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:
Link-State Age
Options
Link-State ID
Advertising Router
Link-State Sequence Number
Link-State Checksum
Length
Network Mask
TOS
Metric
5
Forwarding Address
External Route Tag
E
AS External Link Advertisements
Figure 4- 49. 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.
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
