The MPLS-OPS Archive

["Eric Osborne" <eosborne@cisco.com>]

On Tue, 18 May 2004 10:41:03 +0000, M. ELK <elkou141061@hotmail.com> wrote:

> Hi
>
> Despite it was explained to me previously still facing some difficulty.
> Consider the following :
>
>
> R1-a-----R2-----R3-----R4
>
> P1 prefix associated with R1 ,P2 with R2 ...etc .
>
> All link with 10 as cost . "a" is the interface of R1
>
> currently at R1 :
>
> Prefix      Cost    Via
> P2           10       a
> P3           20       a
> p4           30       a
>
> Set T1 from R1 to R4 with absolute metric 1 :
>
> Case 1 :
>
> Prefix         Cost         Via
> P2              10          a
> P3              20          a
> p4              1           T1
>
> Case 2 :
>
> Prefix      Cost    Via
> P2           10       a
> P3           11      T1
> p4           1        T1
>
> Is it case 1 or case 2 (mainly for P3) ??

The details are probably going to be implementation-specific, but in the  
case of the implementation I know the most about, it will be case 1.   
Costs are changed *after* the IGP SPF is computed, which means that the  
cost to p4 has no impact whatsoever on p2 and p3.  This is, of course,  
assuming you are talking about our autoroute feature.  We have this other  
thing, forwarding-adjacency, that doesn't behave like this at all, and  
other vendors are free to do whatever they like with metrics.

> My guess it will be case 1 to follow draft-ietf-rtgwg-igp-shortcut-00.txt

I just looked at this draft, and it basically explains how our autoroute  
works.  This leads me to believe that at least one other implementation  
works basically the same way.

As an aside, the special case in section 5 of the draft looks a *lot* like
http://www/en/US/products/sw/iosswrel/ps1612/products_feature_guide09186a0080080850.html#1019729.

> , but what are the reason
> to prefer this approach , is it to avoid any possible loop ??? If yes :  
> could U pls provide an
> example of topology where loop could exist .
>

I've not seen too many people play with autoroute metrics; they are  
confusing to most people, and tend to be used to solve the last 5% or 10%  
of problems after you've applied lots of other tricks.  But that's just  
me, others may have different exposure.

As far as loops, in the autoroute metric case there won't be any loops,  
because of where the cost adjustment is done.




eric

> .
>
> Now :
> Config T2 from R4 to R1 with absolute metric 1 .
> R1 and R4 are config to advertise T1,T2  as link in the IGP .
>
> at R1 :  could we expect that it is now case 2 ??? .
>
>
> Brgds
>
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