Peter,
comments inline...
JMcL
----- Forwarded by Jenny Mcleod/NSO/CSDA on 08/04/2002 09:14 am -----
"Peter van Oene"
Sent by: [EMAIL PROTECTED]
06/04/2002 01:17 am
Please respond to "Peter van Oene"
To: [EMAIL PROTECTED]
cc:
Subject: Re: OSPF design [7:40269]
New theory! might work :)
My assumptions:
1) R1 and R2 are your ABR's, R2's link into the backbone is a dial on
demand link only used when R1's link fails.
JMcL: Correct /JMcL
2) Due to the above, the primary problem is that when the non-backbone
area
becomes partitioned, R1 will not be able to deliver to certain nets south
of R2 as it does not see R2 as a valid hop toward those nets (since it
doesn't see the type1/2 advertisements from that area). In this case, R1
either forwards via default toward the core and loops traffic for those
unreachable nets, or matches a null0 route for the summary and discards.
JMcL: R1 does not see the type 1/2 advertisements, but it DOES see the
type 3 advertisements for the summary from R2 via the area 0 connection.
IF there is no null0 route generated by R1 (either because generation is
turned off or because the IOS version is such that the default is not to
generate one), I would have expected this summary to be used to direct
traffic for unreachable nets to R2. According to the support guys, this
is not the case as the problem happened even before the null0 route was
generated. /JMcL
3) R2 will have this problem only when R1 loses connectivity to the core
_and_ the non backbone area becomes partitioned. Hence, fixing this
problem is less important that fixing #2.
JMcL: Yes, if you mean that nets south of R1 (I like the phrasing - very
descriptive) will be "lost" if R1 loses connectivity to the core and the
non-backbone area is partitioned. /JMcL
Solution:
Disable the creation of a null0 route for the aggregate on R1 and instead
add a static route for the aggregate on R1 toward R2.
JMcL: I suspect this would work, but I am still confused as to why the
type 3 advertisement from R2 does not provide reachability if there is no
null0 route. As far as I can see, the static shouldn't be necessary (but
probably is).
As a side note, has anyone actually used the "no discard-route" command?
That's the only way I've spotted to turn off the creation of the null0
route, and it's basically undocumented - does it work as expected? /JMcL
With this config, if the area becomes partitioned, while R1's ethernet
toward the core is live, when R1 pulls traffic based on the summary toward
unreachable nets behind R2, this route will push the traffic toward
R2. Should R2 not be able to reach those nets, the can be safely
considered unreadable and R2's null0 route will discard the traffic
thereby
eliminating loops. The only downside is that some truly unreachable
traffic might transit the R1-R2 link before being eliminated.
This will not help the situation where the area is partitioned and R1
loses
core connectivity, but this is a much less likely occurrence. Plus, in
this case your dialup link might be strained anyway so dropping a bunch of
traffic might be helpful :)
JMcL: It's a big dialup link :-) But you're right, that situation is much
less likely. /JMcL
In summary, assume 192.168/16 is the summary
R1
ip route 192.168.0.0 255.255.0.0 R2
R2
ip route 192.168.0.0 255.255.0.0 null0
Adding the cable is also helpful, but costs money and requires you to
touch
a bunch of routers.
At 09:04 AM 4/5/2002 -0500, Peter van Oene wrote:
>Adding a point to point link between ABR's would enhance the resiliency
>between the two and tend to protect against Area partitioning. Depending
>on the capabilities of the backbone routers, letting more specifics into
>the backbone might be helpful as well as it would deliver more optimal
>routing and also help solve this problem.
>
>Shorter answer is, ya, thats a good idea in my opinion :)
>
>Pete
>
>
[snipped]
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=40774&t=40269
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