A.2 The Options field

The OSPF Options field is present in OSPF Hello packets, Database Description packets and all link state advertisements. The Options field enables OSPF routers to support (or not support) optional capabilities, and to communicate their capability level to other OSPF routers. Through this mechanism routers of differing capabilities can be mixed within an OSPF routing domain.

When used in Hello packets, the Options field allows a router to reject a neighbor because of a capability mismatch. Alternatively, when capabilities are exchanged in Database Description packets a router can choose not to forward certain link state advertisements to a neighbor because of its reduced functionality. Lastly, listing capabilities in link state advertisements allows routers to route traffic around reduced functionality routers, by excluding them from parts of the routing table calculation.

Two capabilities are currently defined. For each capability, the effect of the capability's appearance (or lack of appearance) in Hello packets, Database Description packets and link state advertisements is specified below. For example, the ExternalRoutingCapability (below called the E-bit) has meaning only in OSPF Hello Packets. Routers should reset (i.e. clear) the unassigned part of the capability field when sending Hello packets or Database Description packets and when originating link state advertisements.

Additional capabilities may be assigned in the future. Routers encountering unrecognized capabilities in received Hello Packets, Database Description packets or link state advertisements should ignore the capability and process the packet/advertisement normally.

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                             The Options field

T-bit

This describes the router's TOS capability. If the T-bit is reset, then the router supports only a single TOS (TOS 0). Such a router is also said to be incapable of TOS-routing, and elsewhere in this document referred to as a TOS-0-only router. The absence of the T-bit in a router links advertisement causes the router to be skipped when building a non-zero TOS shortest- path tree (see Section 16.9). In other words, routers incapable of TOS routing will be avoided as much as possible when forwarding data traffic requesting a non-zero TOS. The absence of the T-bit in a summary link advertisement or an AS external link advertisement indicates that the advertisement is describing a TOS 0 route only (and not routes for non-zero TOS).

E-bit

This bit reflects the associated area's ExternalRoutingCapability. AS external link advertisements are not flooded into/through OSPF stub areas (see Section 3.6). The E-bit ensures that all members of a stub area agree on that area's configuration. The E-bit is meaningful only in OSPF Hello packets. When the E-bit is reset in the Hello packet sent out a particular interface, it means that the router will neither send nor receive AS external link state advertisements on that interface (in other words, the interface connects to a stub area). Two routers will not become neighbors unless they agree on the state of the E-bit.