ignored.
At this point, an attempt is made to match the source of the
Hello Packet to one of the receiving interface's neighbors. If
the receiving interface connects to a broadcast, Point-to-
MultiPoint or NBMA network the source is identified by the IP
source address found in the Hello's IP header. If the receiving
interface connects to a point-to-point link or a virtual link,
the source is identified by the Router ID found in the Hello's
OSPF packet header. The interface's current list of neighbors
is contained in the interface's data structure. If a matching
neighbor structure cannot be found, (i.e., this is the first
time the neighbor has been detected), one is created. The
initial state of a newly created neighbor is set to Down.
When receiving an Hello Packet from a neighbor on a broadcast,
Point-to-MultiPoint or NBMA network, set the neighbor
structure's Neighbor ID equal to the Router ID found in the
packet's OSPF header. For these network types, the neighbor
structure's Router Priority field, Neighbor's Designated Router
field, and Neighbor's Backup Designated Router field are also
set equal to the corresponding fields found in the received
Hello Packet; changes in these fields should be noted for
possible use in the steps below. When receiving an Hello on a
point-to-point network (but not on a virtual link) set the
neighbor structure's Neighbor IP address to the packet's IP
source address.
Now the rest of the Hello Packet is examined, generating events
to be given to the neighbor and interface state machines. These
state machines are specified either to be executed or scheduled
(see Section 4.4). For example, by specifying below that the
neighbor state machine be executed in line, several neighbor
state transitions may be effected by a single received Hello:
o Each Hello Packet causes the neighbor state machine to be
executed with the event HelloReceived.
o Then the list of neighbors contained in the Hello Packet is
examined. If the router itself appears in this list, the
neighbor state machine should be executed with the event 2-
WayReceived. Otherwise, the neighbor state machine should
be executed with the event 1-WayReceived, and the processing
of the packet stops.
o Next, if a change in the neighbor's Router Priority field
was noted, the receiving interface's state machine is
scheduled with the event NeighborChange.
o If the neighbor is both declaring itself to be Designated
Router (Hello Packet's Designated Router field = Neighbor IP
address) and the Backup Designated Router field in the
packet is equal to 0.0.0.0 and the receiving interface is in
state Waiting, the receiving interface's state machine is
scheduled with the event BackupSeen. Otherwise, if the
neighbor is declaring itself to be Designated Router and it
had not previously, or the neighbor is not declaring itself
Designated Router where it had previously, the receiving
interface's state machine is scheduled with the event
NeighborChange.
o If the neighbor is declaring itself to be Backup Designated
Router (Hello Packet's Backup Designated Router field =
Neighbor IP address) and the receiving interface is in state
Waiting, the receiving interface's state machine is
scheduled with the event BackupSeen. Otherwise, if the
neighbor is declaring itself to be Backup Designated Router
and it had not previously, or the neighbor is not declaring
itself Backup Designated Router where it had previously, the
receiving interface's state machine is scheduled with the
event NeighborChange.
On NBMA networks, receipt of an Hello Packet may also cause an
Hello Packet to be sent back to the neighbor in response. See
Section 9.5.1 for more details.
10.6. Receiving Database Description Packets
This section explains the detailed processing of a received
Database Description Packet. The incoming Database Description
Packet has already been associated with a neighbor and receiving
interface by the generic input packet processing (Section 8.2).
Whether the Database Description packet should be accepted, and
if so, how it should be further processed depends upon the
neighbor state.
If a Database Description packet is accepted, the following
packet fields should be saved in the corresponding neighbor data
structure under "last received Database Description packet":
the packet's initialize(I), more (M) and master(MS) bits,
Options field, and DD sequence number. If these fields are set
identically in two consecutive Database Description packets
received from the neighbor, the second Database Description
packet is considered to be a "duplicate" in the processing
described below.
If the Interface MTU field in the Database Description packet
indicates an IP datagram size that is larger than the router can
accept on the receiving interface without fragmentation, the
Database Description packet is rejected. Otherwise, if the
neighbor state is:
Down
The packet should be rejected.
