D-Link DGS-3600 Series User Manual page 209

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:
Field
Network Mask
E - bit
Forwarding
Address
TOS
Metric
External Route
Tag
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:
®
xStack DGS-3600 Series Layer 3 Gigabit Ethernet Managed Switch
AS External Link Advertisements
Link-State Age
Link-State Sequence Number
Link-State Checksum
E
TOS
Figure 8- 41. AS External Link Advertisements
Description
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.
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).
A 32-bit field attached to each external route. This is not used by the OSPF protocol
itself.
Options
Link-State ID
Advertising Router
Length
Network Mask
Metric
Forwarding Address
External Route Tag
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