9. The Interface Data Structure
-
The Interface Data Structure
An OSPF interface is the connection between a router and a network. We assume a single OSPF interface to each attached network/subnet, although supporting multiple interfaces on a single network is considered in Appendix F. Each interface structure has at most one IP interface address.
An OSPF interface can be considered to belong to the area that contains the attached network. All routing protocol packets originated by the router over this interface are labelled with the interface's Area ID. One or more router adjacencies may develop over an interface. A router's LSAs reflect the state of its interfaces and their associated adjacencies.
The following data items are associated with an interface. Note that a number of these items are actually configuration for the attached network; such items must be the same for all routers connected to the network.
Type The OSPF interface type is either point-to-point, broadcast, NBMA, Point-to-MultiPoint or virtual link.
State The functional level of an interface. State determines whether or not full adjacencies are allowed to form over the interface. State is also reflected in the router's LSAs.
IP interface address The IP address associated with the interface. This appears as the IP source address in all routing protocol packets originated over this interface. Interfaces to unnumbered point-to-point networks do not have an associated IP address.
IP interface mask Also referred to as the subnet mask, this indicates the portion of the IP interface address that identifies the attached network. Masking the IP interface address with the IP interface mask yields the IP network number of the attached network. On point-to-point networks and virtual links, the IP interface mask is not defined. On these networks, the link itself is not assigned an IP network number, and so the addresses of each side of the link are assigned independently, if they are assigned at all.
Area ID The Area ID of the area to which the attached network belongs. All routing protocol packets originating from the interface are labelled with this Area ID.
HelloInterval The length of time, in seconds, between the Hello packets that the router sends on the interface. Advertised in Hello packets sent out this interface.
RouterDeadInterval The number of seconds before the router's neighbors will declare it down, when they stop hearing the router's Hello Packets. Advertised in Hello packets sent out this interface.
InfTransDelay The estimated number of seconds it takes to transmit a Link State Update Packet over this interface. LSAs contained in the Link State Update packet will have their age incremented by this amount before transmission. This value should take into account transmission and propagation delays; it must be greater than zero.
Router Priority An 8-bit unsigned integer. When two routers attached to a network both attempt to become Designated Router, the one with the highest Router Priority takes precedence. A router whose Router Priority is set to 0 is ineligible to become Designated Router on the attached network. Advertised in Hello packets sent out this interface.
Hello Timer An interval timer that causes the interface to send a Hello packet. This timer fires every HelloInterval seconds. Note that on non-broadcast networks a separate Hello packet is sent to each qualified neighbor.
Wait Timer A single shot timer that causes the interface to exit the Waiting state, and as a consequence select a Designated Router on the network. The length of the timer is RouterDeadInterval seconds.
List of neighboring routers The other routers attached to this network. This list is formed by the Hello Protocol. Adjacencies will be formed to some of
these neighbors. The set of adjacent neighbors can be
determined by an examination of all of the neighbors' states.Designated Router The Designated Router selected for the attached network. The Designated Router is selected on all broadcast and NBMA networks by the Hello Protocol. Two pieces of identification are kept for the Designated Router: its Router ID and its IP interface address on the network. The Designated Router advertises link state for the network; this network-LSA is labelled with the Designated Router's IP address. The Designated Router is initialized to 0.0.0.0, which indicates the lack of a Designated Router.
Backup Designated Router The Backup Designated Router is also selected on all broadcast and NBMA networks by the Hello Protocol. All routers on the attached network become adjacent to both the Designated Router and the Backup Designated Router. The Backup Designated Router becomes Designated Router when the current Designated Router fails. The Backup Designated Router is initialized to 0.0.0.0, indicating the lack of a Backup Designated Router.
Interface output cost(s) The cost of sending a data packet on the interface, expressed in the link state metric. This is advertised as the link cost for this interface in the router-LSA. The cost of an interface must be greater than zero.
RxmtInterval The number of seconds between LSA retransmissions, for adjacencies belonging to this interface. Also used when retransmitting Database Description and Link State Request Packets.
AuType The type of authentication used on the attached network/subnet. Authentication types are defined in Appendix D. All OSPF packet exchanges are authenticated. Different authentication schemes may be used on different networks/subnets.