Attempt
The packet should be rejected.
Init
The neighbor state machine should be executed with the event
2-WayReceived. This causes an immediate state change to
either state 2-Way or state ExStart. If the new state is
ExStart, the processing of the current packet should then
continue in this new state by falling through to case
ExStart below.
2-Way
The packet should be ignored. Database Description Packets
are used only for the purpose of bringing up adjacencies.[7]
ExStart
If the received packet matches one of the following cases,
then the neighbor state machine should be executed with the
event NegotiationDone (causing the state to transition to
Exchange), the packet's Options field should be recorded in
the neighbor structure's Neighbor Options field and the
packet should be accepted as next in sequence and processed
further (see below). Otherwise, the packet should be
ignored.
o The initialize(I), more (M) and master(MS) bits are set,
the contents of the packet are empty, and the neighbor's
Router ID is larger than the router's own. In this case
the router is now Slave. Set the master/slave bit to
slave, and set the neighbor data structure's DD sequence
number to that specified by the master.
o The initialize(I) and master(MS) bits are off, the
packet's DD sequence number equals the neighbor data
structure's DD sequence number (indicating
acknowledgment) and the neighbor's Router ID is smaller
than the router's own. In this case the router is
Master.
Exchange
Duplicate Database Description packets are discarded by the
master, and cause the slave to retransmit the last Database
Description packet that it had sent. Otherwise (the packet
is not a duplicate):
o If the state of the MS-bit is inconsistent with the
master/slave state of the connection, generate the
neighbor event SeqNumberMismatch and stop processing the
packet.
o If the initialize(I) bit is set, generate the neighbor
event SeqNumberMismatch and stop processing the packet.
o If the packet's Options field indicates a different set
of optional OSPF capabilities than were previously
received from the neighbor (recorded in the Neighbor
Options field of the neighbor structure), generate the
neighbor event SeqNumberMismatch and stop processing the
packet.
o Database Description packets must be processed in
sequence, as indicated by the packets' DD sequence
numbers. If the router is master, the next packet
received should have DD sequence number equal to the DD
sequence number in the neighbor data structure. If the
router is slave, the next packet received should have DD
sequence number equal to one more than the DD sequence
number stored in the neighbor data structure. In either
case, if the packet is the next in sequence it should be
accepted and its contents processed as specified below.
o Else, generate the neighbor event SeqNumberMismatch and
stop processing the packet.
Loading or Full
In this state, the router has sent and received an entire
sequence of Database Description Packets. The only packets
received should be duplicates (see above). In particular,
the packet's Options field should match the set of optional
OSPF capabilities previously indicated by the neighbor
(stored in the neighbor structure's Neighbor Options field).
Any other packets received, including the reception of a
packet with the Initialize(I) bit set, should generate the
neighbor event SeqNumberMismatch.[8] Duplicates should be
discarded by the master. The slave must respond to
duplicates by repeating the last Database Description packet
that it had sent.
When the router accepts a received Database Description Packet
as the next in sequence the packet contents are processed as
follows. For each LSA listed, the LSA's LS type is checked for
validity. If the LS type is unknown (e.g., not one of the LS
types 1-5 defined by this specification), or if this is an AS-
external-LSA (LS type = 5) and the neighbor is associated with a
stub area, generate the neighbor event SeqNumberMismatch and
stop processing the packet. Otherwise, the router looks up the
LSA in its database to see whether it also has an instance of
the LSA. If it does not, or if the database copy is less recent
(see Section 13.1), the LSA is put on the Link state request
list so that it can be requested (immediately or at some later
time) in Link State Request Packets.
When the router accepts a received Database Description Packet
as the next in sequence, it also performs the following actions,
depending on whether it is master or slave:
Master
Increments the DD sequence number in the neighbor data
structure. If the router has already sent its entire
sequence of Database Description Packets, and the just
accepted packet has the more bit (M) set to 0, the neighbor
event ExchangeDone is generated. Otherwise, it should send
a new Database Description to the slave.
