Regards,
Sujay
On 5/15/07, *Roch Guerin* <[EMAIL PROTECTED]
<mailto:[EMAIL PROTECTED]>> wrote:
I guess this is a question of defining what "optimal" means. If
optimal means that packets should be forwarded along the VL, then
clearly there are cases where that may not happen, although as
Acee pointed out, this wont create (permanent0 routing loops
since no matter what packets are always forwarded in a manner
that consistently decreases their cost to the destination.
However, note that the cost decrease rule is not equivalent to
always selecting the shortest path. In particular, there are
many cases where actual packet forwarding follows a different
path from that the initial router computed. In some cases those
paths will be shorter than the original one (the VL case under
discussion is one such example), but not always. This is because
of the many situations where different routers have access to
different information, and the fact that there are non-cost based
precedence rules when selecting paths.
For example, an internal router R0 could pick an ABR, say ABR1,
as its exit point from its area for a remote prefix /p/ because
the sum of its cost to ABR1 and the cost of the T3 for /p/
advertised by ABR1 is the smallest among all possible choices.
However, if there is another ABR, say ABR2, on the (shortest)
path from R0 to ABR1, then ABR2 will hijack all the packets to
/p/ and will proceed to forward them onto its own shortest path
to /p/, even though this wont be the shortest path originally
selected by R0. And there are many other examples one can
construct, where actual packet forwarding is not along true
end-to-end shortest paths and where different routers have
differing views on what paths packets actually follow. None of
these behaviors create the risk of routing loops.
Roch
Hi,
I fail to see as to how a non-optimal path will be picked up,
for all
ABR's adjoining the transit area will do a transit area summary lsa
checking and route table modification if required.
Noting that the only difference prior to transit area summary
lsa checking and
after will be the nexthop modified by the ABR to reach the same
destination, and
as IP is hop by hop forwarding any border router adjoining the
transit area will pick
up the optimum path( be it thru the VL or not).
Am I miss something?
Regds,
Sujay
On 4/30/07, *Kui Zhang* <[EMAIL PROTECTED]
<mailto:[EMAIL PROTECTED]>> wrote:
Hi Acee,
Yes, a simple way to prevent this from happening is to
configure a full mesh
of virtual links between all ABRs in the transit area.
And this makes IP FRR works well too.
Thanks,
Kui
-----Original Message-----
From: Acee Lindem [mailto:[EMAIL PROTECTED]
<mailto:[EMAIL PROTECTED]>]
Sent: Monday, April 30, 2007 8:28 PM
To: 章魁
Cc: 'Anton Smirnov'; 'OSPF List'; 'Jin Wang'
Subject: Re: [OSPF] Doubt for virtual link cost ?
Hi Kui,
Yes - there could be an ABR for the transit area that takes a
"shortcut" across the transit area for a non-virtual link
path. Other
routers in the backbone will not know about this path and
may take
other paths. However, this is not a problem since there is no
possibility of a routing loop. Additionally, the situation
can easily
be remedied by configuring a virtual link between the
transit area
routers across the "shortcut" path.
Thanks,
Acee
On Apr 30, 2007, at 2:48 AM, 章魁 wrote:
> Hi Acee,
> I guess Jin is asking that a non-optimal path might be
chosen when
> other
> routers in backbone area can't check the transit area's
summary lsa to
> update the intra-area route. This should be a drawback of
virtual
> link, I
> think.
>
> Kui
> -----Original Message-----
> From: Acee Lindem [mailto:[EMAIL PROTECTED]
<mailto:[EMAIL PROTECTED]>]
> Sent: Monday, April 30, 2007 4:11 AM
> To: Anton Smirnov
> Cc: OSPF List; Jin Wang
> Subject: Re: [OSPF] Doubt for virtual link cost ?
>
> Hi Anton, Jin,
> I guess I don't really understand the point of controversy
here. I
> agree with Anton. Note that if there is not an active
virtual link
> then the non-backbone area is not a transit area.
> Thanks,
> Acee
> On Apr 29, 2007, at 8:23 AM, Anton Smirnov wrote:
>
>> Jin,
>> right. In network design where such scenario is
possible you
>> always
>> can create VL between this ABR and ABR providing best path via
>> transit
>> area. This will make the best path 'visible' to other
routers in
>> backbone.
>>
>> Anton
>>
>>
>> Jin Wang wrote:
>>>
>>> Thanks for anton's response.
>>> But if so,only the traffic going through this ABR(determined
>>> before 16.3
>>> calculating result) is forwarded by the new optimal
path.For those
>>> other
>>> traffic not forwarded via this ABR(determined by 16.1 &
16.2) in the
>>> backbone will not consider the new path(via ABR) even path is
>>> shorter.right?
>>>
>>>
>>> Best regards,
>>>
>>> Wangjin
>>> Accton Technology China Company Ltd.
>>> Shanghai R&D Center
>>> TEL:+86-021-64859922*6227
>>> E-mail:[EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]>
>>> Web Site: www.accton.com.cn <http://www.accton.com.cn>
>>>
>>
>>
>> _______________________________________________
>> OSPF mailing list
>> [email protected] <mailto:[email protected]>
>> https://www1.ietf.org/mailman/listinfo/ospf
>
>
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