routing
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An OSPF routing domain can be split into several subdomains, called
areas, which limit the scope of LSA ...
... topology is that OSPF inter-area routing uses the
distance-vector approach and a strict area hierarchy permits
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... OSPF prevents
inter-area routing loops by implementing a split-horizon mechanism,
allowing ABRs to inject into the backbone ...
... LSAs from the backbone when building the
routing table (according to section 16.2 of [Ref1]), so it will not
have any inter-area ...
... Ref1]), so it will not
have any inter-area routes in its routing table, but only intra-area
routes from both Area 1 and Area 2. Consequently, according to
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... a situation where separate OSPF processes are run for each area and
supply routes to the routing table. It remedies the situation
described in the examples above by not dropping transit traffic.
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2. The algorithm for the routing table calculation is changed to
allow the router to consider the summary-LSAs ...
... area border routers. The precise
summary routes to advertise into an area are determined by
examining the routing table structure (see Section 11) in
accordance with the algorithm described below. Note that while
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... router, a summary-LSA should be originated if and only if the
routing table entry describes the preferred path to the AS
boundary router (see Step 3 of Section 16.4). If so, a Type 4
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... router's Router ID and metric
equal to the routing table entry's cost. If the ABR
performing this algorithm ...
... host bits set; see Appendix E for
details) and metric equal to the routing table cost."
The changes in the ABR ...
... algorithm in 16.1 of [Ref1] when a router routing entry is added, but
every time a vertex is added to the SPT in step 3 of the same
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... of the router-to-router cooperation and do not create routing loops,
and hence are fully compatible with standard OSPF. Proof of
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... router connected to multiple areas without a backbone
attachment, but can lead to unexpected routing asymmetry, as
described below.
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Figure 4. Inter-area routing asymmetry
Assume that R3 uses the approach described in this document. In this
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... ABR behavior described as IBM ABR approach was implemented by IBM
in IBM Nways Multiprotocol Routing Services (MRS) 3.3.
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