backbone
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... domains the area topology is restricted so that there must be
a backbone area (area 0) and all other areas must have either
physical or virtual connections ...
... physical or virtual connections to the backbone. The reason for this
star-like topology is that OSPF ...
... routing loops by implementing a split-horizon mechanism,
allowing ABRs to inject into the backbone only Summary-LSAs derived
from the
...
... OSPF, an ABR does not need to be attached to the
backbone). Consider a sample OSPF domain depicted in the Figure 1.
...
... ABR, R3 can
only consider summary-LSAs from the backbone when building the
routing table (according to section 16.2 of [Ref1 ...
... At the same time, router R2, as an ABR connected to the backbone,
will inject into Area 2 summary-LSAs describing the destinations ...
... LSAs describing the destinations in
Area 0 (the backbone), Area 1 and other areas reachable through the
backbone.
...
... Area 0 (the backbone), Area 1 and other areas reachable through the
backbone.
This results in a situation where internal router ...
... router R4 calculates its
routes to destinations in the backbone and areas other than Area 1
via R2. The topology of Area 2 itself can be such that the best path
...
... topology of Area 2 itself can be such that the best path
from R4 to R2 is via R3, so all traffic destined for the backbone and
backbone-attached areas goes through R3. Router ...
... traffic destined for the backbone and
backbone-attached areas goes through R3. Router R3 in turn, having
only intra-area ...
... traffic not
destined for the areas directly attached to it. The same problem can
occur when a backbone-connected ABR loses all of its adjacencies in
the backbone ...
... backbone-connected ABR loses all of its adjacencies in
the backbone---even if there are other normally functioning ABRs in
the attached areas, all traffic ...
... ABRs in
the attached areas, all traffic going to the backbone (destined for
it or for other areas) will be dropped.
...
... Ref1] for more details). In
this case, router R3 will have a virtual backbone connection, will
form an adjacency over it, will receive all LSAs ...
... form an adjacency over it, will receive all LSAs directly from a
backbone-attached router (R1 or R2, or both in our example) and will
...
...
While being an unavoidable technique for repairing a partitioned
backbone area, the use of virtual links in the described situation
adds extra configuration headaches and system traffic ...
... These solutions are targeted to the situation when an ABR has no
backbone connection. They imply that a router connected to multiple
...
... connection. They imply that a router connected to multiple
areas without a backbone connection is not an ABR and should function
...
... methods described here improve the behavior
of a router connecting two or more backbone-attached areas.
...
... backbone connection if
the backbone area is actively attached and there is at least one
fully adjacent neighbor in it.
...
... 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:
...
... router is considered to be an ABR if it has more than one
Actively Attached area and the backbone area Configured.
...
... ABR, but has more than one attached area,
or it does not have an Active Backbone Connection. Definitions
of the terms used in this paragraph are given in section 2.1.
...
... ABR but doesn't
have an Active Backbone Connection (and, consequently, examines
summaries from all attached areas). The algorithm ...
... algorithm described below. Note that while
only intra-area routes are advertised into the backbone, if the
router has an Active ...
... performing this algorithm does not have an Active Backbone
Connection, it can originate Type 4 summary-LSA ...
... route traffic destined
for the backbone and other areas. Note that if the router does not
have a backbone ...
... backbone and other areas. Note that if the router does not
have a backbone area Configured it does not actively attract
inter-area traffic ...
... it, as described in section 1.2.
By processing all summaries when the backbone is not active, we
prevent the ABR ...
... active, we
prevent the ABR, which has just lost its last backbone adjacency,
from dropping any packets going through the ABR in question to
...
... ABR in question to
another ABR and destined towards the backbone or other areas not
connected to the ABR directly.
...
... have at least one active interface in the backbone area. This
requirement may cause problems with virtual links ...
... requirement may cause problems with virtual links in those rare
situations where the backbone area is purely virtual, as shown in
Figure 3, and the state of the VL ...
... methods improves the
behavior of a router connected to multiple areas without a backbone
attachment, but can lead to unexpected routing asymmetry, as
...
... inter-area routes
via R3. So, traffic from N to M will always go through the backbone
while traffic from M to N will cross the areas directly via R3 and,
...
... traffic from M to N will cross the areas directly via R3 and,
in this example, will not use a more optimal path through the
backbone.
Note that this problem is not caused by the fact that R3 uses the
...
... Cisco ABR implementation (slightly different from the one
described in this memo, requiring non-backbone areas to be
configured, and not necessarily actively attached in the ABR
...