D-Link DGS-3612G Product Manual - Page 229

xStack® DGS-3600 Series Layer 3 Managed Gigabit Ethernet Switch 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 4- 49. AS External Link Advertisements Field Description Network Mask E - bit Forwarding Address The IP address mask for the advertised destination. The type of external metric. If the E-bit is set, the metric specified is a Type 2 external metric. This means the metric is considered larger than any link state path. If the E-bit is zero, the specified metric is a Type 1 external metric. This means that is comparable directly to the link state metric. Data traffic for the advertised destination will be forwarded to this address. If the Forwarding Address is set to 0.0.0.0, data traffic will be forwarded instead to the advertisement's originator. TOS Metric The Type of Service that the following cost is relevant to. The cost of this route. The interpretation of this metric depends on the external type indication (the E - bit above). External Route A 32-bit field attached to each external route. This is not used by the OSPF protocol Tag itself. Including the NSSA The NSSA or Not So Stubby Area is a feature that has been added to OSPF so external routes from ASs (Autonomous Systems) can be imported into the OSPF area. As an extension of stub areas, the NSSA feature uses a packet translation system used by BRs (Border Routers) to translate outside routes into the OSPF area. Consider the following example: 213