Connected: An Internet Encyclopedia
9. The Interface Data Structure

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9. The Interface Data Structure

9. The Interface Data Structure An OSPF interface is the connection between a router and a network. There is a single OSPF interface structure for each attached network; each interface structure has at most one IP interface address (see below). The support for multiple addresses on a single network is a matter for future consideration.

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 link state advertisements 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; those items must be the same for all routers connected to the network.


The kind of network to which the interface attaches. Its value is either broadcast, non-broadcast yet still multi-access, point-to-point or virtual link.


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 link state advertisements.

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.


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.


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.


The estimated number of seconds it takes to transmit a Link State Update Packet over this interface. Link state advertisements 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. On multi-access networks, 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 multi-access 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 link state advertisement is labelled with the Designated Router's IP address. The Designated Router is initialized to, which indicates the lack of a Designated Router.

Backup Designated Router

The Backup Designated Router is also selected on all multi- access 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, 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 links advertisement. There may be a separate cost for each IP Type of Service. The cost of an interface must be greater than zero.


The number of seconds between link state advertisement retransmissions, for adjacencies belonging to this interface. Also used when retransmitting Database Description and Link State Request Packets.

Authentication key

This configured data allows the authentication procedure to generate and/or verify the Authentication field in the OSPF header. 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 password. This key would be inserted directly into the OSPF header when originating routing protocol packets, and there could be a separate password for each network.

Next: 9.1. Interface states

Connected: An Internet Encyclopedia
9. The Interface Data Structure