D-Link DGS-3324SR Product Manual - Page 193
Autonomous Systems External Link Advertisements, EAS External Link Advertisements
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xStack DGS/DXS-3300 Series Layer 3 Stackable Gigabit Ethernet Switch User Manual 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 Stat ID is always set 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 6- 139. AS External Link Advertisements Field Description Network Mask The IP address mask for the advertised destination. E - bit 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. Forwarding Address 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 The Type of Service that the following cost is relevant to. Metric The cost of this route. The interpretation of this metric depends on the external type indication (the E - bit above). External Route Tag A 32-bit field attached to each external route. This is not used by the OSPF protocol itself. 178