Authentication key This configured data allows the authentication procedure to generate and/or verify OSPF protocol packets. The Authentication key can be configured on a per-interface basis. For example, if the AuType indicates simple password, the Authentication key would be a 64-bit clear password which is inserted into the OSPF packet header. If instead Autype indicates Cryptographic authentication, then the Authentication key is a shared secret which enables the generation/verification of message digests which are appended to the OSPF protocol packets. When Cryptographic authentication is used, multiple simultaneous keys are supported in order to achieve smooth key transition (see Section D.3).
9.1. Interface states
The various states that router interfaces may attain is
documented in this section. The states are listed in order of
progressing functionality. For example, the inoperative state
is listed first, followed by a list of intermediate states
before the final, fully functional state is achieved. The
specification makes use of this ordering by sometimes making
references such as "those interfaces in state greater than X".
Figure 11 shows the graph of interface state changes. The arcs
of the graph are labelled with the event causing the state
change. These events are documented in Section 9.2. The
interface state machine is described in more detail in Section
9.3.
Down
This is the initial interface state. In this state, the
lower-level protocols have indicated that the interface is
unusable. No protocol traffic at all will be sent or
received on such a interface. In this state, interface
parameters should be set to their initial values. All
interface timers should be disabled, and there should be no
adjacencies associated with the interface.
Loopback
In this state, the router's interface to the network is
+----+ UnloopInd +--------+
|Down|<--------------|Loopback|
+----+ +--------+
|
|InterfaceUp
+-------+ | +--------------+
|Waiting|<-+------------->|Point-to-point|
+-------+ +--------------+
|
WaitTimer|BackupSeen
|
|
| NeighborChange
+------+ +-+<---------------- +-------+
|Backup|<----------|?|----------------->|DROther|
+------+---------->+-+<-----+ +-------+
Neighbor | |
Change | |Neighbor
| |Change
| +--+
+---->|DR|
+--+
Figure 11: Interface State changes
In addition to the state transitions pictured,
Event InterfaceDown always forces Down State, and
Event LoopInd always forces Loopback State
looped back. The interface may be looped back in hardware
or software. The interface will be unavailable for regular
data traffic. However, it may still be desirable to gain
information on the quality of this interface, either through
sending ICMP pings to the interface or through something
like a bit error test. For this reason, IP packets may
still be addressed to an interface in Loopback state. To
facilitate this, such interfaces are advertised in router-
LSAs as single host routes, whose destination is the IP
interface address.[4]
Waiting
In this state, the router is trying to determine the
identity of the (Backup) Designated Router for the network.
To do this, the router monitors the Hello Packets it
receives. The router is not allowed to elect a Backup
Designated Router nor a Designated Router until it
transitions out of Waiting state. This prevents unnecessary
changes of (Backup) Designated Router.
Point-to-point
In this state, the interface is operational, and connects
either to a physical point-to-point network or to a virtual
link. Upon entering this state, the router attempts to form
an adjacency with the neighboring router. Hello Packets are
sent to the neighbor every HelloInterval seconds.
DR Other
The interface is to a broadcast or NBMA network on which
another router has been selected to be the Designated
Router. In this state, the router itself has not been
selected Backup Designated Router either. The router forms
adjacencies to both the Designated Router and the Backup
Designated Router (if they exist).
Backup
In this state, the router itself is the Backup Designated
Router on the attached network. It will be promoted to
Designated Router when the present Designated Router fails.
The router establishes adjacencies to all other routers
attached to the network. The Backup Designated Router
performs slightly different functions during the Flooding
Procedure, as compared to the Designated Router (see Section
13.3). See Section 7.4 for more details on the functions
performed by the Backup Designated Router.
DR In this state, this router itself is the Designated Router
on the attached network. Adjacencies are established to all
other routers attached to the network. The router must also
originate a network-LSA for the network node. The network-
LSA will contain links to all routers (including the
Designated Router itself) attached to the network. See
Section 7.3 for more details on the functions performed by
the Designated Router.9.2. Events causing interface state changes
State changes can be effected by a number of events. These
events are pictured as the labelled arcs in Figure 11. The
label definitions are listed below. For a detailed explanation
of the effect of these events on OSPF protocol operation,
consult Section 9.3.
InterfaceUp
Lower-level protocols have indicated that the network
interface is operational. This enables the interface to
transition out of Down state. On virtual links, the
interface operational indication is actually a result of the
shortest path calculation (see Section 16.7).
