Each router in the Autonomous System originates one or more link state advertisements. There are five distinct types of link state advertisements, which are described in Section 4.3. The collection of link state advertisements forms the link state or topological database. Each separate type of advertisement has a separate function. Router links and network links advertisements describe how an area's routers and networks are interconnected. Summary link advertisements provide a way of condensing an area's routing information. AS external advertisements provide a way of transparently advertising externally-derived routing information throughout the Autonomous System.
Type Dest Area Path Type Cost Next Adv.
Hop(s) Router(s)
________________________________________________________________
N Ib 0 intra-area 16 RT3 *
N Ia 0 intra-area 21 RT3 *
BR RT3 0 intra-area 1 * *
BR RT10 0 intra-area 16 RT3 *
BR RT11 0 intra-area 19 RT3 *
________________________________________________________________
N N9-N11,H1 0 inter-area 20 RT3 RT11
Table 14: Changes resulting from an
additional virtual link.
Each link state advertisement begins with a standard 20-byte header. This link state advertisement header is discussed below.