Slave
Sets the DD sequence number in the neighbor data structure
to the DD sequence number appearing in the received packet.
The slave must send a Database Description Packet in reply.
If the received packet has the more bit (M) set to 0, and
the packet to be sent by the slave will also have the M-bit
set to 0, the neighbor event ExchangeDone is generated.
Note that the slave always generates this event before the
master.
10.7. Receiving Link State Request Packets
This section explains the detailed processing of received Link
State Request packets. Received Link State Request Packets
specify a list of LSAs that the neighbor wishes to receive.
Link State Request Packets should be accepted when the neighbor
is in states Exchange, Loading, or Full. In all other states
Link State Request Packets should be ignored.
Each LSA specified in the Link State Request packet should be
located in the router's database, and copied into Link State
Update packets for transmission to the neighbor. These LSAs
should NOT be placed on the Link state retransmission list for
the neighbor. If an LSA cannot be found in the database,
something has gone wrong with the Database Exchange process, and
neighbor event BadLSReq should be generated.
10.8. Sending Database Description Packets
This section describes how Database Description Packets are sent
to a neighbor. The Database Description packet's Interface MTU
field is set to the size of the largest IP datagram that can be
sent out the sending interface, without fragmentation. Common
MTUs in use in the Internet can be found in Table 7-1 of
[Ref22]. Interface MTU should be set to 0 in Database
Description packets sent over virtual links.
The router's optional OSPF capabilities (see Section 4.5) are
transmitted to the neighbor in the Options field of the Database
Description packet. The router should maintain the same set of
optional capabilities throughout the Database Exchange and
flooding procedures. If for some reason the router's optional
capabilities change, the Database Exchange procedure should be
restarted by reverting to neighbor state ExStart. One optional
capability is defined in this specification (see Sections 4.5
and A.2). The E-bit should be set if and only if the attached
network belongs to a non-stub area. Unrecognized bits in the
Options field should be set to zero.
The sending of Database Description packets depends on the
neighbor's state. In state ExStart the router sends empty
Database Description packets, with the initialize (I), more (M)
and master (MS) bits set. These packets are retransmitted every
RxmtInterval seconds.
In state Exchange the Database Description Packets actually
contain summaries of the link state information contained in the
router's database. Each LSA in the area's link-state database
(at the time the neighbor transitions into Exchange state) is
listed in the neighbor Database summary list. Each new Database
Description Packet copies its DD sequence number from the
neighbor data structure and then describes the current top of
the Database summary list. Items are removed from the Database
summary list when the previous packet is acknowledged.
In state Exchange, the determination of when to send a Database
Description packet depends on whether the router is master or
slave:
Master
Database Description packets are sent when either a) the
slave acknowledges the previous Database Description packet
by echoing the DD sequence number or b) RxmtInterval seconds
elapse without an acknowledgment, in which case the previous
Database Description packet is retransmitted.
Slave
Database Description packets are sent only in response to
Database Description packets received from the master. If
the Database Description packet received from the master is
new, a new Database Description packet is sent, otherwise
the previous Database Description packet is resent.
In states Loading and Full the slave must resend its last
Database Description packet in response to duplicate Database
Description packets received from the master. For this reason
the slave must wait RouterDeadInterval seconds before freeing
the last Database Description packet. Reception of a Database
Description packet from the master after this interval will
generate a SeqNumberMismatch neighbor event.
10.9. Sending Link State Request Packets
In neighbor states Exchange or Loading, the Link state request
list contains a list of those LSAs that need to be obtained from
the neighbor. To request these LSAs, a router sends the
neighbor the beginning of the Link state request list, packaged
in a Link State Request packet.
When the neighbor responds to these requests with the proper
Link State Update packet(s), the Link state request list is
truncated and a new Link State Request packet is sent. This
process continues until the Link state request list becomes
empty. LSAs on the Link state request list that have been
requested, but not yet received, are packaged into Link State
Request packets for retransmission at intervals of RxmtInterval.