WaitTimer
The Wait Timer has fired, indicating the end of the waiting
period that is required before electing a (Backup)
Designated Router.
BackupSeen
The router has detected the existence or non-existence of a
Backup Designated Router for the network. This is done in
one of two ways. First, an Hello Packet may be received
from a neighbor claiming to be itself the Backup Designated
Router. Alternatively, an Hello Packet may be received from
a neighbor claiming to be itself the Designated Router, and
indicating that there is no Backup Designated Router. In
either case there must be bidirectional communication with
the neighbor, i.e., the router must also appear in the
neighbor's Hello Packet. This event signals an end to the
Waiting state.
NeighborChange
There has been a change in the set of bidirectional
neighbors associated with the interface. The (Backup)
Designated Router needs to be recalculated. The following
neighbor changes lead to the NeighborChange event. For an
explanation of neighbor states, see Section 10.1.
o Bidirectional communication has been established to a
neighbor. In other words, the state of the neighbor has
transitioned to 2-Way or higher.
o There is no longer bidirectional communication with a
neighbor. In other words, the state of the neighbor has
transitioned to Init or lower.
o One of the bidirectional neighbors is newly declaring
itself as either Designated Router or Backup Designated
Router. This is detected through examination of that
neighbor's Hello Packets.
o One of the bidirectional neighbors is no longer
declaring itself as Designated Router, or is no longer
declaring itself as Backup Designated Router. This is
again detected through examination of that neighbor's
Hello Packets.
o The advertised Router Priority for a bidirectional
neighbor has changed. This is again detected through
examination of that neighbor's Hello Packets.
LoopInd
An indication has been received that the interface is now
looped back to itself. This indication can be received
either from network management or from the lower level
protocols.
UnloopInd
An indication has been received that the interface is no
longer looped back. As with the LoopInd event, this
indication can be received either from network management or
from the lower level protocols.
InterfaceDown
Lower-level protocols indicate that this interface is no
longer functional. No matter what the current interface
state is, the new interface state will be Down.9.3. The Interface state machine
A detailed description of the interface state changes follows.
Each state change is invoked by an event (Section 9.2). This
event may produce different effects, depending on the current
state of the interface. For this reason, the state machine
below is organized by current interface state and received
event. Each entry in the state machine describes the resulting
new interface state and the required set of additional actions.
When an interface's state changes, it may be necessary to
originate a new router-LSA. See Section 12.4 for more details.
Some of the required actions below involve generating events for
the neighbor state machine. For example, when an interface
becomes inoperative, all neighbor connections associated with
the interface must be destroyed. For more information on the
neighbor state machine, see Section 10.3.
State(s): Down
Event: InterfaceUp
New state: Depends upon action routine
Action: Start the interval Hello Timer, enabling the
periodic sending of Hello packets out the interface.
If the attached network is a physical point-to-point
network, Point-to-MultiPoint network or virtual
link, the interface state transitions to Point-to-
Point. Else, if the router is not eligible to
become Designated Router the interface state
transitions to DR Other.
Otherwise, the attached network is a broadcast or
NBMA network and the router is eligible to become
Designated Router. In this case, in an attempt to
discover the attached network's Designated Router
the interface state is set to Waiting and the single
shot Wait Timer is started. Additionally, if the
network is an NBMA network examine the configured
list of neighbors for this interface and generate
the neighbor event Start for each neighbor that is
also eligible to become Designated Router.
State(s): Waiting
Event: BackupSeen
New state: Depends upon action routine.
Action: Calculate the attached network's Backup Designated
Router and Designated Router, as shown in Section
9.4. As a result of this calculation, the new state
of the interface will be either DR Other, Backup or
DR.
State(s): Waiting
Event: WaitTimer
New state: Depends upon action routine.
Action: Calculate the attached network's Backup Designated
Router and Designated Router, as shown in Section
9.4. As a result of this calculation, the new state
of the interface will be either DR Other, Backup or
DR.
State(s): DR Other, Backup or DR
Event: NeighborChange
New state: Depends upon action routine.
Action: Recalculate the attached network's Backup Designated
Router and Designated Router, as shown in Section
9.4. As a result of this calculation, the new state
of the interface will be either DR Other, Backup or
DR.
State(s): Any State
Event: InterfaceDown
New state: Down
Action: All interface variables are reset, and interface
timers disabled. Also, all neighbor connections
associated with the interface are destroyed. This
is done by generating the event KillNbr on all
associated neighbors (see Section 10.2).
