D-Link DES-6500 - Switch User Manual page 167

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:
Field
Network Mask
E - bit
Forwarding Address
TOS
Metric
External Route Tag
xStack DES-6500 Modular Layer 3 Chassis Ethernet Switch User Manual
AS External Link Advertisements
Link-State Age
Link-State Checksum
E
TOS
Figure 6- 131. 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
Link-State Sequence Number
Network Mask
Metric
Forwarding Address
External Route Tag
152
5
Length