There should be at most one Link State Request packet
outstanding at any one time.
When the Link state request list becomes empty, and the neighbor
state is Loading (i.e., a complete sequence of Database
Description packets has been sent to and received from the
neighbor), the Loading Done neighbor event is generated.
10.10. An Example
Figure 14 shows an example of an adjacency forming. Routers RT1
and RT2 are both connected to a broadcast network. It is
assumed that RT2 is the Designated Router for the network, and
that RT2 has a higher Router ID than Router RT1.
The neighbor state changes realized by each router are listed on
the sides of the figure.
At the beginning of Figure 14, Router RT1's interface to the
network becomes operational. It begins sending Hello Packets,
although it doesn't know the identity of the Designated Router
or of any other neighboring routers. Router RT2 hears this
hello (moving the neighbor to Init state), and in its next Hello
Packet indicates that it is itself the Designated Router and
that it has heard Hello Packets from RT1. This in turn causes
RT1 to go to state ExStart, as it starts to bring up the
adjacency.
RT1 begins by asserting itself as the master. When it sees that
RT2 is indeed the master (because of RT2's higher Router ID),
RT1 transitions to slave state and adopts its neighbor's DD
sequence number. Database Description packets are then
exchanged, with polls coming from the master (RT2) and responses
from the slave (RT1). This sequence of Database Description
+---+ +---+
|RT1| |RT2|
+---+ +---+
Down Down
Hello(DR=0,seen=0)
------------------------------>
Hello (DR=RT2,seen=RT1,...) Init
<------------------------------
ExStart D-D (Seq=x,I,M,Master)
------------------------------>
D-D (Seq=y,I,M,Master) ExStart
<------------------------------
Exchange D-D (Seq=y,M,Slave)
------------------------------>
D-D (Seq=y+1,M,Master) Exchange
<------------------------------
D-D (Seq=y+1,M,Slave)
------------------------------>
...
...
...
D-D (Seq=y+n, Master)
<------------------------------
D-D (Seq=y+n, Slave)
Loading ------------------------------>
LS Request Full
------------------------------>
LS Update
<------------------------------
LS Request
------------------------------>
LS Update
<------------------------------
Full
Figure 14: An adjacency bring-up example
Packets ends when both the poll and associated response has the
M-bit off.
In this example, it is assumed that RT2 has a completely up to
date database. In that case, RT2 goes immediately into Full
state. RT1 will go into Full state after updating the necessary
parts of its database. This is done by sending Link State
Request Packets, and receiving Link State Update Packets in
response. Note that, while RT1 has waited until a complete set
of Database Description Packets has been received (from RT2)
before sending any Link State Request Packets, this need not be
the case. RT1 could have interleaved the sending of Link State
Request Packets with the reception of Database Description
Packets.
11. The Routing Table Structure
The routing table data structure contains all the information
necessary to forward an IP data packet toward its destination. Each
routing table entry describes the collection of best paths to a
particular destination. When forwarding an IP data packet, the
routing table entry providing the best match for the packet's IP
destination is located. The matching routing table entry then
provides the next hop towards the packet's destination. OSPF also
provides for the existence of a default route (Destination ID =
DefaultDestination, Address Mask = 0x00000000). When the default
route exists, it matches all IP destinations (although any other
matching entry is a better match). Finding the routing table entry
that best matches an IP destination is further described in Section
11.1.
There is a single routing table in each router. Two sample routing
tables are described in Sections 11.2 and 11.3. The building of the
routing table is discussed in Section 16.
The rest of this section defines the fields found in a routing table
entry. The first set of fields describes the routing table entry's
destination.
Destination Type
Destination type is either "network" or "router". Only network
entries are actually used when forwarding IP data traffic.
Router routing table entries are used solely as intermediate
steps in the routing table build process.
A network is a range of IP addresses, to which IP data traffic
may be forwarded. This includes IP networks (class A, B, or C),
IP subnets, IP supernets and single IP hosts. The default route
also falls into this category.