State(s): Any State
Event: LoopInd
New state: Loopback
Action: Since this interface is no longer connected to the
attached network the actions associated with the
above InterfaceDown event are executed.
State(s): Loopback
Event: UnloopInd
New state: Down
Action: No actions are necessary. For example, the
interface variables have already been reset upon
entering the Loopback state. Note that reception of
an InterfaceUp event is necessary before the
interface again becomes fully functional.9.4. Electing the Designated Router
This section describes the algorithm used for calculating a
network's Designated Router and Backup Designated Router. This
algorithm is invoked by the Interface state machine. The
initial time a router runs the election algorithm for a network,
the network's Designated Router and Backup Designated Router are
initialized to 0.0.0.0. This indicates the lack of both a
Designated Router and a Backup Designated Router.
The Designated Router election algorithm proceeds as follows:
Call the router doing the calculation Router X. The list of
neighbors attached to the network and having established
bidirectional communication with Router X is examined. This
list is precisely the collection of Router X's neighbors (on
this network) whose state is greater than or equal to 2-Way (see
Section 10.1). Router X itself is also considered to be on the
list. Discard all routers from the list that are ineligible to
become Designated Router. (Routers having Router Priority of 0
are ineligible to become Designated Router.) The following
steps are then executed, considering only those routers that
remain on the list:
(1) Note the current values for the network's Designated Router
and Backup Designated Router. This is used later for
comparison purposes.
(2) Calculate the new Backup Designated Router for the network
as follows. Only those routers on the list that have not
declared themselves to be Designated Router are eligible to
become Backup Designated Router. If one or more of these
routers have declared themselves Backup Designated Router
(i.e., they are currently listing themselves as Backup
Designated Router, but not as Designated Router, in their
Hello Packets) the one having highest Router Priority is
declared to be Backup Designated Router. In case of a tie,
the one having the highest Router ID is chosen. If no
routers have declared themselves Backup Designated Router,
choose the router having highest Router Priority, (again
excluding those routers who have declared themselves
Designated Router), and again use the Router ID to break
ties.
(3) Calculate the new Designated Router for the network as
follows. If one or more of the routers have declared
themselves Designated Router (i.e., they are currently
listing themselves as Designated Router in their Hello
Packets) the one having highest Router Priority is declared
to be Designated Router. In case of a tie, the one having
the highest Router ID is chosen. If no routers have
declared themselves Designated Router, assign the Designated
Router to be the same as the newly elected Backup Designated
Router.
(4) If Router X is now newly the Designated Router or newly the
Backup Designated Router, or is now no longer the Designated
Router or no longer the Backup Designated Router, repeat
steps 2 and 3, and then proceed to step 5. For example, if
Router X is now the Designated Router, when step 2 is
repeated X will no longer be eligible for Backup Designated
Router election. Among other things, this will ensure that
no router will declare itself both Backup Designated Router
and Designated Router.[5]
(5) As a result of these calculations, the router itself may now
be Designated Router or Backup Designated Router. See
Sections 7.3 and 7.4 for the additional duties this would
entail. The router's interface state should be set
accordingly. If the router itself is now Designated Router,
the new interface state is DR. If the router itself is now
Backup Designated Router, the new interface state is Backup.
Otherwise, the new interface state is DR Other.
(6) If the attached network is an NBMA network, and the router
itself has just become either Designated Router or Backup
Designated Router, it must start sending Hello Packets to
those neighbors that are not eligible to become Designated
Router (see Section 9.5.1). This is done by invoking the
neighbor event Start for each neighbor having a Router
Priority of 0.
(7) If the above calculations have caused the identity of either
the Designated Router or Backup Designated Router to change,
the set of adjacencies associated with this interface will
need to be modified. Some adjacencies may need to be
formed, and others may need to be broken. To accomplish
this, invoke the event AdjOK? on all neighbors whose state
is at least 2-Way. This will cause their eligibility for
adjacency to be reexamined (see Sections 10.3 and 10.4).
The reason behind the election algorithm's complexity is the
desire for an orderly transition from Backup Designated Router
to Designated Router, when the current Designated Router fails.
This orderly transition is ensured through the introduction of
hysteresis: no new Backup Designated Router can be chosen until
the old Backup accepts its new Designated Router
responsibilities.
