>Let me state first of al that I do not know the genesis of this question. It
>originated on another news group, and there were a number of responses that
>stated "use virtual links"
There is no general solution to solving partitioned nonzero areas in
OSPF. That being said, there are some hacks that may be tried,
although the "right way" is to build adequate cross-connections
inside an area.
Some ugly things can be done with GRE tunnels and on-demand OSPF.
OSPF would see the tunnel, but the underlying IP route for the tunnel
would be statically routed through area 0.0.0.0.
Virtual links are more appropriate for healing partitions in area 0.0.0.0.
From an Internet-Draft I really need to update,
3.4 Virtual Links
A virtual link is a tunnel that has at least one
end in Area 0. Virtual
Links (VL) are a mechanism that can be used
within OSPF to handle
certain connectivity patterns. The standard is
a bit "soft" on their
applications, and support engineers have seen a
variety of VL
applications. For some of the problems being raised, it
appears better
OSPF solutions may exist.
A matter of particular
interest is the potential advantages of NSSAs
over some of the VL
solutions to bringing in a new "community of
interest." See the
discussion below of VL applicability in other than
backbone
robustness.
The perception of virtual links' untility seems to be as
a means of
accomodating specical connectivity requirements "inside" a
single "OSPF
domain," the latter defined as a set of an Area 0 and
some number of
nonzero areas.
In some of these requirements, virtual
links may be completely
appropriate, one of several potential
solutions, or definitiely not an
appropriate solution.
Some
designers may use virtual links to avoid other mechanisms that
they
do not like, such as defining the enterprise's network with
multiple
interconnected OSPF domains.
Virtual links are not
necessarily the appropriate solution, but cases
have been seen where
they are used for:
--- Protecting against Area 0 partitioning
--- Making one area 0 following the merger of two enterprises,
both of which ran independent OSPF
--- Providing connectivity to
a newly acquired enterprise
whose best connectivity is to a
router in a nonzero area
of the acquiring enterprise
[snip]
5. Increasing Backbone Reliability
A failure in Area 0 is
critical.
A single Area 0 has a single point of failure. Hopefully,
the ASCII
graphic shows this. Area 0 has two interconnected Border
Routers, BR1
and BR2. To avoid single router points of failure,
there are two ABRs,
each with three interfaces: Area 0, Area 1, and
Area 2.
5.1 VL Solution
If the BR1-BR2 link fails, a VL can be
defined between the Area 1
interface of ABR-1 and between the Area 1
interface of ABR-2. This
reconstitutes backbone connectivity with a
tunnel through Area 1.
BR1------------------------------------------BR2
\
/
\ ...to BR2 /
ABR-1 ABR-2--/ ABR-3
=========================================================
| | | * |
| |-------
* |
| VL? *
|
| * |
v * v
Area 0.0.0.1
* Area 0.0.0.2
5.2 Alternative using Adding Circuit(s)
The
preferred way to solve this problem is adding a parallel link
between
BR1 and BR2. Especially if these links can be per-packet
load
balanced, convergence would be extremely fast.
This solution,
however, incurs additional cost for the additional
circuit. Balanced
against this cost is the performance impact the VL
would have on the
routers and links in the nonzero area through which
the VL is
tunneled. Those routers and links may have been engineered for
the
traffic estimates and performance goals under the workload of
that
single area, not of that area with backbone traffic added to it.
-------------------------
Now, let's talk about a partition in area 0.0.0.1.
BR1------------------------------------------BR2
\ /
\
...to BR2 /
ABR-1 ABR-2--/
ABR-3
=========================================================
| | *
| | *
IR1---X-----IR2 *
| *
v
*
Area 0.0.0.1 *
At "X", there is a break between interior routers IR1 and IR2.
Without summarization, this will work.
Assume that 172.16.0.0/16 has been assigned to the area, and this is the
only summary announced to the backbone. Further assume that 172.16.0.0/17
is assigned to the "west" side of the area and 172.16.128.0/17 is assigned
to the "east."
With Cisco's implementation, both ABR's will continue to advertise the
entire summary, so half the addresses are advertised by the wrong router.
A reasonable argument can be made that this is a good thing, because
even if the routes are unreachable, the stability of the overall OSPF
system is improved by suppressing more-specific routes and avoiding
flaps.
Bay RS interprets OSPF summarization differently. If any
more-specific of the summary becomes unreachable, a Bay router will
stop announcing the aggregate and announce the more-specifics. This
approach encourages optimal routing over routing stablity.
RFC2328 is silent on the expected behavior in this situation. Both
interpretations have pros and cons.
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