290 CHAPTER 28: OSPF CONFIGURATION
n
A Type-3 LSA can be used to advertise a default route, having the Link State ID
and Network Mask set to 0.0.0.0.
1 AS external LSA
An AS external LSA originates from an ASBR, describing routing information to a
destination outside the AS.
Figure 91 AS external LSA format
Major fields:
■ 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. Refer to “Route
types” on page 280 for description about external route types
■ metric: The metric to the destination
■ Forwarding Address: Data traffic for the advertised destination will be
forwarded to this address
■ External Route Tag: A tag attached to each external route. This is not used by
the OSPF protocol itself. It may be used to manage external routes.
■ NSSA external LSA
An NSSA external LSA originates from the ASBR in a NSSA and is flooded in the
NSSA area only. It has the same format as the AS external LSA.
Network mask
Forwarding address
LS age
Linke state ID
Advertising Router
Options 5
LS sequence number
LS checksum Length
Metric
External route tag
...
E 0
Forwarding address
TOS metric
External route tag
E TOS
0 7 15 31
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