Re: [OSPF] Doubt for virtual link cost ?

Wed, 16 May 2007 05:16:45 -0700 

Hi Sujay,

On May 16, 2007, at 2:05 AM, sujay gupta wrote:


Hi Roch,
Agree with your explanation, "optimal" definitely in this context does not mean that the packet should be forwarded by the VL, in fact that is the reason why we perform the transit area summary lsa checks to rectify the reachability thru a better path.
Again going with your example, as the next forwarding decision or the path is taken by the intermediate routers and not by the internal router R0, there should not arise a case when a packet forwarded by an internal router in the backbone reaching a ABR adjacent to a transit area will not be forwarded using the best path, which I believe was the point of discussion in 
this thread.( I am yet to read the link suggested by Pierre )
Here is a simple case. Consider a transit area with 3 ABRs: ABR-1, ABR-2, and ABR-3. ABR-1 and ABR-3 are connected via a virtual link but ABR-2 is not. A backbone area internal router BBIR-1 is adjacent to ABR-2. There may be shorter paths for BBIR-1 through ABR-2 via the transit area but it will not take these paths since they are not advertised. If BBIR-1 happens to select a path that includes ABR-2, ABR=2 may use the shorter path through the transit area even though that was not the path BBIR-1 computed. 

Hope this helps,
Acee
P.S. Don't take the link to paper unless you are an ACM member since the paper requires a login and subscription. I let my ACM membership lapse a long time ago since I found I didn't even have time to look at the pictures, let alone read the articles :^) 







Regards,
Sujay

On 5/15/07, Roch Guerin <[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] > 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] ]
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]
> 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]
>>> Web Site: www.accton.com.cn
>>>
>>
>>
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>
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