-127-
Autonomous Systems External Link Advertisements
Autonomous Systems (AS) link advertisements are Type 5 link state advertisements. These advertise-
ments 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 net-
work 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
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 ad-
dress. 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.