Router entries are kept for area border routers and AS boundary
routers. Routing table entries for area border routers are used
when calculating the inter-area routes (see Section 16.2), and
when maintaining configured virtual links (see Section 15).
Routing table entries for AS boundary routers are used when
calculating the AS external routes (see Section 16.4).
Destination ID
The destination's identifier or name. This depends on the
Destination Type. For networks, the identifier is their
associated IP address. For routers, the identifier is the OSPF
Router ID.[9]
Address Mask
Only defined for networks. The network's IP address together
with its address mask defines a range of IP addresses. For IP
subnets, the address mask is referred to as the subnet mask.
For host routes, the mask is "all ones" (0xffffffff).
Optional Capabilities
When the destination is a router this field indicates the
optional OSPF capabilities supported by the destination router.
The only optional capability defined by this specification is
the ability to process AS-external-LSAs. For a further
discussion of OSPF's optional capabilities, see Section 4.5.
The set of paths to use for a destination may vary based on the OSPF
area to which the paths belong. This means that there may be
multiple routing table entries for the same destination, depending
on the values of the next field.
Area
This field indicates the area whose link state information has
led to the routing table entry's collection of paths. This is
called the entry's associated area. For sets of AS external
paths, this field is not defined. For destinations of type
"router", there may be separate sets of paths (and therefore
separate routing table entries) associated with each of several
areas. For example, this will happen when two area border
routers share multiple areas in common. For destinations of
type "network", only the set of paths associated with the best
area (the one providing the preferred route) is kept.
The rest of the routing table entry describes the set of paths to
the destination. The following fields pertain to the set of paths
as a whole. In other words, each one of the paths contained in a
routing table entry is of the same path-type and cost (see below).
Path-type
There are four possible types of paths used to route traffic to
the destination, listed here in decreasing order of preference:
intra-area, inter-area, type 1 external or type 2 external.
Intra-area paths indicate destinations belonging to one of the
router's attached areas. Inter-area paths are paths to
destinations in other OSPF areas. These are discovered through
the examination of received summary-LSAs. AS external paths are
paths to destinations external to the AS. These are detected
through the examination of received AS-external-LSAs.
Cost
The link state cost of the path to the destination. For all
paths except type 2 external paths this describes the entire
path's cost. For Type 2 external paths, this field describes
the cost of the portion of the path internal to the AS. This
cost is calculated as the sum of the costs of the path's
constituent links.
Type 2 cost
Only valid for type 2 external paths. For these paths, this
field indicates the cost of the path's external portion. This
cost has been advertised by an AS boundary router, and is the
most significant part of the total path cost. For example, a
type 2 external path with type 2 cost of 5 is always preferred
over a path with type 2 cost of 10, regardless of the cost of
the two paths' internal components.
Link State Origin
Valid only for intra-area paths, this field indicates the LSA
(router-LSA or network-LSA) that directly references the
destination. For example, if the destination is a transit
network, this is the transit network's network-LSA. If the
destination is a stub network, this is the router-LSA for the
attached router. The LSA is discovered during the shortest-path
tree calculation (see Section 16.1). Multiple LSAs may
reference the destination, however a tie-breaking scheme always
reduces the choice to a single LSA. The Link State Origin field
is not used by the OSPF protocol, but it is used by the routing
table calculation in OSPF's Multicast routing extensions
(MOSPF).
When multiple paths of equal path-type and cost exist to a
destination (called elsewhere "equal-cost" paths), they are stored
in a single routing table entry. Each one of the "equal-cost" paths
is distinguished by the following fields:
Next hop
The outgoing router interface to use when forwarding traffic to
the destination. On broadcast, Point-to-MultiPoint and NBMA
networks, the next hop also includes the IP address of the next
router (if any) in the path towards the destination.
Advertising router
Valid only for inter-area and AS external paths. This field
indicates the Router ID of the router advertising the summary-
LSA or AS-external-LSA that led to this path.
