9. The Interface Data Structure

9. 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.