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4.4.3.2. Router-LSAs

The LS type of a router-LSA is set to the value 0x2001. Router-LSAs have area flooding scope. A router MAY originate one or more router-LSAs for a given area. Each router-LSA contains an integral number of interface descriptions. Taken together, the collection of router-LSAs originated by the router for an area describes the collected states of all the router's interfaces attached to the area. When multiple router-LSAs are used, they are distinguished by their Link State ID fields.

To the left of the Options field, the router capability bits V, E, and B should be set according to Section 12.4.1 of [OSPFV2].

Each of the router's interfaces to the area is then described by appending "link descriptions" to the router-LSA. Each link description is 16 bytes long, consisting of five fields: (link) Type, Metric, Interface ID, Neighbor Interface ID, and Neighbor Router ID (see Appendix A.4.3). Interfaces in the state "Down" or "Loopback" are not described (although looped back interfaces can contribute prefixes to intra-area-prefix-LSAs), nor are interfaces without any full adjacencies described (except in the case of multiple Standby Interfaces as described in Section 4.9). All other interfaces to the area add zero, one, or more link descriptions. The number and content of these depend on the interface type. Within each link description, the Metric field is always set to the interface's output cost, and the Interface ID field is set to the interface's OSPF Interface ID.

Point-to-point interfaces
If the neighboring router is fully adjacent, add a Type 1 link description (point-to-point). The Neighbor Interface ID field is set to the Interface ID advertised by the neighbor in its Hello packets, and the Neighbor Router ID field is set to the neighbor's Router ID.

Broadcast and NBMA interfaces
If the router is fully adjacent to the link's Designated Router or if the router itself is the Designated Router and is fully adjacent to at least one other router, add a single Type 2 link description (transit network). The Neighbor Interface ID field is set to the Interface ID advertised by the Designated Router in its Hello packets, and the Neighbor Router ID field is set to the Designated Router's Router ID.

Virtual links
If the neighboring router is fully adjacent, add a Type 4 link description (virtual). The Neighbor Interface ID field is set to the Interface ID advertised by the neighbor in its Hello packets, and the Neighbor Router ID field is set to the neighbor's Router ID. Note that the output cost of a virtual link is calculated during the routing table calculation (see Section 4.7).

Point-to-Multipoint interfaces
For each fully adjacent neighbor associated with the interface, add a separate Type 1 link description (point-to-point) with the Neighbor Interface ID field set to the Interface ID advertised by the neighbor in its Hello packets and the Neighbor Router ID field set to the neighbor's Router ID.

As an example, consider the router-LSA that router RT3 would originate for Area 1 in Figure 1. Only a single interface must be described, namely, that which connects to the transit network N3. It assumes that RT4 has been elected the Designated Router of Network N3.

; RT3's router-LSA for Area 1

LS age = 0                     ; newly (re)originated
LS type = 0x2001               ; router-LSA
Link State ID = 0              ; first fragment
Advertising Router = 192.0.2.3 ; RT3's Router ID
bit E = 0                      ; not an AS boundary router
bit B = 1                      ; area border router
Options = (V6-bit|E-bit|R-bit)
    Type = 2                     ; connects to N3
    Metric = 1                   ; cost to N3
    Interface ID = 1             ; RT3's Interface ID on N3
    Neighbor Interface ID = 1    ; RT4's Interface ID on N3
    Neighbor Router ID = 192.0.2.4 ; RT4's Router ID

For example, if another router was added to Network N4, RT3 would have to advertise a second link description for its connection to (the now transit) network N4. This could be accomplished by reoriginating the above router-LSA, this time with two link descriptions. Or, a separate router-LSA could be originated with a separate Link State ID (e.g., using a Link State ID of 1) to describe the connection to N4.

Host routes for stub networks no longer appear in the router-LSA. Rather, they are included in intra-area-prefix-LSAs.

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