11.1. Routing table lookup
When an IP data packet is received, an OSPF router finds the
routing table entry that best matches the packet's destination.
This routing table entry then provides the outgoing interface
and next hop router to use in forwarding the packet. This
section describes the process of finding the best matching
routing table entry.
Before the lookup begins, "discard" routing table entries should
be inserted into the routing table for each of the router's
active area address ranges (see Section 3.5). (An area range is
considered "active" if the range contains one or more networks
reachable by intra-area paths.) The destination of a "discard"
entry is the set of addresses described by its associated active
area address range, and the path type of each "discard" entry is
set to "inter-area".[10]
Several routing table entries may match the destination address.
In this case, the "best match" is the routing table entry that
provides the most specific (longest) match. Another way of
saying this is to choose the entry that specifies the narrowest
range of IP addresses.[11] For example, the entry for the
address/mask pair of (128.185.1.0, 0xffffff00) is more specific
than an entry for the pair (128.185.0.0, 0xffff0000). The
default route is the least specific match, since it matches all
destinations. (Note that for any single routing table entry,
multiple paths may be possible. In these cases, the calculations
in Sections 16.1, 16.2, and 16.4 always yield the paths having
the most preferential path-type, as described in Section 11).
If there is no matching routing table entry, or the best match
routing table entry is one of the above "discard" routing table
entries, then the packet's IP destination is considered
unreachable. Instead of being forwarded, the packet should then
be discarded and an ICMP destination unreachable message should
be returned to the packet's source.
11.2. Sample routing table, without areas
Consider the Autonomous System pictured in Figure 2. No OSPF
areas have been configured. A single metric is shown per
outbound interface. The calculation of Router RT6's routing
table proceeds as described in Section 2.2. The resulting
routing table is shown in Table 12. Destination types are
abbreviated: Network as "N", Router as "R".
There are no instances of multiple equal-cost shortest paths in
this example. Also, since there are no areas, there are no
inter-area paths.
Routers RT5 and RT7 are AS boundary routers. Intra-area routes
have been calculated to Routers RT5 and RT7. This allows
external routes to be calculated to the destinations advertised
by RT5 and RT7 (i.e., Networks N12, N13, N14 and N15). It is
assumed all AS-external-LSAs originated by RT5 and RT7 are
advertising type 1 external metrics. This results in type 1
external paths being calculated to destinations N12-N15.
11.3. Sample routing table, with areas
Consider the previous example, this time split into OSPF areas.
An OSPF area configuration is pictured in Figure 6. Router
RT4's routing table will be described for this area
configuration. Router RT4 has a connection to Area 1 and a
backbone connection. This causes Router RT4 to view the AS as
the concatenation of the two graphs shown in Figures 7 and 8.
The resulting routing table is displayed in Table 13.
Again, Routers RT5 and RT7 are AS boundary routers. Routers
RT3, RT4, RT7, RT10 and RT11 are area border routers. Note that
there are two routing entries for the area border router RT3,
since it has two areas in common with RT4 (Area 1 and the
backbone).
Backbone paths have been calculated to all area border routers.
These are used when determining the inter-area routes. Note
that all of the inter-area routes are associated with the
backbone; this is always the case when the calculating router is
itself an area border router. Routing information is condensed
at area boundaries. In this example, we assume that Area 3 has
been defined so that networks N9-N11 and the host route to H1
Type Dest Area Path Type Cost Next Adv.
