Inter-Area
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... avoidance of the "counting to infinity" problem. OSPF prevents
inter-area routing loops by implementing a split-horizon mechanism,
allowing ABRs ...
... routing table (according to section 16.2 of [Ref1]), so it will not
have any inter-area routes in its routing table, but only intra-area
...
... router (R1 or R2, or both in our example) and will
install intra- or inter-area routes.
While being an unavoidable technique for repairing a partitioned
...
... routers. The changes are in the ABR definition and
inter-area route calculation. Any other parts of standard OSPF are
...
... interpreted as follows:
"The inter-area routes are calculated by examining summary-LSAs.
If the router ...
... Connection, both intra-area and
inter-area routes are advertised into the other areas; otherwise,
the router only advertises intra-area ...
... Destination type is network. If this is an
inter-area route and the ABR performing this algorithm ...
... have a backbone area Configured it does not actively attract
inter-area traffic, because it does not consider itself an ABR and
...
... intra-area routes in case other
routers in corresponding areas have the best inter-area paths over
it, as described in section 1.2.
...
... ........... ..........
Figure 4. Inter-area routing asymmetry
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...
Assume that R3 uses the approach described in this document. In this
case R2 will have inter-area routes to network M via ABR R1 ...
... route it directly to it (because
intra-area routes are always preferred over inter-area ones).
Traffic going back from network ...
... network M will pass through R2
and will be routed to R1, as R2 will not have any inter-area routes
via R3. So, traffic from N to M will always go through the backbone ...
... behavior is used, i.e., it does not inject summary-LSAs into the
attached areas, but inter-area traffic can still go through it.
...