LevelOne GTL-2691 - Manual (Page 1519)

C HAPTER 49 | IP Routing Commands
Open Shortest Path First (OSPFv3)
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D EFAULT S ETTING
cisco
C OMMAND U SAGE
◆ The basic criteria for a router to serve as an ABR is shown below:
■
Cisco Systems Interpretation: A router is considered to be an ABR if
it has more than one area actively attached and one of them is the
backbone area.
■
IBM Interpretation: A router is considered to be an ABR if it has
more than one actively attached area and the backbone area is
configured.
■
Standard Interpretation: A router is considered to be an ABR if it is
attached to two or more areas. It does not have to be attached to
the backbone area.
◆ To successfully route traffic to inter-area and AS external destinations,
an ABR must be connected to the backbone. If an ABR has no backbone
connection, all traffic destined for areas not connected to it or outside
the AS will be dropped. This situation is normally resolved, by
configuring a virtual link from the ABR to the backbone area.
◆ In both the Cisco and IBM interpretation, a router connected to more
than one area cannot issue a Type 1 router LSA declaring itself as an
ABR unless it meets the other criteria listed above.
Routing table calculations are changed to allow the router to consider
summary-LSAs from all attached areas if it is not an ABR, but has more
than one attached area, or it does not have an active backbone
connection.
In other words, inter-area routes are calculated by examining
summary-LSAs. If the router is an ABR and has an active backbone
connection, only backbone summary-LSAs are examined. Otherwise
(when either the router is not an ABR or it has no active backbone
connection), the router should consider summary-LSAs from all actively
attached areas.
This ensures that the summary-LSAs originated by area border routers
advertise only intra-area routes into the backbone if the router has an
active backbone connection, and advertises both intra-area and inter-
area routes into the other areas. Otherwise, the router only advertises
intra-area routes into non-backbone areas.
E XAMPLE
Console(config-router)#abr-type ibm
Console(config-router)#