Hop(s) Router(s)
____________________________________________________________
N N1 0 intra-area 10 RT3 *
N N2 0 intra-area 10 RT3 *
N N3 0 intra-area 7 RT3 *
N N4 0 intra-area 8 RT3 *
N Ib 0 intra-area 7 * *
N Ia 0 intra-area 12 RT10 *
N N6 0 intra-area 8 RT10 *
N N7 0 intra-area 12 RT10 *
N N8 0 intra-area 10 RT10 *
N N9 0 intra-area 11 RT10 *
N N10 0 intra-area 13 RT10 *
N N11 0 intra-area 14 RT10 *
N H1 0 intra-area 21 RT10 *
R RT5 0 intra-area 6 RT5 *
R RT7 0 intra-area 8 RT10 *
____________________________________________________________
N N12 * type 1 ext. 10 RT10 RT7
N N13 * type 1 ext. 14 RT5 RT5
N N14 * type 1 ext. 14 RT5 RT5
N N15 * type 1 ext. 17 RT10 RT7
Table 12: The routing table for Router RT6
(no configured areas).
are all condensed to a single route when advertised into the
backbone (by Router RT11). Note that the cost of this route is
the maximum of the set of costs to its individual components.
There is a virtual link configured between Routers RT10 and
RT11. Without this configured virtual link, RT11 would be
unable to advertise a route for networks N9-N11 and Host H1 into
the backbone, and there would not be an entry for these networks
in Router RT4's routing table.
In this example there are two equal-cost paths to Network N12.
However, they both use the same next hop (Router RT5).
Router RT4's routing table would improve (i.e., some of the
paths in the routing table would become shorter) if an
additional virtual link were configured between Router RT4 and
Router RT3. The new virtual link would itself be associated
with the first entry for area border router RT3 in Table 13 (an
intra-area path through Area 1). This would yield a cost of 1
for the virtual link. The routing table entries changes that
would be caused by the addition of this virtual link are shown
Type Dest Area Path Type Cost Next Adv.
Hops(s) Router(s)
__________________________________________________________________
N N1 1 intra-area 4 RT1 *
N N2 1 intra-area 4 RT2 *
N N3 1 intra-area 1 * *
N N4 1 intra-area 3 RT3 *
R RT3 1 intra-area 1 * *
__________________________________________________________________
N Ib 0 intra-area 22 RT5 *
N Ia 0 intra-area 27 RT5 *
R RT3 0 intra-area 21 RT5 *
R RT5 0 intra-area 8 * *
R RT7 0 intra-area 14 RT5 *
R RT10 0 intra-area 22 RT5 *
R RT11 0 intra-area 25 RT5 *
__________________________________________________________________
N N6 0 inter-area 15 RT5 RT7
N N7 0 inter-area 19 RT5 RT7
N N8 0 inter-area 18 RT5 RT7
N N9-N11,H1 0 inter-area 36 RT5 RT11
__________________________________________________________________
N N12 * type 1 ext. 16 RT5 RT5,RT7
N N13 * type 1 ext. 16 RT5 RT5
N N14 * type 1 ext. 16 RT5 RT5
N N15 * type 1 ext. 23 RT5 RT7
Table 13: Router RT4's routing table
in the presence of areas.
in Table 14.
12. Link State Advertisements (LSAs)
Each router in the Autonomous System originates one or more link
state advertisements (LSAs). This memo defines five distinct types
of LSAs, which are described in Section 4.3. The collection of LSAs
forms the link-state database. Each separate type of LSA has a
separate function. Router-LSAs and network-LSAs describe how an
area's routers and networks are interconnected. Summary-LSAs
provide a way of condensing an area's routing information. AS-
external-LSAs provide a way of transparently advertising
externally-derived routing information throughout the Autonomous
System.
Each LSA begins with a standard 20-byte header. This LSA header is
discussed below.
Type Dest Area Path Type Cost Next Adv.
Hop(s) Router(s)
________________________________________________________________
N Ib 0 intra-area 16 RT3 *
N Ia 0 intra-area 21 RT3 *
R RT3 0 intra-area 1 * *
R RT10 0 intra-area 16 RT3 *
R RT11 0 intra-area 19 RT3 *
________________________________________________________________
N N9-N11,H1 0 inter-area 30 RT3 RT11
Table 14: Changes resulting from an
additional virtual link.
12.1. The LSA Header
The LSA header contains the LS type, Link State ID and
Advertising Router fields. The combination of these three
fields uniquely identifies the LSA.