Re: [mpls] Question regarding MPLS reserved label with ECMP

[Curtis Villamizar <[e-mail]>]

In message <[e-mail]>
Kireeti Kompella writes:
 
> On Aug 18, 2012, at 4:45 , Vivek Kumar wrote:
>  
> > Hi ,
> >  I have one question regarding whether MPLS reserved label should be used or skipped when Transit LSR is doing hashing on full label stack which has some reserved label.
>  
> draft-ietf-mpls-entropy-label, section 4.3:
>  
>    If a transit LSR recognizes the ELI, it MAY choose to load balance
>    solely on the following label (the EL); otherwise, it SHOULD use as
>    much of the whole label stack as feasible as keys for the load
>    balancing function, with the exception that reserved labels MUST NOT
>    be used.
>  
> >   RFC 6391 , section 7 , says " Note that, depending on the number of labels hashed by the LSR, the
> >   inclusion of the Router Alert label may cause the OAM packet to be
> >   load-balanced to a different path from that taken by the data packets
> >   with identical flow and PW labels".
> >
> >  The above comment implies that reserved label is used by LSR when doing hashing for ECMP.
> >
> >  Is there any other RFC which states what should be the correct behavior.
> >
> >  The draft " draft-ietf-mpls-entropy-label-05" section 4.3 , says reserved label should not be used by LSR when doing hashing on label stack.
> >
> >
> > Regards,
> > Vivek


Kireeti,

entropy-label may be the first place where this advice has been given
and it is good that it is finally somewhere.

If there is a GAL that is also a reason not to hash on a reserved
label.  The path with or without GAL should be the same.

The word "may" in "may cause" in the quoted statement from RFC 6391 is
significant.  Some routers hash on everything they find.  This is a
bad practice, but exists.

Curtis