D-Link Xstack DGS-3300 Series User Manual page 192

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:
E
Field
Network Mask
E - bit
Forwarding Address
TOS
Metric
External Route Tag
AS External Link Advertisements
Link-State Age
Advertising Router
Link-State Sequence Number
Link-State Checksum
Network Mask
TOS
Forwarding Address
External Route Tag
Figure 6- 139. 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
Length
Metric
177
5