C.3 Router interface parameters

Some of the configurable router interface parameters (such as IP interface address and subnet mask) actually imply properties of the attached networks, and therefore must be consistent across all the routers attached to that network. The parameters that must be configured for a router interface are:

IP interface address

The IP protocol address for this interface. This uniquely identifies the router over the entire internet. An IP address is not required on serial lines. Such a serial line is called "unnumbered".

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.

Interface output cost(s)

The cost of sending a packet on the interface, expressed in the link state metric. This is advertised as the link cost for this interface in the router's router links advertisement. There may be a separate cost for each IP Type of Service. The interface output cost(s) must always be greater than 0.

RxmtInterval

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. This should be well over the expected round-trip delay between any two routers on the attached network. The setting of this value should be conservative or needless retransmissions will result. It will need to be larger on low speed serial lines and virtual links. Sample value for a local area network: 5 seconds.

InfTransDelay

The estimated number of seconds it takes to transmit a Link State Update Packet over this interface. Link state advertisements contained in the update packet must have their age incremented by this amount before transmission. This value should take into account the transmission and propagation delays of the interface. It must be greater than 0. Sample value for a local area network: 1 second.

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. If there is still a tie, the router with the highest Router ID takes precedence. A router whose Router Priority is set to 0 is ineligible to become Designated Router on the attached network. Router Priority is only configured for interfaces to multi-access networks.

HelloInterval

The length of time, in seconds, between the Hello Packets that the router sends on the interface. This value is advertised in the router's Hello Packets. It must be the same for all routers attached to a common network. The smaller the HelloInterval, the faster topological changes will be detected, but more OSPF routing protocol traffic will ensue. Sample value for a X.25 PDN network: 30 seconds. Sample value for a local area network: 10 seconds.

RouterDeadInterval

After ceasing to hear a router's Hello Packets, the number of seconds before its neighbors declare the router down. This is also advertised in the router's Hello Packets in their RouterDeadInterval field. This should be some multiple of the HelloInterval (say 4). This value again must be the same for all routers attached to a common network.

Authentication key

This configured data allows the authentication procedure to generate and/or verify the authentication field in the OSPF header. This value again must be the same for all routers attached to a common network. 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. There could be a separate password for each network.