The above procedure may elect the same router to be both
Designated Router and Backup Designated Router, although that
router will never be the calculating router (Router X) itself.
The elected Designated Router may not be the router having the
highest Router Priority, nor will the Backup Designated Router
necessarily have the second highest Router Priority. If Router
X is not itself eligible to become Designated Router, it is
possible that neither a Backup Designated Router nor a
Designated Router will be selected in the above procedure. Note
also that if Router X is the only attached router that is
eligible to become Designated Router, it will select itself as
Designated Router and there will be no Backup Designated Router
for the network.9.5. Sending Hello packets
Hello packets are sent out each functioning router interface.
They are used to discover and maintain neighbor
relationships.[6] On broadcast and NBMA networks, Hello Packets
are also used to elect the Designated Router and Backup
Designated Router.
The format of an Hello packet is detailed in Section A.3.2. The
Hello Packet contains the router's Router Priority (used in
choosing the Designated Router), and the interval between Hello
Packets sent out the interface (HelloInterval). The Hello
Packet also indicates how often a neighbor must be heard from to
remain active (RouterDeadInterval). Both HelloInterval and
RouterDeadInterval must be the same for all routers attached to
a common network. The Hello packet also contains the IP address
mask of the attached network (Network Mask). On unnumbered
point-to-point networks and on virtual links this field should
be set to 0.0.0.0.
The Hello packet's Options field describes the router's optional
OSPF capabilities. One optional capability is defined in this
specification (see Sections 4.5 and A.2). The E-bit of the
Options field should be set if and only if the attached area is
capable of processing AS-external-LSAs (i.e., it is not a stub
area). If the E-bit is set incorrectly the neighboring routers
will refuse to accept the Hello Packet (see Section 10.5).
Unrecognized bits in the Hello Packet's Options field should be
set to zero.
In order to ensure two-way communication between adjacent
routers, the Hello packet contains the list of all routers on
the network from which Hello Packets have been seen recently.
The Hello packet also contains the router's current choice for
Designated Router and Backup Designated Router. A value of
0.0.0.0 in these fields means that one has not yet been
selected.
On broadcast networks and physical point-to-point networks,
Hello packets are sent every HelloInterval seconds to the IP
multicast address AllSPFRouters. On virtual links, Hello
packets are sent as unicasts (addressed directly to the other
end of the virtual link) every HelloInterval seconds. On Point-
to-MultiPoint networks, separate Hello packets are sent to each
attached neighbor every HelloInterval seconds. Sending of Hello
packets on NBMA networks is covered in the next section.
9.5.1. Sending Hello packets on NBMA networks
Static configuration information may be necessary in order
for the Hello Protocol to function on non-broadcast networks
(see Sections C.5 and C.6). On NBMA networks, every
attached router which is eligible to become Designated
Router becomes aware of all of its neighbors on the network
(either through configuration or by some unspecified
mechanism). Each neighbor is labelled with the neighbor's
Designated Router eligibility.
The interface state must be at least Waiting for any Hello
Packets to be sent out the NBMA interface. Hello Packets
are then sent directly (as unicasts) to some subset of a
router's neighbors. Sometimes an Hello Packet is sent
periodically on a timer; at other times it is sent as a
response to a received Hello Packet. A router's hello-
sending behavior varies depending on whether the router
itself is eligible to become Designated Router.
If the router is eligible to become Designated Router, it
must periodically send Hello Packets to all neighbors that
are also eligible. In addition, if the router is itself the
Designated Router or Backup Designated Router, it must also
send periodic Hello Packets to all other neighbors. This
means that any two eligible routers are always exchanging
Hello Packets, which is necessary for the correct operation
of the Designated Router election algorithm. To minimize
the number of Hello Packets sent, the number of eligible
routers on an NBMA network should be kept small.
If the router is not eligible to become Designated Router,
it must periodically send Hello Packets to both the
Designated Router and the Backup Designated Router (if they
exist). It must also send an Hello Packet in reply to an
Hello Packet received from any eligible neighbor (other than
the current Designated Router and Backup Designated Router).
This is needed to establish an initial bidirectional
relationship with any potential Designated Router.
When sending Hello packets periodically to any neighbor, the
interval between Hello Packets is determined by the
neighbor's state. If the neighbor is in state Down, Hello
Packets are sent every PollInterval seconds. Otherwise,
Hello Packets are sent every HelloInterval seconds.