4.5. Optional OSPF capabilities

The OSPF protocol defines several optional capabilities. A router indicates the optional capabilities that it supports in its OSPF Hello packets, Database Description packets and in its link state advertisements. This enables routers supporting a mix of optional capabilities to coexist in a single Autonomous System.

Some capabilities must be supported by all routers attached to a specific area. In this case, a router will not accept a neighbor's Hello Packet unless there is a match in reported capabilities (i.e., a capability mismatch prevents a neighbor relationship from forming). An example of this is the ExternalRoutingCapability (see below).

Other capabilities can be negotiated during the Database Exchange process. This is accomplished by specifying the optional capabilities in Database Description packets. A capability mismatch with a neighbor in this case will result in only a subset of link state advertisements being exchanged between the two neighbors.

The routing table build process can also be affected by the presence/absence of optional capabilities. For example, since the optional capabilities are reported in link state advertisements, routers incapable of certain functions can be avoided when building the shortest path tree. An example of this is the TOS routing capability (see below).

The current OSPF optional capabilities are listed below. See Section A.2 for more information.

ExternalRoutingCapability

Entire OSPF areas can be configured as "stubs" (see Section 3.6). AS external advertisements will not be flooded into stub areas. This capability is represented by the E-bit in the OSPF options field (see Section A.2). In order to ensure consistent configuration of stub areas, all routers interfacing to such an area must have the E-bit clear in their Hello packets (see Sections 9.5 and 10.5).

TOS capability

All OSPF implementations must be able to calculate separate routes based on IP Type of Service. However, to save routing table space and processing resources, an OSPF router can be configured to ignore TOS when forwarding packets. In this case, the router calculates routes for TOS 0 only. This capability is represented by the T-bit in the OSPF options field (see Section A.2). TOS-capable routers will attempt to avoid non-TOS-capable routers when calculating non-zero TOS paths.