D-Link DGS-3620-28TC-SI Product Manual - Page 190

xStack® DGS-3620 Series Managed Switch Web UI Reference Guide For stub area, Type 3 summary link advertisements can also be used to describe a default route on a per-area basis. Default summary routes are used in stub area instead of flooding a complete set of external routes. When describing a default summary route, the advertisement's Link State ID is always set to the Default Destination  0.0.0.0, and the Network Mask is set to 0.0.0.0. Separate costs may be advertised for each IP Type of Service. Note that the cost for ToS 0 must be included, and is always listed first. If the T-bit is reset in the advertisement's Option field, only a route for ToS 0 is described by the advertisement. Otherwise, routes for the other ToS values are also described. If a cost for a certain ToS is not included, its cost defaults to that specified for ToS 0. Parameter Network Mask ToS Metric Description For Type 3 link state advertisements, this indicates the destination network's IP address mask. For example, when advertising the location of a class A network the value 0xff000000. The Type of Service that the following cost is relevant to. The cost of this route. Expressed in the same units as the interface costs in the router links advertisements. Autonomous Systems External Link Advertisements Autonomous Systems (AS) link advertisements are Type 5 link state advertisements. These advertisements are originated by AS boundary routers. A separate advertisement is made for each destination known to the router that is external to the AS. AS external link advertisements usually describe a particular external destination. For these advertisements the Link State ID field specifies an IP network number. AS external link advertisements are also used to describe a default route. Default routes are used when no specific route exists to the destination. When describing a default route, the Link State ID is always set with the Default Destination address (0.0.0.0) and the Network Mask is set to 0.0.0.0. The format of the AS External Link Advertisement is shown below: AS External Link Advertisements Link-State Age Options 5 Link-State ID Advertising Router Link-State Sequence Number Link-State Checksum Length E TOS Network Mask Metric Forwarding Address External Route Tag Figure 5-31 AS External Link Advertisements 180