3Com Switch 4800G 24-Port Configuration Manual page 290

290 C 28: OSPF C
HAPTER
n
ONFIGURATION
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
0
LS checksum
E 0
E TOS
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.
7 15
LS age
Linke state ID
Advertising Router
LS sequence number
Network mask
Forwarding address
External route tag
Forwarding address
External route tag
...
Options
Length
Metric
TOS metric
31
5