document fixes routing loops which can occur in RFC 1583 when
the same external destination is advertised by AS boundary routers in
separate areas. There are two manifestations of this problem. The
first, discovered by Dennis Ferguson, occurs when an aggregated
forwarding address is in use. In this case, the desirability of the
forwarding address can change for the worse as a packet crosses an
area aggregation boundary on the way to the forwarding address, which
in turn can cause the preference of AS-external-LSAs to change,
resulting in a routing loop.
The second manifestation was discovered by Richard Woundy. It is
caused by an incomplete application of OSPF's preference of intra-
area routes over inter-area routes: paths to any given
ASBR/forwarding address are selected first based on intra-area
preference, while the comparison between separate ASBRs/forwarding
addresses is driven only by cost, ignoring intra-area preference. His
example is replicated in Figure 19. Both router A3 and router B3 are
originating an AS-external-LSA for 10.0.0.0/8, with the same type 2
metric. Router A1 selects B1 as its next hop towards 10.0.0.0/8,
based on shorter cost to ASBR B3 (via B1->B2->B3). However, the
shorter route to B3 is not available to B1, due to B1's preference
for the (higher cost) intra-area route to B3 through Area A. This
leads B1 to select A1 as its next hop to 10.0.0.0/8, resulting in a
routing loop.
Moy Standards Track [Page 207]
RFC 2178 OSPF Version 2 July 1997
The following two changes have been made to prevent these routing
loops:
o When originating a type 3 summary-LSA for a configured area
address range, the cost of the summary-LSA is now set to the
maximum cost of the range's component networks (instead of the
previous algorithm which set the cost to the minimum component
cost). This change affects Sections 3.5 and 12.4.3, Figures 7
and 8, and Tables 6 and 13.
o The preference rules for choosing among multiple AS-
external-LSAs have been changed. Where previously cost was the
only determining factor, now the preference is driven first by
type of path (intra-area or inter-area, through non-backbone area
or through backbone) to the ASBR/forwarding address, using cost
only to break ties. This change affects Sections 16.4 and 16.4.1.
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