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An AS external LSA is originated by an ASBR, and describes routing information to a destination outside
the AS.
Figure 35
AS external LSA format
Major fields of the AS external LSA are as follows:
•
Link state ID: The IP address of another AS to be advertised. When describing a default route, the
Link state ID is always set to default destination (0.0.0.0) and the network mask is set to 0.0.0.0
•
Network mask: The IP address mask for the advertised destination
•
E (External Metric): The type of the external metric value, which is set to 1 for type 2 external routes,
and set to 0 for type 1 external routes. See “
“ for a description of external route types.
•
Metric: The metric to the destination.
•
Forwarding address: Data traffic for the advertised destination is forwarded to this address.
•
External route tag: A tag attached to each external route. This is not used by the OSPF protocol. It
may be used to manage external routes.
5.
NSSA external LSA
An NSSA external LSA originates from the ASBR in an NSSA, and is flooded in the NSSA area only. It
has the same format as the AS external LSA.
