These messages have a Type value of 3 in the header. Following the header comes one or more sets of three fields that each identify a link state advertisement (LSA) for which the router is requesting an update, as shown in Figure 187. Each LSA identification has the format described in Table 129.
Link State Type: The type of link state advertisement (LSA) being sought.
Link State ID: The identifier of the LSA, usually the IP address of either the router or network linked.
Advertising Router: The ID of the router that created the LSA whose update is being sought.